MANUFACTURING OF OXIDE-DISPERSION STRENGTHENED ALLOYS BY LIQUID METALLURGY

Abstract

Provided herein is a method of producing an oxide-dispersion strengthened (ODS) alloy that includes providing a master alloy powder comprising a metal or metal alloy and particles of a metal oxide; adding the master alloy powder to a molten diluent alloy to form an oxide-dispersion strengthened (ODS) alloy; and allowing the ODS to solidify. The molten diluent alloy includes a molten metal or metal alloy, and a wetting-enhancing metal is added to the molten diluent alloy either prior to, during, or after adding of the master alloy to the molten diluent alloy. The wetting-enhancing alloy reduces an interfacial energy and vdW attraction between the particles of the metal oxide and the molten alloy diluent to achieve a stable nanoparticulate ODS solid.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority to U.S. Provisional Patent Application No. 63/160,433 filed Mar. 12, 2021, which is hereby incorporated by reference, in its entirety for any and all purposes.

TECHNICAL FIELD

Oxide-dispersion strengthened (ODS) alloys, exhibiting extraordinary properties at high temperature, are in high demand in fields such as aerospace, nuclear power, and other applications with demanding conditions. They may also be used in additive manufacturing and welding. Current fabrication methods for ODS alloys such as ball milling have been limited by the high cost (hundreds of dollars per kilogram) and low production volume. Nanoparticles such as Al₂O₃ and Y₂O₃ cannot be incorporated into molten alloys without severe sintering due poor wettability. Other in-situ methods are also limited by high cost due to the requirement of fast cooling. Similar problems can face other ODS alloys.

SUMMARY

Provided in one aspect is a method of producing an oxide-dispersion strengthened (ODS) alloy, the method including:

• • providing a master alloy powder including a metal or metal alloy and particles of a metal oxide;
  • adding the master alloy powder to a molten diluent alloy to form an oxide-dispersion strengthened (ODS) alloy; and
  • allowing the ODS to solidify;
  • wherein:
    • the molten diluent alloy includes a molten metal or metal alloy;
    • a wetting-enhancing metal is added to the molten diluent alloy either prior to, during, or after adding of the master alloy to the molten diluent alloy; and
    • the wetting-enhancing alloy reduces an interfacial energy and vdW attraction between the particles of the metal oxide and the molten alloy diluent to achieve a stable nanoparticulate ODS solid.

In some embodiments, allowing the ODS to solidify includes casting the ODS alloy.

In some embodiments, the metal or metal alloy of the master alloy powder and the molten diluent alloy are individually at least one of Al, Mg, Fe, Ag, Cu, Mn, Ni, Ti, Cr, Co, Au, or Zn. In some embodiments, the metal or metal alloy of the master alloy powder and the molten diluent alloy include Fe, Mg, Mn, Co, or a mixture of any two or more thereof. In some embodiments, the metal or metal alloy of the master alloy powder and the molten diluent alloy are the same. In some embodiments, the metal or metal alloy of the master alloy powder and the molten diluent alloy both include a steel.

In some embodiments, the particles of metal oxide particles include Al₂O₃, Y₂O₃, ZrO₂, MgAl₂O₄, yttria stabilized zirconia, MgO, or a mixture of any two or more thereof. In some embodiments, the particles of the metal oxide particles are microparticles or nanoparticles. In some embodiments, the particles of the metal oxide particles are nanoparticles.

In some embodiments, the wetting-enhancing metal comprises an alloying element. In some embodiments, the wetting-enhancing metal comprises Nb, Cr, or a mixture thereof.

Provided in another aspect is a bulk metal matrix formed by any one of the methods disclosed herein.

Provided in another aspect is a bulk metal matrix including a metal matrix and a plurality of uniformly dispersed metal oxide particles, wherein the oxide particles include at their surface a wetting-enhancing metal that reduces the interfacial energy and vdW attraction between the oxide particles and the bulk metal in molten metal. In some embodiments, the wetting-enhancing metal comprises an alloying element.

In some embodiments, the metal matrix includes at least one of Al, Mg, Fe, Ag, Cu, Mn, Ni, Ti, Cr, Co, Au, or Zn. In some embodiments, the metal matrix includes a steel.

In some embodiments, the metal oxide particles include Al₂O₃, Y₂O₃, ZrO₂, MgAl₂O₄, yttria stabilized zirconia, MgO, or a mixture of any two or more thereof. In some embodiments, the wetting-enhancing metal comprises Nb, Cr, or a mixture thereof.

Provided in another aspect is a method of manufacturing an ODS master alloy with high oxide nanoparticle loading for dilution to final ODA alloys including rapidly heating one or more metal of the alloy with the oxide nanoparticles for a period of time that less than the amount of time to sinter a majority of the oxide nanoparticles.

In some embodiments, the metal of the alloy with the oxide nanoparticles are rapidly heated using arc melting or laser melting.

In some aspects of the present disclosure includes the following embodiments. These are illustrative embodiments using steels, but it should be understood that the disclosure related to other embodiments as well.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B, 1, and 1D show the various methods for reducing vdW attraction between the oxide particles in the bulk molten metal/alloy.

FIGS. 2A, 2B, and 2C show the 1D main forms of work of adhesion and contact angle variation for non-reactive A-B liquid alloy and oxide systems and the corresponding criteria of determination.

FIG. 3 shows a schematic of the vdW attraction between two oxide nanoparticles with interfacial liquid layer (y_B) in a melt with composition of x_B.

FIGS. 4A, 4B, 4C, 4D, and 4E show the W(xB) and θ (xB) isotherms for (a) Fe—Si/Al₂O₃, (b) Fe—Mn/Al₂O₃, (c) Fe—Cr/Al₂O₃, (d) Fe—Nb/Al₂O₃, and (e) Fe—Nb/Y₂O₃, respectively.

FIGS. 5A, 5B, and 5C show vdW attraction potential vs. interfacial layer thickness plot at nanoparticle radius of 20 nm or 5 nm for (a) Fe—Nb/Al₂O₃, (b) Fe—Cr/Al₂O₃, and (c) Fe—Nb/Y₂O₃, respectively.

FIG. 6 shows a schematic of oxide nanoparticle incorporation in molten steel using powder master pellet.

FIGS. 7A and 7C show SEM micrographs of as cast Fe-1.2Nb-0.12Al₂O₃ (in wt. %) at different magnification. FIG. 7B shows the SEM micrograph of as cast Fe-0.12Al₂O₃.

FIGS. 8A and 8B show SEM micrographs of as cast 316L-1Nb-0.13Y₂O₃ (in wt. %). FIGS. 8C and 8D show SEM micrographs of as cast 316L-1Nb-0.11Al₂O₃. FIGS. 8E and 8F show SEM micrographs of as cast 316L-0Nb-0.11Al₂O₃ (reference sample).

FIGS. 9A and 9B show SEM micrographs of ODS low carbon steel as cast Fe1.2Nb0.62Mn0.15C0.06Si-0.13Y₂O₃ (in wt. %); FIGS. 9C and 9D as cast Fe1.2Nb0.62Mn0.15C0.06Si-0.11Al₂O₃; and FIGS. 9E and 9F as cast Fe0.62Mn0.15C0.06Si-0.11Al₂O₃ (reference sample).

FIG. 10A shows the HAADF STEM image displaying one Y₂O₃ nanoparticle in Fe matrix. The red dashed line indicates a 160 nm segment of line in which the EDS line scan data was taken. FIG. 10B shows the elemental distribution (Fe, Nb, and Y) along the red dashed line.

FIG. 11 shows a schematic of ODS master alloy manufacturing by arc melting using a TIG welding torch.

FIG. 12 shows a schematic of ODS master alloy manufacturing by selective laser melting.

FIG. 13A shows the SEM micrograph of as-solidified Fe7Mn1Nb1C1Si-4Y₂O₃ ODS master alloy. FIG. 13B shows a zoomed-in view of selected area. FIG. 13C shows the size distribution of the Y₂O₃ nanoparticles in FIG. 13B. FIG. 13D shows the SEM micrograph of as-solidified Fe7Mn1Nb1C1Si-6.8Y₂O₃ ODS master alloy. FIG. 13E shows a zoomed-in view of selected area. FIG. 13F shows the size distribution of the Y₂O₃ nanoparticles in FIG. 13E.

FIG. 14A shows the SEM micrograph of as-solidified Fe1Nb-4Y₂O₃ ODS master alloy. FIG. 14B shows a zoomed-in view of selected area. FIG. 14C shows the size distribution of the Y₂O₃ nanoparticles in FIG. 14B. FIG. 14D shows the SEM micrograph of as-solidified Fe1Nb-6.8Y₂O₃ ODS ODS master alloy. FIG. 14E shows a zoomed-in view of selected area. FIG. 14F shows the size distribution of the Y₂O₃ nanoparticles in FIG. 14E.

FIGS. 15A and 15B shows the SEM micrograph of as-printed Fe7Mn1Nb1C1Si-4Y₂O₃ ODS master alloy. FIGS. 15C and 15D shows the SEM micrograph of as-printed Fe7Mn1Nb1C1Si-6.8Y₂O₃ ODS master alloy.

FIGS. 16A, 16B, and 16C show the SEM micrographs of as-cast ODS low carbon steels: (16A.1) Fe1.2Nb0.5Mn0.14C0.04i-0.32Y₂O₃ produced by dilution of arc master containing 6.8% Y₂O₃, (16A.2) zoomed-in view of the nanoparticles of selected area in (16A.1), and (16A.3) particle size distribution of (16A.2). FIG. 16B.1 shows the Fe1.2Nb0.5Mn0.14C0.041-0.32Y₂O₃ produced by dilution of laser master containing 6.8% Y₂O₃. FIG. 16B.2 shows the zoomed-in view of the nanoparticles of selected area (16B.1), and FIG. 16B.3 shows the particle size distribution of (16B.2). FIG. 16C.1 shows the Fe1.2Nb0.88Mn0.19C0.1Si-0.41Y₂O₃ produced by dilution of laser master containing 4% Y₂O₃. FIG. 16C.2 shows the zoomed-in view of the nanoparticles of selected area (16C.1). FIG. 16C.3 shows the particle size distribution of (16C.2).

FIGS. 17A and 17B: SEM micrographs of as-cast ODS stainless steels: FIG. 17A.1 shows the 316L-1Nb-0.14Y₂O₃ produced by dilution of arc master containing 6.8% Y₂O₃. FIG. 17A.2 shows the zoomed-in view of the nanoparticles of selected area (17A.1), and FIG. 17A.3 shows the particle size distribution of (17A.2). FIG. 17B.1 shows the 316L-1Nb-0.14Y₂O₃ produced by dilution of laser master containing 6.8% Y₂O₃. FIG. 17B.2 shows the zoomed-in view of the nanoparticles of selected area (17B.1). FIG. 17B.3 shows the particle size distribution of (17B.2).

FIG. 18A shows the Vickers hardness of as-cast low carbon steel (LCS) blank references (LCS 1 and LCS 2) and ODS LCSs, and FIG. 18B shows the stainless steel 316L blank references and ODS 316L.

DESCRIPTION

Various embodiments are described hereinafter. It should be noted that the specific embodiments are not intended as an exhaustive description or as a limitation to the broader aspects discussed herein. One aspect described in conjunction with a particular embodiment is not necessarily limited to that embodiment and may be practiced with any other embodiment(s).

As used herein, the singular terms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to an object can include multiple objects unless the context clearly dictates otherwise.

As used herein, the terms “substantially,” “substantial,” “approximately,” and “about” are used to describe and account for small variations. When used in conjunction with an event or circumstance, the terms can refer to instances in which the event or circumstance occurs precisely as well as instances in which the event or circumstance occurs to a close approximation. When used in conjunction with a numerical value, the terms can refer to a range of variation of less than or equal to ±10% of that numerical value, such as less than or equal to ±5%, less than or equal to ±4%, less than or equal to ±3%, less than or equal to ±2%, less than or equal to ±1%, less than or equal to ±0.5%, less than or equal to ±0.1%, or less than or equal to ±0.05%. For example, a first numerical value may be deemed to be “substantially” the same or equal to a second numerical value if the first numerical value is within a range of variation of less than or equal to ±10% of the second numerical value, such as less than or equal to ±5%, less than or equal to ±4%, less than or equal to ±3%, less than or equal to ±2%, less than or equal to #1%, less than or equal to ±0.5%, less than or equal to ±0.1%, or less than or equal to ±0.05%.

Additionally, amounts, ratios, and other numerical values are sometimes presented herein in a range format. It is to be understood that such range format is used for convenience and brevity and should be understood flexibly to include numerical values explicitly specified as limits of a range, but also to include all individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly specified. For example, a ratio in the range of about 1 to about 200 should be understood to include the explicitly recited limits of about 1 and about 200, but also to include individual ratios such as about 2, about 3, and about 4, and sub-ranges such as about 10 to about 50, about 20 to about 100, and so forth.

Disclosed herein are methods relating to a manufacturing method for incorporating and dispersing oxide nanoparticles in molten alloys (such as steels), and thus provide a novel and cost-effective way to manufacture oxide-dispersion strengthened (ODS) alloys (such as steels). ODS alloys (such as steels) exhibit higher strength, better thermal stability, better creep and irradiation resistance over alloys (such as steels) counterparts that are not reinforced by oxide nanoparticles (unreinforced alloys). Conventionally, ODS steels are produced primarily through solid-state processes, in which the oxide nanoparticles or oxygen carriers are ball milled (for dispersion) with steel powders at the desirable concentration, and the ball milled powders are consolidated to form bulk ODS steel. This method is time consuming and costly due to the long duration needed for ball milling and the high-cost metal powders, as well as the thermomechanical processing procedures to make plates, rods, or tubes. In contrast to the conventional method of manufacturing ODS steels, this disclosure described methods to enable the production of ODS alloys (such as steels) by liquid metallurgy more quickly and at lower cost.

The present disclosure addresses at least one of two key challenges of manufacturing ODS alloys (such as steels) by liquid metallurgy: 1) Molten alloys (such as steels) have naturally poor wettability with oxide nanoparticles, which will force the nanoparticles to sinter instead of dispersing in molten steels. 2) During the incorporation process, oxide nanoparticles can easily sinter together before having the chance to disperse in molten steel due to the very high temperature they experience. The present disclosure improves the interfacial adhesion, and thus wettability, between the molten steel and the oxide nanoparticles, and reduces the van der Waals attraction significantly between oxide nanoparticles by adding suitable element(s) into the melt. In some aspects, the methods prevent the sintering of oxide nanoparticles during incorporation by protecting them within a master alloy.

The manufacturing methods described herein may be applied to various types of alloys (such as steels) with various oxide nanoparticles. Specifically, the manufacturing methods described herein are scalable due to the conventional liquid metallurgy process used to achieve bulk ODS steels, provide bulk ODS steels with uniformly dispersed small nanoparticles (e.g., 10-50 nm), are relatively low cost, compatible with various steel/oxide systems, and allow for tunable volume percentage of oxide nanoparticles. For example, some embodiments include Y₂O₃ nanoparticle strengthened stainless steel, Al₂O₃ nanoparticle strengthened stainless steel, Y₂O₃ nanoparticle strengthened low carbon steel, and Al₂O₃ nanoparticle strengthened low carbon steel. The manufacturing methods described herein are advantageous over current methods as nanoparticles such as Al₂O₃ and Y₂O₃ cannot be incorporated into molten steels without severe sintering due poor wettability.

Provided in one aspect is a method of producing an oxide-dispersion strengthened (ODS) alloy. The method includes providing a master alloy powder including a metal or metal alloy and particles of a metal oxide; adding the master alloy powder to a molten diluent alloy to form an oxide-dispersion strengthened (ODS) alloy; and allowing the ODS to solidify. In the method, the the molten diluent alloy includes a molten metal or metal alloy. Additionally, a wetting-enhancing metal may be added to the molten diluent alloy either prior to, during, or after adding of the master alloy to the molten diluent alloy, where the the wetting-enhancing alloy reduces an interfacial energy and vdW attraction between the particles of the metal oxide and the molten alloy diluent to achieve a stable nanoparticulate ODS solid. As used herein, the master alloy is typically a powder of intimately mixed metal or metal alloy with the metal oxide. This can be prepared by a variety of methods including ball milling. The master alloy is typically more concentrated in the metal oxide prior to dilution with the diluent metal or metal alloy. Master alloys are typically used in the industry and their production is not particularly limited here.

In some embodiments, the allowing the ODS to solidify includes casting the ODS alloy. In some embodiments, the ODS is allowed to solidify slowly (e.g., cooling rate <5 K/s).

In some embodiments, the metal or metal alloy of the master alloy powder and the molten diluent alloy are individually at least one of Al, Mg, Fe, Ag, Cu, Mn, Ni, Ti, Cr, Co, Au, or Zn. In some embodiments, the metal or metal alloy of the master alloy powder and the molten diluent alloy include Fe, Mg, Mn, Co, or a mixture of any two or more thereof.

Examples of suitable sources for the master alloy powder include but are not limited to commercial stainless steel 316L, medium manganese steel (Fe-7Mn-1Nb-1C-1Si in wt. %), Fe—Nb binary alloy (Fe-1Nb), and pure iron. In some embodiments, the metal or metal alloy of the master alloy powder is at least one of Al, Mg, Fe, Ag, Cu, Mn, Ni, Ti, Cr, Co, Au, or Zn. In some embodiments, the metal or metal alloy of the master alloy powder includes Fe, Mg, Mn, Co, or a mixture of any two or more thereof. In some embodiments, the metal or metal alloy of the master alloy powder includes Al. In some embodiments, the metal or metal alloy of the master alloy powder includes Mg. In some embodiments, the metal or metal alloy of the master alloy powder includes Fe. In some embodiments, the metal or metal alloy of the master alloy powder includes Ag. In some embodiments, the metal or metal alloy of the master alloy powder includes Cu. In some embodiments, the metal or metal alloy of the master alloy powder includes Mn. In some embodiments, the metal or metal alloy of the master alloy powder includes Ni. In some embodiments, the metal or metal alloy of the master alloy powder includes Cr. In some embodiments, the metal or metal alloy of the master alloy powder includes Co. In some embodiments, the metal or metal alloy of the master alloy powder includes Au. In some embodiments, the metal or metal alloy of the master alloy powder includes Zn.

In some embodiments, the molten diluent alloy is at least one of Al, Mg, Fe, Ag, Cu, Mn, Ni, Ti, Cr, Co, Au, or Zn. In some embodiments, the molten diluent alloy includes Fe, Mg, Mn, Co, or a mixture of any two or more thereof. In some embodiments, the molten diluent alloy includes Al. In some embodiments, the molten diluent alloy includes Mg. In some embodiments, molten diluent alloy includes Fe. In some embodiments, the molten diluent alloy includes Ag. In some embodiments, the molten diluent alloy includes Cu. In some embodiments, the molten diluent alloy includes Mn. In some embodiments, the molten diluent alloy includes Ni. In some embodiments, the molten diluent alloy includes Cr. In some embodiments, the molten diluent alloy includes Co. In some embodiments, the molten diluent alloy includes Au. In some embodiments, the molten diluent alloy includes Zn.

In some embodiments, the metal or metal alloy of the master alloy powder and the molten diluent alloy are the same. In some embodiments, the metal or metal alloy of the master alloy powder and the molten diluent alloy are different. In some embodiments, the metal or metal alloy of the master allow powder and the molten diluent alloy both include a steel (e.g. stainless steel and low carbon steel).

Examples of suitable oxide nanoparticles include but not limited to Al₂O₃, Y₂O₃, and yttria stabilized zirconia (YSZ). In some embodiments, the particles of metal oxide particles include Al₂O₃, Y₂O₃, ZrO₂, MgAl₂O₄, yttria stabilized zirconia, MgO, or a mixture of any two or more thereof. In some embodiments, the particles of metal oxide particles include Al₂O₃. In some embodiments, the particles of metal oxide particles include Y₂O₃. In some embodiments, the particles of metal oxide particles include ZrO₂. In some embodiments, the particles of metal oxide particles include MgAl₂O₄. In some embodiments, the particles of metal oxide particles include MgAl₂O₄. In some embodiments, the particles of metal oxide particles include yttria stabilized zirconia. In some embodiments, the particles of metal oxide particles include MgO.

In some embodiments, the particles of the metal oxide particles are microparticles or nanoparticles. In some embodiments, the particles of the metal oxide particles are nanoparticles. In some embodiments, the particles of the metal oxide particles have a particle size distribution of from about 1 to about 100 nm, from about 1 to about 75 nm, and from about 1 to about 50 nm, including about 1 nm, about 10 nm, about 20 nm, about 30 nm, about 40 nm, about 50 nm, about 60 nm, about 70 nm, about 75 nm, about 80 nm, about 90 nm, and about 100 nm. In some embodiments, the particles of the metal oxide particles have a particle size distribution of from about 1 to about 50 nm. In some embodiments, the particles of the metal oxide particles have a particle size distribution of from about 10 to about 50 nm.

Suitable wetting-enhancing agents may be alloying elements, such as Mn, Si, Cr, and Nb. Such wetting agents may be effective in reducing the van der Waals (vdW) attraction between oxide nanoparticles, because they are likely to form a thin interfacial liquid layer around the oxide nanoparticles. In some embodiments, the wetting-enhancing metal may be any alloying element. In some embodiments, the wetting-enhancing metal includes Nb, Cr, or a mixture thereof. In some embodiments, the wetting-enhancing metal includes Nb. In some embodiments, the wetting-enhancing metal includes Cr.

Provided in another aspect is a bulk metal matrix formed by any one of the methods disclosed herein.

Provided in another aspect is a bulk metal matrix including a metal matrix and a plurality of uniformly dispersed metal oxide particles, wherein the oxide particles include at their surface a wetting-enhancing metal that reduces the interfacial energy and vdW attraction between the oxide particles and the bulk metal in molten metal.

In some embodiments, the metal matrix includes at least one of Al, Mg, Fe, Ag, Cu, Mn, Ni, Ti, Cr, Co, Au, or Zn. In some embodiments, the metal matrix includes Al. In some embodiments, the metal matrix includes Mg. In some embodiments, the metal matrix includes Fe. In some embodiments, the metal matrix includes Ag. In some embodiments, the metal matrix includes Cu. In some embodiments, the metal matrix includes Mn. In some embodiments, the metal matrix includes Ni. In some embodiments, the metal matrix includes Ti. In some embodiments, the metal matrix includes Cr. In some embodiments, the metal matrix includes Co. In some embodiments, the metal matrix includes Au. In some embodiments, the metal matrix includes Zn. In some embodiments, the metal matrix includes a steel.

In some embodiments, the metal oxide particles include Al₂O₃, Y₂O₃, ZrO₂, MgAl₂O₄, yttria stabilized zirconia, MgO, or a mixture of any two or more thereof. In some embodiments, the metal oxide particles include Al₂O₃. In some embodiments, the metal oxide particles include Y₂O₃. In some embodiments, the metal oxide particles include ZrO₂. In some embodiments, the metal oxide particles include MgAl₂O₄. In some embodiments, the metal oxide particles include yttria stabilized zirconia. In some embodiments, the metal oxide particles include MgO.

In some embodiments, the wetting-enhancing metal may be any alloying element. In some embodiments, the wetting-enhancing metal comprises Nb, Cr, or a mixture thereof. In some embodiments, the wetting-enhancing metal includes Nb. In some embodiments, the wetting-enhancing metal includes Cr.

Provided in another aspect is a method of manufacturing an ODS master alloy with high oxide nanoparticle loading for dilution to final ODA alloys including rapidly heating one or more metal of the alloy with the oxide nanoparticles for a period of time that less than the amount of time to sinter a majority of the oxide nanoparticles.

For the manufacturing methods described herein, rapid melting with an extreme heat source prevents sintering of the oxide nanoparticles. In particular, this disclosure recognizes that the combination of rapid heating (>1000 K/s), extreme processing temperature (>2200 K), and short processing duration (<5 s) for important for minimizing sintering. In some embodiments, the metal of the alloy with the oxide nanoparticles are rapidly heated using arc melting or laser melting. In some embodiments, the metal of the alloy with the oxide nanoparticles are rapidly heated using arc melting. In some embodiments, the metal of the alloy with the oxide nanoparticles are rapidly heated using laser melting.

Some embodiments include a method of producing an oxide-dispersion strengthened (ODS) metal/alloy. The method includes providing a bulk molten metal/alloy comprising one or more metals; adding a plurality of oxide particles that are surface enriched with an element that reduces interfacial energy and vdW attraction between the oxide particles and the bulk molten metal/alloy; and casting the ODS metal/alloy.

The ODS metal/alloy may be melted or welded without substantially affecting the oxide-dispersion. In some embodiments, the metal/alloy comprises one or more selected from Al, Mg, Fe, Ag, Cu, Mn, Ni, Ti, Cr, Co, Au, and Zn. In some embodiments, the bulk molten metal/alloy is steel, such as stainless steel or low carbon steel. In some embodiments, the oxide particles are surface enriched at nanoscale with Nb, Cr, which should be carefully selected for each nanoparticle and metal/alloy system. In some embodiments, the oxide particles comprise a higher concentration of the element that reduces interfacial energy and vdW attraction within a few atomic layers (e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10 layers) or about 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8 or 2.0 nm from surface than the concentration of the alloying element that reduces interfacial energy between nanoparticle and metal/alloy and vdW attraction between nanoparticles in the high temperature melt. In some embodiments, the oxide particles comprise at least one of Al₂O₃, Y₂O₃, ZrO₂, MgAl₂O₄, yttria stabilized zirconia and MgO.

In some embodiments, the oxide particles are microparticles or nanoparticles. In some embodiments, the oxide particles are nanoparticles. In some embodiments, the nanoparticles can have at least one dimension (e.g., an effective diameter which is twice an effective radius) or at least one average or median dimension (e.g., an average effective diameter which is twice an average effective radius) in a range of about 1 nm to about 1000 nm, such as about 1 nm to about 100 nm, about 1 nm to about 80 nm, about 1 nm to about 60 nm, about 1 nm to about 40 nm, about 1 nm to about 20 nm, or about 1 nm to about 10 nm, although other ranges within about 1 nm to about 1000 nm are contemplated, such as about 1 nm to about 500 nm or about 1 nm to about 200 nm).

Reducing vdW attraction between the oxide particles in the bulk molten metal/alloy may be demonstrated in FIGS. 1A-1D.

Additional embodiments include a bulk metal/alloy matrix comprising a metal/alloy matrix and a plurality of uniformly dispersed oxide particles that are surface enriched with an element that reduces interfacial energy and vdW attraction between the oxide particles and the bulk metal/alloy when molten metal/alloy. In some embodiments, the bulk metal/alloy matrix is formed by the method of any one of the embodiments herein.

Additional embodiments include a method of producing an oxide-dispersion strengthened (ODS) metal/alloy, comprising providing a bulk molten metal/alloy comprising one or more metal/alloy; adding an alloy comprising a metal/alloy and a plurality of oxide particles uniformly dispersed therein; and casting the ODS metal/alloy.

In some embodiments, the alloy comprises one or metal/alloy found in the bulk molten metal/alloy. In some embodiments, the metal/alloy comprises one or more alloys selected from Al, Mg, Fe, Ag, Cu, Mn, Ni, Ti, Cr, Co, Au, and Zn. In some embodiments, the bulk molten metal/alloy is steel. In some embodiments, the alloy comprises Nb or Cr.

In some embodiments, the oxide particles are microparticles or nanoparticles. In some embodiments, the oxide particles are nanoparticle. In some embodiments, oxide particles are nanoparticles that have at least one dimension (e.g., an effective diameter which is twice an effective radius) or have at least one average or median dimension (e.g., an average effective diameter which is twice an average effective radius) in a range of about 1 nm to about 1000 nm, such as about 1 nm to about 100 nm, about 1 nm to about 80 nm, about 1 nm to about 60 nm, about 1 nm to about 40 nm, about 1 nm to about 20 nm, or about 1 nm to about 10 nm, although other ranges within about 1 nm to about 1000 nm are contemplated, such as about 1 nm to about 500 nm or about 1 nm to about 200 nm).

Additional embodiments include a method of manufacturing ODS master alloy with high oxide nanoparticle loading, comprising: rapidly heating one or more metal/alloy of the alloy with the oxide nanoparticles for a period of time that less than the amount of time to sinter a majority of the oxide nanoparticles. In some embodiments, the metal/alloy of the alloy with the oxide nanoparticles are rapidly heated using arc melting or laser melting. In some embodiments, the oxide particles are microparticles or nanoparticles. In some embodiments, the oxide particles are nanoparticles. In some embodiments, the oxide particles are nanoparticles that have at least one dimension (e.g., an effective diameter which is twice an effective radius) or have at least one average or median dimension (e.g., an average effective diameter which is twice an average effective radius) in a range of about 1 nm to about 1000 nm, such as about 1 nm to about 100 nm, about 1 nm to about 80 nm, about 1 nm to about 60 nm, about 1 nm to about 40 nm, about 1 nm to about 20 nm, or about 1 nm to about 10 nm, although other ranges within about 1 nm to about 1000 nm are contemplated, such as about 1 nm to about 500 nm or about 1 nm to about 200 nm).

Manufacturing of ODS Master Alloy Containing Low Volume Percentage of Oxide Nanoparticles

Some aspects of the present disclosure relate to manufacturing ODS alloys (such as steels) by liquid metallurgy is a two-step process. First, the ODS master alloy may be fabricated. The ODS master alloy is an alloy that contains higher volume percentage of oxide nanoparticles than what is needed in ODS steel. The ODS master alloy may be diluted in a steel melt that contains no oxide nanoparticles to create the desired volume percentage of oxide nanoparticles in the final steel product. Depending on the designed volume percentage of nanoparticles in the ODS master alloy, it may be manufactured differently.

Illustrative matrix steels for the ODS master alloy may include, but are not limited to, commercial stainless steel 316L, medium manganese steel (Fe-7Mn-1Nb-1C-1Si in wt. %), Fe—Nb binary alloy (Fe-1Nb), and pure iron. Illustrative oxide nanoparticles include, but are not limited to, Al₂O₃, Y₂O₃, and yttria stabilized zirconia (YSZ).

At low volume percentage of nanoparticles (<2 vol. %), the ODS master alloy may be referred to as a “powder master.” To manufacture the powder master, steel powders are mixed with 2 vol. % of oxide nanoparticles and organic solvent (pure ethanol or acetone) to create a slurry mixture. The particle size distribution of the steel powders is between 15 to 45 μm, and of oxide nanoparticles is between 1-50 nm. The slurry mixture is placed in a glass container and treated in an ultrasonic bath for 10 min. The ultrasonic bath breaks up the oxide nanoparticle clusters and improves the uniformity of mixing, while the organic solvent absorbs the moisture. The slurry mixture is then dried at 60° C. to evaporate the organic solvent. The dried powder mixture is rigorously ground in a mortar and pestle set for 0.5 to 1 hr for uniform mixing. The ground powders are transferred to a stainless steel mold for cold compaction. The cold compaction pressure is between 200 to 400 MPa in uniaxial compression.

One Example of Manufacturing of ODS Master Alloy Containing High Volume Percentage of Oxide Nanoparticles by Using Rapid Melting to Prevent Sintering

Liquid state processing routes that require a temperature above the melting point of steel are not able to prevent sintering, due to the temperature requirement being significantly higher than the sintering temperature of the oxides. Rather, the present disclosure provides a combination of rapid heating (>1000 K/s), extreme processing temperature (>2200 K), and short processing duration (<5 seconds) to bypass the sintering issue in the master alloys containing a high volume percentage of oxide nanoparticles. Without being bound by theory, it is believed that rapid heating allows the process to reach desired temperature in an extremely short time window (within 2 seconds). This means that the sintering that could take place during a prolonged heating stage may be mitigated. Also, extreme processing temperature significantly enhances the wettability between the oxide and the molten steel. It also significantly increases the thermal energy (kT) available for the oxide nanoparticles to move around in the liquid. Finally, the short processing duration allows the system to solidify before any sintering can happen. Nanoparticle sintering in high temperature melt is a statistical event, in which long duration results in more sintering. Therefore short processing duration is necessary to eliminate the sintering.

The combination of the three features can only be offered by extreme heat source, such as an electrical arc, plasma, or laser.

Therefore, at high volume percentage of nanoparticles (2 vol. % or above), the ODS master alloy is manufactured by a rapid melting method, including, but not limited to, by arc/plasma melting or by laser melting, in which they are referred to as “arc master” and “laser master”, respectively.

To manufacture the arc master or the laser master, the starting powders may be prepared as described above. However, as the volume percentage of nanoparticle rises, the uniform mixing becomes more difficult to achieve. To this end, powder-mixing methods, such as ball milling, may be used to achieve better mixing. In a ball-milling step, steel powders are mixed with a high volume percent of oxide nanoparticles and placed in a steel jar. Hardened steel balls are added. The weight ratio between the hardened steel balls and the powder is 10 to 1. The steel jar is sealed and then shaken by a mechanical shaker for 8 up to 24 hr. The uniformly mixed powder is referred to as the “feedstock powder”.

For an arc master, the feedstock powder is cold compacted to form a pellet. The compacted pellet is placed in an electrically grounded pure copper melting bowl or a conductive ceramic crucible. Then, the compacted pellet is melted with a tungsten inert gas (TIG) welding or plasma torch placed 1-2 mm above pellet. The electrical arc or plasma torch can heat up and melt the pellet extremely quickly, and as it is turned off, the molten droplet quickly solidifies. In this process, almost no particle sintering will happen. For laser master, the feedstock powder is directly laid flat in an alumina crucible. The feedstock powder is then melted by a laser melting equipment. The lase melting process takes place with laser power of 100 to 200 W, laser scanning speed of 50 to 200 mm/s, and a hatch space of 0.1 mm. Similar to the arc melting process, the laser melting process heats up, melts, and cools the powder at very high rate, which results in almost no particle sintering.

Dilution of ODS Master Alloy and Dispersion and Stabilization of Oxide Nanoparticle in Molten Alloys (Such as Steels)

The ODS master alloy contains high percentage of oxide nanoparticles, beyond the practical percentage that is needed for ODS alloys (such as steels). Therefore, the ODS master alloy may be diluted in a molten steel base that does not contain any oxide nanoparticles to create the ODS steel with desired percentage of oxide nanoparticles by adjusting the ratio between the master and the base.

During this process, the oxide nanoparticles in the master alloy can still sinter in the melt. Some aspects of the present disclosure address this issue by modifying the melt chemistry through the addition of Nb element. In a high temperature melt, the dispersion or sintering of nanoparticles is determined by the potential energy balance between the vdW attraction potential of the nanoparticles, the interfacial energy barrier due to wetting, and the thermal energy due to temperature. To prevent sintering, the interfacial energy barrier due to wetting needs to be high. But, while the interfacial energy barrier prevents sintering, the nanoparticles can still aggregate in pseudo-clusters. To achieve dispersion is to allow the nanoparticles to break free from the vdW energy well in a pseudo-cluster. The addition of Nb to the melt achieves both goals. The Nb improves the wettability between the nanoparticles and the melt, therefore, the interfacial energy barrier is increased, reducing the chance of sintering. Additionally, the Nb reduces the vdW attraction between the nanoparticles by forming a thin Nb-enriched layer around each nanoparticle, which allows the nanoparticles to break free from pseudo-clusters.

The parameters involved in the dilution process are summarized in Table 1. A steel with desired composition is melted at 1560° C. under protective atmosphere. To adjust the wettability of the molten steel with the oxide nanoparticles, 0.8 to 1.2 wt. % Nb is added to the melt. Stirring is applied to promote the uniform mixing of the Nb in the melt. The ODS master alloy, either in the form of small powder pellet (powder master), small ingot (arc master), or thin sheet (laser master), is added to the molten steel.

Stirring is applied to promote the dissolution of the ODS master, and the uniform mixing of the oxide nanoparticles. Due to the high wettability between the molten steel and the oxide nanoparticles, the nanoparticles will disperse in the molten steel rather than sinter. The furnace is then turned off, allowing the molten steel to solidify slowly (cooling rate <5 K/s).

TABLE 1
Weight-based and volume-based parameters used in the dilution of ODS
master.
Parameter	Weight-based	Volume-based
Oxide nanoparticle percentage in the master (WM or VM)	$W_M\%=\frac{{\mathrm{wt}}_{\mathrm{oxide}}}{{\mathrm{wt}}_{\mathrm{oxide}}+{\mathrm{wt}}_{\mathrm{metal}}}$   wtoxide + wtmetal = wtmaster	$V_M\%=\frac{V_{\mathrm{oxide}}}{V_{\mathrm{oxide}}+V_{\mathrm{metal}}}$   Voxide + Vmetal = Vmaster
Final oxide percentage (WF or VF)	$W_F\%=\frac{{\mathrm{wt}}_{\mathrm{oxide}}}{{\mathrm{wt}}_{\mathrm{master}}+{\mathrm{wt}}_{\mathrm{base}\mathrm{metal}}}$	$V_F\%=\frac{V_{\mathrm{oxide}}}{V_{\mathrm{master}}+V_{\mathrm{base}\mathrm{metal}}}$
Dilution ratio (DR)	$\mathrm{DR}=\frac{{\mathrm{wt}}_{\mathrm{base}\mathrm{metal}}}{{\mathrm{wt}}_{\mathrm{master}}}$
Relation	$W_F\%=\frac{W_M\%}{\mathrm{DR}+1}$	$V_F\%=\frac{V_M\%}{\mathrm{DR}+1}$

Manufacturing of ODS 316L Stainless Steel by Liquid Metallurgy

ODS stainless alloys (such as steels) may be used in high temperature and chemically aggressive service environment such as nuclear reactor fuel cladding. To manufacture ODS 316L stainless steel, the ODS master alloy needs to be fabricated first. The ODS master alloy is consisted of Fe-1Nb-6.76Y₂O₃ (in wt. %), equivalent to 10 vol. % Y₂O₃. Pure Fe powders (APS 50 μm), pure Nb powders (APS 50 μm) and Y₂O₃ nanoparticles (APS 10 nm) are uniformly mixed using the method introduced in 1. The mixed powders are compacted into pellets and then melted with electrical arc using a TIG welding torch at 2.5 kW of power for 5 s, producing the arc master. Instead of producing arc master, producing the laser master is also a viable alternative. Commercial 316L ingot (Fe-17Cr-13Ni-2.2Mo-0.8Si-0.2Mn-0.03C in wt. %) is melted at 1560° C. under protective atmosphere. The molten 316L is added with 1 wt. % of pure Nb pieces. The melt is stirred and held at the same temperature for 10 minutes to ensure homogeneity. To create 316L-1Nb-0.13 wt. %Y₂O₃, the ODS master alloy is added at DR=50, based on Table 1. To create 316L-1Nb-0.26 wt. %Y₂O₃, the ODS master alloy is added at DR=25. Upon addition of the master at designed DR, the melt is stirred for 3 minutes to ensure the full dissolution of the master. The furnace is then turned off, allowing the melt to solidify slowly.

Manufacturing of ODS Low Carbon Steel by Liquid Metallurgy

Low carbon steel is the most versatile and commonly used type of steel in the world. The manufacturing of ODS low carbon steel by liquid metallurgy is an indication of the robustness and the wide-ranging applicability of some aspects of the present disclosure. In this case, the ODS master alloy is consisted of Fe-7Mn-1Nb-1C-1Si-6.76Y₂O₃ (in wt. %). Medium manganese steel (MMS) powders (Fe7Mn1Nb1C1Si, APS 20 μm) is commercially made by gas atomization. The MMS powders and Y₂O₃ nanoparticles (APS 10 nm) are uniformly mixed using the method introduced in 1. The mixed powders are compacted into pellets and then melted with electrical arc using a TIG welding torch at 2.5 kW of power for 5 seconds, producing the arc master. Instead of producing arc master, producing the laser master is also a viable alternative.

A steel with chemical composition of Fe1.2Nb0.2Mn0.1C0.1Si is melted at 1560° C. under protective atmosphere. The melt is stirred and held at the same temperature for 10 minutes to ensure homogeneity. The ODS master alloy is added at DR=25. Upon addition of the master at designed DR, the melt is stirred for 3 minutes to ensure the full dissolution of the master. The furnace is then turned off, allowing the melt to solidify slowly. The final composition of the steel is Fe1.19Nb0.5Mn0.14C0.14Si-0.31Y₂O₃, in which the base composition of the steel resembles that of a common low carbon steel. The final composition may be easily tuned by changing the starting composition of the melt, the composition of the ODS master, and controlling DR.

The present invention, thus generally described, will be understood more readily by reference to the following examples, which are provided by way of illustration and are not intended to be limiting of the present invention.

EXAMPLES

Ionocovalent oxides have higher thermodynamic stability than the metal-like ceramics. Molten steels have very limited reactivity with ionocovalent oxides. If oxide nanoparticles may be incorporated into liquid steel, the reaction issue encountered in previous chapters will be largely bypassed. However, ionocovalent oxides, due to having partial ionic and partial covalent bond, do not tend to form strong bonds with molten steel, resulting in poor interfacial adhesion. The poor interfacial adhesion manifests in the measurable engineering parameters such as work of adhesion and contact angle. In addition, oxides and alloys have large difference in their Hamaker constants, which are in the range of 100-150 zJ for oxides and 300-400 zJ for alloys. Such difference in Hamaker constants resulted in high vdW attraction potential between the oxide nanoparticles in molten steel, which can cause the nanoparticles to be trapped in pseudo-clusters or be sintered if the interfacial energy barrier is overcome.

To this end, the oxide nanoparticle incorporation and dispersion in molten steel can only be achieved when two conditions are satisfied. First, the oxide nanoparticles have a good wettability, due to a strong interfacial adhesion, with molten steel. This gives rise to large interfacial energy barrier against particle sintering. Second, the vdW attraction between the oxide nanoparticles is low. The thermal energy would allow the nanoparticles to break free from pseudo-clusters caused by vdW potential well, and disperse uniformly in the melt.

The wetting behavior between oxides and molten steels is well studied in conventional steel metallurgy. Oxides, such as MgO, Al₂O₃, CaO, and SiO₂, are the main components of slag, which is considered detrimental to the steel product. Therefore, in a conventional steelmaking process, to separate the slag from the molten steel, a poor wetting (high contact angle) between the two is often favored. In this work, the reverse is favored. This chapter examines the effect of several alloying elements on the interfacial adhesion and wettability between the oxide and the molten steel using theoretical modeling approach, then validate their effect using experiments. The vdW attraction is also studied under the same theoretical framework.

Theoretical Study of the Work of Adhesion and Wetting Angle in Fe-x (x=Cr, Si, Mn, or Nb)/Oxide Systems

Pure liquid iron has poor wettability and low work of adhesion with most of metal oxides. Oxides such as Cr₂O₃, TiO₂, and NiO exhibit better wettability with pure liquid iron (θ˜60-90°), but these oxides have low thermodynamic stability and are generally not selected as reinforcing phase in ODS steels.

TABLE 2
Contact angle and work of adhesion between
oxides and pure liquid iron at 1823 K.
	Contact angle,	Work of adhesion,
	θ (deg)	W (J/m2)
	Al2O3	115-135	0.65-1.20
	Y2O3	105	~1.35
	ZrO2	115	1.01
	MgO	125	0.90

Some literature works suggest that the contact angle between liquid may be tuned when certain alloying elements are added. Verhiest et al. demonstrated the effect of Cr and Si addition on the contact angle between liquid iron and oxides (Y₂O₃ and Al₂O₃), and found that 20 wt. % Cr reduces the contact angle by 20° in pure iron while 0.5 wt. % Si reduces the contact angle by 10° in Fe-9Cr melt. Yu et al. studied the effect of Nb (0-1.48%) addition on the contact angle between liquid medium Mn steel (8% Mn, 1% C) and Al₂O₃. It was found that 1.15% Nb reduces the contact angle to below 50° [Journal of Materials Science and Technology, vol. 19, no. 6. pp. 625-627, 2003]. Due to the different chemical composition in the liquid steels and the experimental setup including temperature, holding duration, atmosphere, etc., it is difficult to determine the relative potency of each alloying element in improving the wettability. Therefore, a theoretical modeling of the contact angle and work of adhesion between an ideal binary Fe-x (x=Cr, Si, Mn, or Nb) and Al₂O₃ (or Y₂O₃) is performed. Element Cr is selected because it is the main alloying element for stainless steel, while Mn and Si are the most common alloying element in plain carbon steels. Element Nb is selected due to its reported potency for improving wettability at as low as about 1 wt. %.

Effect of Minor Alloying Element on the Wettability Between Molten Metal/Alloy and Oxides

Common engineering alloys such as steels, Al alloys, and Mg alloys typically contain one or more alloying elements. The interaction between multiple alloying elements and nanoparticles becomes highly complicated. Li et al. laid groundwork for understanding and predicting the influence of alloying element on the wetting behavior of oxide particles in a binary alloy system (A-B system). To understand the effect of adding B solute into A on the oxide wettability, the surface tension between the melt and the oxide (σ_SL) needs to be calculated. When the molar fraction of B (x_B) is small, the change of the surface tension (σ_SL) is also small and may be approximated with a linear relation. The slope of this linear relation is given by Eqn. 1:

$\begin{array}{cc}{\left(\frac{d{\sigma }_{SL}}{d{x}_B}\right)}_{x_B\to 0}=\frac{RT}{{\Omega }_M}\left[1-e^{-\frac{{E_{SL}\left(B\right)}_A}{RT}}\right]& \mathrm{Eqn}.1\end{array}$ $\begin{array}{cc}{\Omega }_M=N\omega =1.091{N\left(\frac{V_M}{N}\right)}^{\frac{2}{3}}& \mathrm{Eqn}.2\end{array}$

The term

${\left(\frac{d{\sigma }_{SL}}{d{x}_B}\right)}_{x_B\to 0}$

is the slope of surface tension change when solute B is infinitely diluted in A. R is gas constant and T is temperature. Ω_M is the molar interfacial area given by Eqn. 2, in which N is Avogadro's number and ω is the average area of a metallic atom at the interface, V_M is the molar volume of the metal. The term E_SL(B)_A is the energy of adsorption of atom B at the metal A-oxide interface in AB alloy with infinitely diluted B, and is given by Eqn. 3:

$\begin{array}{cc}{E_{\mathrm{SL}}\left(B\right)}_A=\left({\sigma }_{\mathrm{LV}}^B-{\sigma }_{\mathrm{LV}}^A\right){\Omega }_M-\left(W^B-W^A\right){\Omega }_M-m\lambda & \mathrm{Eqn}.3\end{array}$ $\begin{array}{cc}\lambda =\mathrm{zN}\left[{\epsilon }_{\mathrm{AB}}-\frac{{\epsilon }_{\mathrm{AA}}+{\epsilon }_{\mathrm{BB}}}{2}\right]& \mathrm{Eqn}.4\end{array}$ $\begin{array}{cc}{E_{\mathrm{LV}}\left(B\right)}_A=\left({\sigma }_{\mathrm{LV}}^B-{\sigma }_{\mathrm{LV}}^A\right){\Omega }_M-m\lambda & \mathrm{Eqn}.5\end{array}$ $\begin{array}{cc}{\left(\frac{y_B}{x_B}\right)}_{x_b\to 0}^{\mathrm{SL}}=e^{-\frac{{E_{\mathrm{SL}}\left(B\right)}_A}{\mathrm{RT}}}& \mathrm{Eqn}.6\end{array}$

where σ_LV^A and σ_LV^B are the liquid surface tension of pure A and pure B, W^A and W^B are the work of adhesion of pure A and pure B on the oxide, m is a structure parameter equal to 0.25 for liquid alloys, λ is the molar exchange energy of the liquid AB alloy, given by Eqn. 4, in which ε_AB, ε_AA, ε_BB are the interaction energy of the AB, AA, BB atom pair, and z is the coordination number in the liquid (number of nearest neighbors). The term E_SL(B)_A describes the tendency of atom B to adsorb onto the surface of the oxide in liquid AB, creating an interfacial layer with enriched B composition (y_B). The ratio between interfacial liquid layer composition (y_B) and bulk liquid composition (x_B) is given by Eqn. 6. Eqn. 5 is the energy of adsorption of atom B at the surface of the metal A from AB alloy infinitely diluted in B, which is used to calculate the variation in surface tension oσ_LV due to the addition of metal B, as given by Eqn. 7.

$\begin{array}{cc}{\left(\frac{d{\sigma }_{LV}}{d{x}_B}\right)}_{x_B\to 0}=\frac{RT}{{\Omega }_M}\left[1-e^{-\frac{{E_{LV}\left(B\right)}_A}{RT}}\right]& \mathrm{Eqn}.7\end{array}$

Finally, the variation of work of adhesion between the oxide and the metal A with infinitely diluted metal B is given by Eqn. 8, and the variation of contact angle (θ) in Eqn. 9.

$\begin{array}{cc}{\left(\frac{dW}{d{x}_B}\right)}_{x_B\to 0}={\left(\frac{d{\sigma }_{LV}}{d{x}_B}\right)}_{x_B\to 0}-{\left(\frac{d{\sigma }_{SL}}{d{x}_B}\right)}_{x_B\to 0}=\frac{RT}{{\Omega }_M}\left[e^{-\frac{{E_{SL}\left(B\right)}_A}{RT}}-e^{-\frac{{E_{LV}\left(B\right)}_A}{RT}}\right]& \mathrm{Eqn}.8\end{array}$ $\begin{array}{cc}{\left(\frac{d\cos \theta }{d{x}_B}\right)}_{x_B\to 0}=-\frac{1}{{\sigma }^{A_{LV}}}\left[{\left(\frac{d{\sigma }_{SL}}{d{x}_B}\right)}_{x_B\to 0}+{\left(\frac{d{\sigma }_{LV}}{d{x}_B}\right)}_{x_B\to 0}\cos {\theta }^A\right]=-\frac{RT}{{\sigma }_{\mathrm{LV}}^A{\Omega }_M}\left\{\left[1-e^{-\frac{{E_{SL}\left(B\right)}_A}{RT}}\right]+\left[1-e^{-\frac{{E_{LV}\left(B\right)}_A}{RT}}\right]\cos {\theta }^A\right\}& \mathrm{Eqn}.9\end{array}$

Where θ^A is the contact angle between the oxide and pure metal A. Based on this set of equation, the change of the work of adhesion (W(x_B))and wetting angle (θ(x_B)) between AB alloy and oxide is summarized in FIGS. 2A-2C. Specifically, FIGS. 2A-2C shown the main forms of work of adhesion and contact angle variation for non-reactive A-B liquid alloy and oxide systems, and the corresponding criteria of determination.

Theoretical Framework

The theoretical framework to predict the change in work of adhesion and contact angle between a liquid binary alloy A-B and an oxide was established by Li et al., J. Mater. Sci., vol. 24, no. 3, pp. 1109-1116, 1989. This framework is based on three basic assumptions: 1) the binary alloy A-B has a very low concentration of metal B, that is, element B is infinitely diluted in metal A; 2) the binary alloy A-B is considered as a regular solution, that is, the enthalpy of mixing is non-zero; 3) the liquid A-B alloy does not react with the oxide. The parameters and their definitions used in this model are shown in Table 3. The molar exchange energy of the liquid AB, λ, as given by Eqn. 4, is difficult to calculate due to the lack of literature data on the bond energy, ε_AB, for some of the systems, such as Fe—Mn, Fe—Nb. Instead, this parameter is evaluated from the tabulated values of partial mixing enthalpies of binary alloys AB at infinite dilution.

TABLE 3
Parameter symbols and their definitions
Parameter
symbol   Definition
σLVA     surface energy of liquid pure metal A
σLVB     surface energy of liquid pure metal B
WA       work of adhesion between pure metal A and oxide
WB       work of adhesion between pure metal B and oxide
θA       contact angle between liquid pure metal A and oxide
θB       contact angle between liquid pure metal B and oxide
xB       molar fraction of B in bulk liquid
yB       molar fraction of B in interfacial liquid layer
σSL      interfacial energy between liquid AB and oxide
σSV      surface energy of solid oxide
λ        molar exchange energy of the liquid AB
ΩM       molar surface area of the liquid AB
ESL (B)A energy of adsorption of metal B at the metal A-oxide
         interface in liquid AB
ELV (B)A energy of adsorption of metal B at the surface of metal
         A in liquid AB

The same theoretical framework is also used to calculate the potential enrichment of metal B on the surface of oxide in liquid AB by using Eqn. 6. Under this framework, it is proposed that a monoatomic interfacial layer with composition y_B will exist on the surface of the oxide. The presence of the interfacial layer can serve as a “dynamic coating” for the oxides that effectively reduces its difference in Hamaker constant with the melt. The schematic of the vdW attraction between two nanoparticles without interfacial layers in a melt, and may be calculated using Eqn. 10.

$\begin{array}{cc}{W}_{\mathrm{vdW}}\left(D\right)=-\frac{{\left(\sqrt{A_S}-\sqrt{A_L}\right)}^2}{6D}\left(\frac{R_1{R}_2}{R_1+R_2}\right)=-\frac{{\left(\sqrt{A_{SiC}}-\sqrt{A_{Mg}}\right)}^2}{6D}\left(\frac{R_1{R}_2}{R_1+R_2}\right)& \mathrm{Eqn}.10\end{array}$

where D is the distance between two nanoparticles in nanometres, A_SiC and A_Mg are the Hamaker constants for the van der Waals interaction and are 248 zJ and 206 zJ for SiC and molten magnesium, respectively. R₁ and R₂ are the radii of two nanoparticles.

The vdW attractions between two nanoparticles with interfacial layers is illustrated in FIG. 3. FIG. 3 is a schematic of the vdW attraction between two oxide nanoparticles with interfacial liquid layer (y_B) in a melt with composition of x_B.

In this scenario, the interfacial liquid layer has a variable thickness T. The Hamaker constants for the solid phase (oxide nanoparticle), interfacial liquid layer, and bulk liquid are A₁₁, A₂₂, and A₃₃, respectively. The vdW attraction potential between the two oxide nanoparticles is given by Eqn. 11, and the associated Hamaker constants are calculated by Eqn. 12, Eqn. 13, and Eqn. 14.

$\begin{array}{cc}{W}_{\mathrm{vdW}}\left(D\right)=\frac{R}{12}\left(-\frac{A_{232}}{D}+\frac{2A_{123}}{D+T}-\frac{A_{121}}{D+2T}\right)& \mathrm{Eqn}.11\end{array}$ $\begin{array}{cc}{A}_{232}\approx {\left(\sqrt{A_{22}}-\sqrt{A_{33}}\right)}^2& \mathrm{Eqn}.12\end{array}$ $\begin{array}{cc}{A}_{123}\approx \left(\sqrt{A_{11}}-\sqrt{A_{22}}\right)\left(\sqrt{A_{33}}-\sqrt{A_{22}}\right)& \mathrm{Eqn}.13\end{array}$ $\begin{array}{cc}{A}_{121}\approx {\left(\sqrt{A_{11}}-\sqrt{A_{22}}\right)}^2& \mathrm{Eqn}.14\end{array}$

Evaluation of Parameters

The AB alloy/oxide system in this work includes Fe—Si/Al₂O₃, Fe—Cr/Al₂O₃, Fe—Mn/Al₂O₃, Fe—Nb/Al₂O₃ and Fe—Nb/Y₂O₃. The numerical values of the parameters required for the theoretical modeling are shown in Table 4. Using Eqn. 3 and Eqn. 5, the E_SL(B)_A and E_LV(B)_A values are calculated for each system, as shown in Table 5. Based on the inequalities between E_SL and E_LV, Σ_LV^B and σ_LV^A, W^B and W^A, the type of change in work of adhesion and contact angle may be assigned to each system as shown in FIGS. 2A-2C.

TABLE 4
Numerical values of parameters in alloy A-B/oxide systems at 1823K.
Alloy		σLVA						λ	ΩM
A-B	Oxide	(J/m2)	σLVB*	σSL	WA	WB	σSV	(kJ/mol)	(104 m2/mol)
Fe—Si	Al2O3	1.85	0.78	2.50	0.65	0.88	0.77	−75	3.4
Si—Fe		0.78	1.85	1.71	0.88	0.65		−67	4.8
Fe—Cr		1.85	1.70	2.50	0.65	2.30**		−6	3.4
Cr—Fe		1.70	1.85	0.20	2.30**	0.65		−6	3.4
Fe—Nb		1.85	1.93	2.50	0.65	2.60		−70	3.4
Nb—Fe		1.93	1.85	0.10***	2.60	0.65		−57	4.5
Fe—Mn		1.85	1.11	2.50	0.65	0.86		1	3.4
Mn—Fe		1.11	1.85	1.02	0.86	0.65		1	3.5
Fe—Nb	Y2O3	1.85	1.93	1.07	1.5**	2.6	0.75	−70	3.4
Nb—Fe		1.93	1.85	0.1	2.6	1.5**		−57	4.5
*Evaluated at melting point of B if it is higher than 1823K.
**Calculated using contact angle and σLV with equation: cos(θ) = W/σLV − 1.
***Approximation using equation: cos(θ) = (σSV − σSL)/σLV

TABLE 5
Evaluation of ESL (B)A and ELV (B)A adsorption energy values in
alloy A-B/oxide systems and their corresponding type of change.
Alloy		ESL (B)A	ELV (B)A
A-B	Oxide	(kJ/mol)	(kJ/mol)	Inequalities	Type
Fe—Si	Al2O3	−25.450	−17.630	ESL < ELV < 0, σLVB <	1
Si—Fe		79.150	68.110	σLVA, WB > WA
Fe—Cr		−59.700	−3.600	ESL < ELV < 0, σLVB <	1
Cr—Fe		62.700	6.600	σLVA, WB > WA
Fe—Nb		−46.080	20.220	ESL < 0 < ELV, σLVB >	3b
Nb—Fe		98.400	10.650	σLVA, WB > WA,
				(σLVB − σLVA) <
				(WB − WA)
Fe—Mn		−32.550	−25.410	ESL < ELV < 0, σLVB <	1
Mn—Fe		33.000	25.650	σLVA, WB > WA
Fe—Nb	Y2O3	−17.180	20.220	ESL < 0 < ELV, σLVB >	3b
Nb—Fe		60.150	10.650	σLVA, WB > WA,
				(σLVB − σLVA) <
				(WB − WA)

Evaluation of the Effect of Different Alloying Elements on Wettability

Using the E_SL(B)_A and E_LV(B)_A adsorption energy values in Table 5, the slopes for the change of the work of adhesion and the contact angle in alloy A-B/oxide systems are calculated using Eqn. 8 and Eqn. 9. The values for the slope for each alloy AB system are summarized in Table 6. The slope values are used to generate W vs. x_B plot and 0 vs. x_B plot for each alloy A-B/oxide system, as shown in FIGS. 4A-4E. Specifically, FIGS. 4A, 4B, 4C, 4D, and 4E show W(x_B) and θ (x_B) isotherms for Fe—Si/Al₂O₃ is shown in FIG. 4A (b) Fe—Mn/Al₂O₃, (c) Fe—Cr/Al₂O₃, (d) Fe—Nb/Al₂O₃, and (e) Fe—Nb/Y₂O₃, respectively. The linear regions near x_B→0 and x_B→1 are limited within 2 at. % of solute in order to satisfy the assumption of high dilution of solute. The transition region beyond the linear region is connected by a smooth curve based on the type of change described in FIGS. 2A-2C.

TABLE 6
$\mathrm{Values}\mathrm{of}{\left(\frac{dW}{d{x}_B}\right)}_{x_B\to 0}\mathrm{and}{\left(\frac{d\cos \theta }{d{x}_B}\right)}_{x_B\to 0}\mathrm{for}\mathrm{each}\mathrm{alloy}A-B/\mathrm{oxide}\mathrm{system}.$
Alloy A-B	Oxide	${\left(\frac{\mathrm{dW}}{d{x}_B}\right)}_{x_B\to 0}\left(J/m^2\right)$	${\left(\frac{d\cos \theta }{d{x}_B}\right)}_{x_B\to 0}$
Fe—Si	Al2O3	0.963	0.710
Si—Fe		−0.002	−0.472
Fe—Cr		22.331	12.094
Cr—Fe		−0.281	−0.290
Fe—Nb		9.204	4.912
Nb—Fe		−0.166	0.204
Fe—Mn		1.434	1.145
Mn—Fe		−0.031	−0.274
Fe—Nb	Y2O3	1.267	0.523
Nb—Fe		−0.166	0.200

The Fe—Si/Al₂O₃ and Fe—Mn/Al₂O₃ systems share the most similarity, in which Si and Mn have slightly higher W value than Fe and the slightly lower θ value than Fe, resulting in the type 1 energy inequalities of E_SL<E_LV<0, σ_LV^B<σ_LV^A, and W^B>W^A. In either case, the work of adhesion and contact angle transitioned slowly between x_B=0 to 1, indicating that Si and Mn will slightly increase the wettability between the liquid and the oxide, but not significant at low concentration. For Fe—Cr/Al₂O₃ system, although the energy inequalities are still type 1, Cr has much higher W value and lower θ value than Fe, resulting in a steeper drop in θ. At 20 at. % Cr, which is the normal Cr concentration in stainless steel, the model predicts the contact angle to be around 90°.

The Fe—Nb/Al₂O₃ and Fe—Nb/Y₂O₃ systems are the only systems with the energy inequalities of type 3b due to Nb having higher liquid surface energy than Fe, resulting in adsorption energy E_LV>0. The type 3b system is unique because the model predicts a specific composition in which a minimum contact angle exist that is lower than the contact angle with either pure A or pure B. Yu et al.'s experimental work seems to confirm this prediction, in which 1.15 wt. % of Nb achieved a lower contact angle (50°@1823K) than 0.83 wt. % of Nb (60°@1823K) or 1.48 wt. % Nb (70°@1823K), in the Fe—Mn—C-xNb(x=0-1.48%)/Al₂O₃ system. The model here deviates from Yu's work in predicting the exact concentration of Nb needed to reach minimum contact angle and the value of the minimum contact angle. Yu's work suggests that an interfacial reaction between Nb and Al₂O₃ takes place and further reduces the contact angle. This theory cannot be accounted for in a model for non-reactive system.

In summary, among the 5 binary alloy systems examined, element Si and Mn showed limited effect in reducing the contact angle. Element Cr is more potent than Si and Mn, but requires high concentration (˜20 at. %) to reach a favourable contact angle (<90°). The Fe—Nb/Al₂O₃ and Fe—Nb/Y₂O₃ are the only type 3b systems indicating that element Nb potentially has the strongest effect in improving the wettability and requires experiment to confirm its effect.

Evaluation of the Effect of Different Alloying Elements on vdW Attraction Between Oxide Particles

Based on the results from previous section, three A-B/oxide systems, Fe—Nb/Al₂O₃, Fe—Cr/Al₂O₃, and Fe—Nb/Y₂O₃, are evaluated for the effect on vdW attraction. The vdW attraction potential between two oxide nanoparticles are calculated using Eqn. 11-Eqn. 14. The Hamaker constants, A₁₁, of Al₂O₃ and Y₂O₃ are 150 zJ and 130 zJ. The Hamaker constant, A₃₃, of the bulk liquid is assumed to equal to that of Fe, 392 zJ , due to the high dilution of Nb or Cr. The Hamaker constant, A₂₂, of the interfacial liquid layer is approximated using a simple rule of mixture based on the interfacial composition, y_B: A₂₂=y_B*A_B+(1−y_B)*A₃₃, in which A_B is the Hamaker constant of the metal B. The nanoparticle distance, D, of 0.4 nm (two atomic layers) is used. The vdW attraction potentials between two oxide nanoparticles are shown in FIGS. 5A-5C. In particular, FIGS. 5A, 5B, and 5C show vdW attraction potential vs. interfacial layer thickness plot at nanoparticle radius of 20 nm or 5 nm for (a) Fe—Nb/Al₂O₃, (b) Fe—Cr/Al₂O₃, and (c) Fe—Nb/Y₂O₃, respectively. Table 7 shows for the y_B/x_B values. The x_B value is set at 0.01 for each system.

TABLE 7
yB/xB values.
	Alloy A-B	Oxide	${\left(\frac{y_B}{x_B}\right)}_{x_{B^{\to 0}}}^{\mathrm{SL}}$
	Fe—Cr	Al2O3	51.4
	Fe—Nb	Al2O3	20.9
	Fe—Nb	Y2O3	3.1

The interfacial layers are highly effective in reducing the vdW attraction between oxide nanoparticles in steel melt. When the interfacial layer is one atomic layer (0.2 nm), the vdW attraction potential is almost reduced by half in all three systems. When the interfacial layer is three atomic layers (0.6 nm), the vdW attraction potential is reduced by about 75%. Despite having very different y_B/x_B values in the three systems (Table 7), the Hamaker constant of the interfacial layer, A₂₂, is relatively unaffected, due to the similar Hamaker constants for Cr (371 zJ), Nb (464 zJ) and Fe (392 zJ). For a nanoparticle with a larger radius (R=20 nm), the thermal energy, kT, is lower than the vdW attraction potential. However, the reduction of vdW attraction potential allows for a significantly higher probability for the nanoparticles to break free from the pseudo-clusters. The probability for a particle to overcome an energy well is proportional to e^−ΔE/kT. Therefore, at a fixed kT of 25 zJ, a particle is 400 times more likely to overcome a 150 zJ well than a 300 zJ well (e^−150/25/e^−300/25=400), or 8100 times more likely to overcome a 75 zJ well (e^−75/25/e^−300/25=8100).

In summary, among the 3 systems examined, Cr and Nb are highly effective in reducing the vdW attraction potential between two Al₂O₃ or Y₂O₃ nanoparticles. The presence of the metallic interfacial liquid layer (y_B) has a strong screening effect to the large Hamaker constant difference between A₁₁ (oxide) and A₃₃ (bulk liquid). The vdW attraction potential is reduced for up to 50% even with only one atomic layer thick of the interfacial liquid layer.

Experimental Study on the Effect of Nb on the Wettability Between Molten Steels and Oxide Nanoparticles

Experimental Design and Procedure

To verify the effect of Nb on the wettability between different steel compositions and oxide nanoparticles, liquid metallurgy experiments were conducted. In molten metal/nanoparticle system, high wettability between the melt and nanoparticles provides a strong interfacial energy barrier that makes nanoparticle contacting and sintering with each other heavily energetically unfavorable. However, for two nanoparticles that are already sintered together, the high wettability offers little help, because the thin liquid film separating them no longer exists. To this end, it is crucial to prevent the sintering of nanoparticles by physically separate them during the incorporation stage, because they have yet to be wetted by the molten metal. For nanoparticles like carbides and borides, molten salt assisted incorporation works well because carbides and borides can form stable colloids in molten salt through the formation of a layer of surface-bound solvent ions which prevents aggregation and sintering, but the minimization of interfacial energy allows carbides and borides to migrate to molten alloys. However, for oxide nanoparticles, molten salt assisted incorporation does not work due to the strong chemical reaction between the two.

In this study, metal micro-powders were used as “spacers” to provide separation between oxide nanoparticles. First, steel micro-powders of different composition were mixed with 2 vol. % oxide nanoparticles. Organic solvents (ethanol or acetone) were added to the mixture creating a slurry. The slurry was treated in an ultrasonic bath for 10 min. The ultrasonic bath helps breaking up the oxide nanoparticle clusters and improves the uniformity of mixing, while the organic solvent absorbs the moisture. The slurry mixture was then dried at 60° C. to evaporate the solvent. The dried powder mixture was rigorously ground in a mortar and pestle set for 0.5-1 hr for uniform mixing. The ground powders were transferred to a stainless steel mold for cold compaction. The cold compaction pressure is ˜400 MPa in uniaxial compression. The compacted powder pellet is referred to as the “powder master” herein.

A steel with desired composition was melted at 1560° C. under argon protective atmosphere. To verify the effect of Nb on the wettability of the molten steel, 1 to 1.2 wt. % Nb was added to the melt. Stirring is applied to promote the dissolution and uniform mixing of the Nb in the melt. The melt is referred to as the “base” herein.

The powder master pellets were added to the molten steel base at a weight ratio of 1:10. Stirring was applied to promote the dissolution of the master, and the uniform mixing of the oxide nanoparticles. The furnace was then turned off, allowing the molten steel to solidify slowly (cooling rate <5 K/s). A schematic of the overall process is shown in FIG. 6, in which, after the addition of the powder master, the dispersion of oxide nanoparticles can only occur in a melt with high wettability.

The different compositions of steel micro-powder, steel base, and oxide nanoparticles tested in this study are summarized in Table 8. The experiments may be divided into 3 categories. In category 1, the micro-powder was pure Fe (APS 100 μm) and the steel base was a binary Fe-1.2Nb alloy. The oxide selected for this category was Al₂O₃ (APS 300 nm). Category 1 was designed to be a simple system that verifies whether Nb improves the wettability and dispersion capability. In category 2, the micro-powder was stainless steel 316L (APS 40 μm) and the steel base was also 316L but with 1% Nb added. Category 2 represents the scenario in which both Cr and Nb are present in the melt. In category 3, the micro-powder was medium Mn steel (MMS) and the steel base was a lean low carbon steel with 1.2% Nb, resulting in a final composition of Fe1.2Nb0.62Mn0.15C0.06Si, closely resembling that of a commercial low carbon steel. Category 3 was designed to test the effect of Nb in the most commonly used and versatile type of steel—low carbon steel. For each category, a steel base that contained no Nb was used as a blank reference. After the melt was solidified, the ingot was cut, polished, and examined under SEM.

TABLE 8
Compositions of steel micro-powder and steel base,
and the oxide nanoparticle type selected.
	Steel micro-powder composition	Steel base
Category	in wt. %	composition	Oxide
1. Binary	Pure Fe	Fe1.2Nb	Al2O3 (300 nm)
Fe—Nb
2. Stainless	316L	316L-1Nb	Al2O3 (50 nm)
steel	(Fe17Cr13Ni2.2Mo0.8Si0.2Mn0.03C)		Y2O3 (50 nm)
3. Low	Medium Mn steel	Fe1.2Nb0.2Mn0.1C	Al2O3 (50 nm)
carbon steel	(Fe7MnNbCSi)		Y2O3 (50 nm)

Results

Binary Fe—Nb/Al₂O₃ System

The microstructures of Fe-1.2Nb-0.12Al₂O₃, equivalent to 0.2 vol. %, are shown in FIGS. 7A and 7C. At low magnification (FIG. 7A), two phases with light contrast may be observed; the one with irregular shape and higher aspect ratio is Fe—Nb intermetallic, formed after solidification due to the poor solubility of Nb in Fe matrix. The phase with equiaxed and circular morphology is Al₂O₃, retaining their size and shape prior to incorporation. High magnification (FIG. 7C) shows that the Al₂O₃ nanoparticles are individually dispersed with no observable sintering issue. Table 9 shows the EDS spot analysis results for two areas selected in FIG. 7C. Area 1 is selected on one Al₂O₃ nanoparticles, showing elevated Al concentration, while Area 2 is selected in the matrix showing no Al signal. In comparison, the reference sample, where the powder master was added directly into pure Fe melt, contains sintered Al₂O₃ phase, as shown in Table 9, due to the poor wettability between pure Fe and Al₂O₃. The comparison between FIG. 7A and FIG. 7B is a clear indication that the addition of Nb drastically improves the wettability between Fe and Al₂O₃.

TABLE 9
EDS analysis for selected areas in FIG. 7C.
	Area 1		Area 2
	Element	wt. %	Element	wt. %
	Fe	91.86	Fe	98.97
	Nb	0.75	Nb	1.03
	Al	7.39	Al	—

Stainless Steel 316L-1Nb/Y₂O₃ and Al₂O₃ Systems

316L is one of the most frequently used stainless steel. Its ODS variant is of great scientific interest. In the literature reviewed, mainly Y₂O₃ are used as the strengthening phase in ODS stainless, due to its excellent high temperature properties. Liquid metallurgy experiments were nonetheless conducted for Al₂O₃ as well as Y₂O₃ to examine the effect of Nb. The as cast microstructures of the samples are shown in FIGS. 8A-8F. In the samples containing Nb, both Al₂O₃ and Y₂O₃ are well dispersed (FIGS. 8B and 8D). In the reference sample containing no Nb, some well dispersed Al₂O₃ nanoparticles are observed, but at a much lower concentration (FIG. 8F). Micron-sized sintered Al₂O₃ are also observed, as shown in FIG. 8E, similar to the case in pure Fe with Al₂O₃. At lower magnification, intermetallics are observed in all samples. In the reference 316L sample, the intermetallics are due to Mo, while in 316L-1Nb samples, the intermetallics contain both Mo and Nb, as they have low solubility at low temperature.

In comparison, the addition of Nb provides clear improvement to the wettability between the 316L matrix and the oxide nanoparticles (Y₂O₃ and Al₂O₃) and nanoparticle dispersion The oxide nanoparticles retained their small size (approximately 50 nm) and equiaxed morphology. Without Nb, the 316L matrix still has limited wettability with the oxides due to the presence of 17% Cr, resulting in a small portion of the oxides being dispersed while the rest being sintered.

Low Carbon Steel/Y₂O₃ and Al₂O₃ Systems

The microstructures of the ODS low carbon steel share similarity with that of ODS stainless steel from previous section, in which, the samples containing Nb have good dispersion of oxide nanoparticles (Y₂O₃ and Al₂O₃), as shown in FIGS. 9B and 9D, with no sintering observed. However, for the same volume percentage of designed oxide (0.2 vol. %), the ODS low carbon steel contained less oxides based on the comparison between the images at the same magnification. This is likely caused by the different wettability between the melt and the oxide nanoparticles, since the stainless steel melt contains high concentrations of Ni and Cr, while the low carbon steel melt contains very little amount of alloying element other than Nb. Therefore, the stainless steel melt is likely to have better wettability with the oxide or better surface solution enrichment, allowing most of the oxides to be incorporated and dispersed, while the low carbon steel melt, still having good wettability, is not as good. In the reference sample (FIGS. 9E and 9F), no dispersed Al₂O₃ nanoparticle is observed, indicating that the wettability is poor between the Nb-free melt and the Al₂O₃ nanoparticles and the vdW potential is high between the Al₂O₃ nanoparticles in the steel melt.

Same sample from FIGS. 9A and 9B was examined using EDS in STEM, as shown in FIGS. 10A and 10B. The HAADF STEM image shows regions with bright contrast surrounding a Y₂O₃ nanoparticle, which are identified to be enriched in Nb by the EDS line scan. The enrichment of Nb is also found in the nanoparticle region, likely due to Nb enriched layer wrapping around the nanoparticle from the back. Since the STEM sample is roughly 150 nm thick, the EDS line scan data contains signals from the entire volume of sample.

Discussion

In the theoretical modeling portion of this study, the effect of Nb, Cr, Mn, and Si on the work of adhesion and contact angle between Fe and Al₂O₃ is predicted. It is found that Mn and Si have a limited effect, Cr has a moderate effect, and Nb has the strongest effect. In the experimental portion, this prediction is verified. The Nb concentration is 1.2% for the Fe—Nb and the low carbon steel systems, and 1% for the stainless steel. The Nb concentration is maintained at this level since Yu's results indicate that the optimal Nb concentration is in the vicinity of 1-1.2%. In the Fe—Nb and the low carbon steel systems, the addition of Nb fundamentally changes the wettability from non-wetting to wetting. Without Nb, the oxides sintered in the melt, leaving no individually dispersed nanoparticle. With 1.2% Nb, the dispersion took place, and no sintered phase is observed. In the stainless steel system, the addition of Nb improves the wettability. Without Nb, the stainless steel melt still offers limited wettability and likely little reduction in vdW potential, which result in a portion being dispersed but a portion of the oxide being sintered. With 1% Nb, the sintering no longer takes place, resulting in individually dispersed nanoparticles. Furthermore, we found that Nb not only improves the wettability for Al₂O₃, but also for Y₂O₃, a more favorable oxide choice for application. In summary, we found that Nb addition improves the dispersion of both Al₂O₃ and Y₂O₃ nanoparticles in various steel compositions, including the simple binary Fe—Nb, the low-alloyed low carbon steel system, and the high-alloyed stainless steel system.

The modeling on vdW attraction potential (FIGS. 5A-5C) suggests that the presence of an interfacial liquid layer will strongly reduce the vdW attraction between two nanoparticles in the steel melt. While smaller nanoparticles (R=5 nm) yield much lower vdW attraction, which may be easily overcome by thermal energy, larger nanoparticles (R=20 nm) still have a good probability breaking free from the vdW potential well. This modeling was experimentally validated in various systems, in which nanoparticle pseudo-clusters were not observed. The STEM image (FIG. 10A) indicates the presence of Nb enriched regions around the Y₂O₃ nanoparticles. At room temperature, Nb has a very low solubility in Fe, primarily forming Fe₂Nb (Laves) intermetallics. Therefore, the Nb enriched regions around the Y₂O₃ nanoparticles may be attribute to the Nb enrichment around the nanoparticle in liquid state due to strong interfacial adhesion, and subsequent solidification allows the Nb beyond solubility limit to precipitate out as Fe₂Nb around the nanoparticle. Therefore, the STEM observation suggests the presence of interfacial liquid layer which significantly reduced the vdW attraction between particles and promoted the uniform dispersion of nanoparticles.

Scalable Manufacturing of Oxide-Dispersion Strengthened (ODS) Steels by Liquid Metallurgy

As described above, non-reactive ionocovalent oxide nanoparticles such as Al₂O₃ and Y₂O₃ are incorporated into various molten alloys (such as steels) containing 1-1.2% Nb, since the addition of Nb greatly improved the wettability between the oxide nanoparticles and the molten alloys (such as steels) while reducing the vdW attraction between the nanoparticles. This chapter proposes methods to fabricate ODS master alloy to enable possible scalable manufacturing of ODS alloys (such as steels) by liquid metallurgy.

Introduction to ODS Master Alloy

While a melt with a good wettability to oxide prevents the oxide nanoparticles sintering by a high interfacial energy barrier, it is unable to pull apart nanoparticles that are already sintered. For metal-like nanoparticles such as WC and TiB₂, molten salts are effective vehicles to carry nanoparticles into the melt, since nanoparticles can form stable colloids without sintering in molten salts. For ionocovalent oxides, similar method is ineffective due to the reaction between oxides and molten salts. Therefore, the oxide nanoparticles are first mixed with metal micro-powders to create physical separation in order to prevent sintering before they are wetted by the melt. However, the metal micro-powder has a very low limit as to how many volume percentages of oxide nanoparticles it can carry. It was found during experiment that metal micro-powder becomes less effective when the volume percentage of oxide rises beyond 2%, in which the oxide nanoparticles start to form thick shells around the metal micro-powders that can no longer be infiltrated by the molten metal, which eventually leads to severe sintering issue during incorporation. To this end, a more effective method to separate the oxide nanoparticles prior to incorporation are necessary.

Xu et al. proposed the concept of metal-nanoparticle superstructure, in which the microstructure changes with the volume percentage of nanoparticle. At low volume percentage, the superstructure takes the form of nanoparticle-coated metal powder. At high volume percentage, the superstructure becomes networks of nanoparticles separated by alloys in-between. These metal-nanoparticle superstructures may be used as masters and diluted into molten alloys to manufacture MMNCs at desired nanoparticle volume percentage. This method of MMNC manufacturing transforms the nanoparticle incorporation, which is normally a high-cost process, into a process that is akin to alloy-making in conventional liquid metallurgy. In conventional liquid metallurgy, necessary elements that are difficult to dissolve, dangerous to handle, or have other reasons making them not suitable for direct addition are often made into master alloy at high concentration and then diluted into the alloy melt to achieve desired composition. Elements such as B, Nb, Li, Ti, V, W, etc. all fit into this category.

For ODS steels, the ideal master alloy should contain high volume percentage of non-sintered oxide nanoparticles, in a form, such as small ingot or thin sheet, that may be easily diluted into molten steel. Currently, there is no mature technology that allows the manufacturing of bulk steel containing high volume percentage of nanoparticles. Kimura et al. demonstrated the production of Fe-24Cr-15Y₂O₃ (in wt. %), equivalent to 22 vol. % Y₂O₃, composite powders by ball milling. The prolonged ball milling process induced super-heavy deformation that caused Y₂O₃ to mechanically decompose in the ferritic matrix. However, the size of Y₂O₃ nanoparticles after re-precipitation hinges upon the heat treatment temperature. An average Y₂O₃ size of 25 nm was reported if the powder is heat treated at 1000 K, but around 150 nm if the heat treated at 1600 K. As such, steel composite powders containing mechanically decomposed oxide phase is unlikely a suitable candidate to be used as master alloy for ODS steel, despite having very high concentration of oxide, because the molten steels will have temperature around 1823 K and significantly higher diffusion kinetics than solid state heat treatment. The decomposed oxide will not be able to retain a small size distribution in this condition. In addition, solid state processing route involving ball milling and hot consolidation have very high cost and requires long processing time, hence not conducive for scalable manufacturing. Majority of the ODS steel produced via this method is limited to below 0.5 vol. % of oxide nanoparticles. Conventional liquid state processing route which needs temperature above the melting point of steel is also unable to manufacture master alloy with high oxide nanoparticle loading, due to the lack of proper incorporation method.

In this work, a novel method of manufacturing ODS master alloy with high oxide nanoparticle loading is described. The sintering issue may be bypassed when nanoparticle loading is high (6-10 vol. %) by the combination of three processing parameters: 1) rapid heating rate (>2000 K/s), 2) extreme melt pool temperature (>2273 K), and 3) short melting duration, for the following reasons. Without being bound by theory it is believed that rapid heating allows the process to reach desired temperature in an extremely short time window (within 1 s). This means that the sintering that could have occurred during a prolonged heating stage in which the nanoparticles are not wetted may be mitigated. Additionally, the extreme melt pool temperature significantly enhances the wettability between the oxide and the molten steel, due to the significant reduction in solid-liquid surface tension (σ_SL) and liquid surface tension (σ_LV). It also significantly increases the thermal energy (kT) available for the oxide nanoparticles to move around in the liquid. Finally, the short melting duration allows the system to solidify before any sintering can happen. Nanoparticle sintering in high temperature melt is a statistical event, in which longer duration increases likelihood of sintering to occur. Therefore, a shorter melt duration is favored when manufacturing ODS master alloy with high nanoparticle loading.

The combination of these processing conditions can only be provided by extreme heat sources, such as an electrical arc or a laser beam, which are known to provide rapid heating rate and melt pool temperature as high as 4000 K. Therefore, ODS master alloys with high nanoparticle load are manufactured using arc melting or laser melting, in which they are referred to “arc master” or “laser master” herein.

Experimental Procedure

ODS Master Alloy Manufacturing

To manufacture the arc master and laser master, metal micro-powders were first mixed with oxide nanoparticles. The mixing method is consistent with that previously shown. For arc master, the mixed powders were cold compacted into pellets about 1 g each. The arc melting was conducted using a tungsten inert gas (TIG) welding equipment. The compacted pellet was placed in an electrically grounded pure Cu bowl for quick heat dissipation after melting. The experimental setup is shown in FIG. 11. For arc master samples, the melting was conducted at 150 A and 16-17 V and lasts for 3-4 seconds. The solidified arc master samples were cut into metallographic samples and examined under SEM.

For laser masters, the mixed powders were cold compacted into thin pellets with thickness around 1 mm. The pellet was placed in an alumina crucible. The surface of the pellet was melted with a selective laser melting (SLM) equipment, as shown in FIG. 12. The printing chamber was purged by Ar gas. The SLM was conducted at laser power of 150 W, scanning speed of 120 mm/s and hatch space of 0.1 mm. After SLM, the pellet was submerged in ethanol and treated in an ultrasonic bath to shake loose the un-melted powders. The laser master in the form of a thin sheet was retrieved. Metallographic samples were prepared and examined under SEM.

Dilution Experiment Using ODS Master Alloy

To verify the effect of the ODS master alloys, they were diluted into various molten steels. A steel with desired composition was melted at 1560° C. under Ar atmosphere. The melt contained 1 wt. % of Nb to improve its wettability with the oxide nanoparticles. Stirring was applied to promote the uniform mixing of the Nb in the melt. The ODS master alloy, either in the form small ingot (arc master) or thin sheet (laser master) was added to the molten steel at calculated ratio; this ratio is referred to as “dilution ratio” (DR) herein. Stirring was applied to promote the dissolution of the ODS master, and the uniform mixing of the oxide nanoparticles. The furnace was then turned off, allowing the molten steel to solidify slowly (cooling rate <5 K/s). The parameters used the control the dilution experiment are summarized in Table 10. The percentage of oxide nanoparticle in the master alloy (W_M or V_M) is related to the final oxide percentage in the ODS steel after dilution by a simple factor of DR+1. The as cast ODS steel samples were prepared into metallographic samples for SEM examination. The Vickers hardness of each sample was measured. Series of control experiments were conducted using the same procedure, however the master alloy used in the control experiments were blanks, which contained no oxide nanoparticles.

TABLE 10
Weight-based and volume-based parameters used in the dilution experiments
of ODS master alloys.
Parameter	Weight-based	Volume-based
Oxide nanoparticle percentage in the master (WM or VM)	$W_M\%=\frac{{\mathrm{wt}}_{\mathrm{oxide}}}{{\mathrm{wt}}_{\mathrm{oxide}}+{\mathrm{wt}}_{\mathrm{metal}}}{\mathrm{wt}}_{\mathrm{oxide}}+{\mathrm{wt}}_{\mathrm{metal}}={\mathrm{wt}}_{\mathrm{master}}$	$V_M\%=\frac{V_{\mathrm{oxide}}}{V_{\mathrm{oxide}}+V_{\mathrm{metal}}}{V}_{\mathrm{oxide}}+V_{\mathrm{metal}}=V_{\mathrm{master}}$
Final oxide percentage (WF or VF)	$W_F\%=\frac{{\mathrm{wt}}_{\mathrm{oxide}}}{{\mathrm{wt}}_{\mathrm{master}}+{\mathrm{wt}}_{\mathrm{base}\mathrm{metal}}}$	$V_F\%=\frac{V_{\mathrm{oxide}}}{V_{\mathrm{master}}+V_{\mathrm{base}\mathrm{metal}}}$
Dilution ratio (DR)	$\mathrm{DR}=\frac{{\mathrm{wt}}_{\mathrm{base}\mathrm{metal}}}{{\mathrm{wt}}_{\mathrm{master}}}$
Relation	$W_F\%=\frac{W_M\%}{\mathrm{DR}+1}$	$V_F\%=\frac{V_M\%}{\mathrm{DR}+1}$

Results

Microstructure of the Arc Masters

Two different matrix compositions were used to manufacture the arc master samples, Fe7Mn1Nb1C1Si and Fe1Nb (in wt. %). Fe7Mn1Nb1C1Si is a medium Mn steel (MMS) that is suitable for dilution to achieve the desired composition of other carbon steels due to its relative high concentration of C (1%) and Mn (7%). Fe1Nb is a simple binary alloy that may be diluted in other low-alloyed steels and stainless steel, since it does not impact the overall composition significantly if a high DR value is used. Y₂O₃ nanoparticle (APS 10 nm) was selected due to its prevalence in literature work. For each matrix composition, arc master samples with two different Y₂O₃ nanoparticle loading were made, 4 and 6.8 wt. %, equivalent to 6 and 10 vol. %, respectively. The microstructures of as-solidified Fe7Mn1Nb1C1Si-4Y₂O₃ ODS and −6.8Y₂O₃ ODS master alloy are shown in FIGS. 13A-13F. For both oxide loading levels (4 and 6.8%), the Y₂O₃ nanoparticles are uniformly dispersed in the matrix. Particle size data is acquired after image processing for the high magnification micrographs (FIGS. 13B and 13E). The APS is 11.6 and 9.7 nm for the 4 and 6.8% Y₂O₃ sample, respectively.

The microstructures of as-solidified Fe1Nb-4Y₂O₃ and -6.8Y₂O₃ ODS master alloy are shown in FIGS. 14A-14F. For both samples, sub-micron Fe₂O₃ inclusions are observed in the matrix (FIGS. 14A and 14D). In the Fe7Mn1Nb1C1Si samples, these inclusions are not observed. Comparatively, the Fe7Mn1Nb1C1Si sample contained high concentration of carbon (C), that can sacrificially burn before other elements are oxidized in the arc melting process, while Fe1Nb sample does not contain a sacrificial element that can prevent Fe from oxidizing. Therefore, the oxygen contamination during the short melting window resulted in the formation of these sub-micro Fe₂O₃ inclusions. However, the presence of these inclusions does not affect the uniform dispersion of Y₂O₃ nanoparticles (FIGS. 14B and 14E). The APS is 11.5 and 12.3 nm for the 4 and 6.8% Y₂O₃ sample, respectively.

Microstructure of the Laser Masters

For laser masters, the matrix composition is Fe7Mn1Nb1C1Si. Y₂O₃ nanoparticle (APS 40-50 nm) was used. Samples with two different Y₂O₃ nanoparticle loadings were prepared, 4 and 6.8 wt. %, equivalent to 6 and 10 vol. %. The microstructures of the as-printed Fe7Mn1Nb1C1Si-4Y₂O₃ and -6.8Y₂O₃ ODS master alloy are shown in FIGS. 15A-D. At low magnification, the laser masters have a distinct two-phase feature, with the matrix having dark contrast and the Y₂O₃ having light contrast. The Y₂O₃ phase distribution strongly resembles that of the laser raster pattern during SLM process. The Y₂O₃ nanoparticles are found in highly concentrated pseudo-clusters (FIGS. 15B and 15D). High magnification view (FIG. 15 insets) suggests that the Y₂O₃ nanoparticles are well-dispersed inside the pseudo-clusters and no sintering is observed. However, the high density of nanoparticles in these regions resulted in the image processing tool being unable to distinguish one nanoparticle from its neighbors. Therefore, particle size statistics cannot be generated. Instead, twenty nanoparticles from the insets of FIGS. 15B and 15D are randomly selected and measured. The APS is 41.3±4.7 nm and 40.7±10.7 nm for the 4 and 6.8% Y₂O₃ sample, respectively.

Microstructure ODS Low Carbon Steel

ODS low carbon steels were produced by diluting the ODS master alloy in a melt with fixed chemical composition. Different DR values were used to control the Y₂O₃ nanoparticle percentage as well as the chemical composition in the final steel. The combination of these parameters is summarized in Table 11. The microstructures of the as-cast ODS low carbon steels are shown in FIGS. 16A-16C.

TABLE 11
Parameters used to produce ODS low carbon steels shown in FIGs. 16A-16C.
		ODS
		master
	Steel base composition	alloy	ODS master alloy composition		Designed final composition
	(wt. %)	type	(wt. %)	DR	(wt. %)
FIG.	Fe1.2Nb0.2Mn0.1C	Arc	Fe7Mn1Nb1C1Si—6.8Y2O3	21	Fe1.2Nb0.5Mn0.14C0.04Si—0.32Y2O3
16A		master	(Y2O3 APS 9.7 nm)
FIG.	Fe1.2Nb0.2Mn0.1C	Laser	Fe7Mn1Nb1C1Si—6.8Y2O3	21	Fe1.2Nb0.5Mn0.14C0.04Si—0.32Y2O3
16B		master	(Y2O3 APS 40.7 nm)
FIG.	Fe1.2Nb0.2Mn0.1C		Fe7Mn1Nb1C1Si—4Y2O3	9	Fe1.2Nb0.88Mn0.19C0.1Si—0.41Y2O3
16C			(Y2O3 APS 41.3 nm)

The primary goal of conducting the dilution experiment is to validate that the oxide nanoparticles can retain a stable self-dispersion after being diluted from high concentration, when in ODS master, to low concentration, when in a steel melt with no nanoparticles. This goal is achieved both using arc master and laser master. The arc master contains very fine Y₂O₃ nanoparticles (APS 9.7 nm), after diluting at DR=21, the resulting ODS low carbon steel is shown in FIGS. 16A.1 and 16A.2. The nanoparticles remained individually dispersed. The APS for this sample was 18.2 nm. However, due to the low volume fraction (0.5 vol. %) and small particle size, the imaging process was challenging. Some very small nanoparticles do not have sufficient contrast to be distinguished from the matrix, which potentially contributed the increase in APS. The laser master contains larger Y₂O₃ nanoparticles (APS ˜40 nm) that are significantly easier to observe after dilution. For samples with DR=9 and 21, as shown in FIGS. 16A.1-2 and FIGS. 16B.1-2, majority of the nanoparticles are individually dispersed, while a few nanoparticles appear to be sintered (FIG. 16A.2). The nanoparticles also retained their size comparing to their as purchased condition (APS 40-50 nm), as printed condition (APS ˜40 nm), and finally as cast condition (APS 40-45 nm).

Microstructure of ODS Stainless Steel

ODS stainless steels were produced by diluting the ODS master alloy in a commercial 316L melt modified with 1 wt. % Nb. The ODS master alloy used in this experiment were Fe1Nb-6.8Y₂O₃ with smaller Y₂O₃ nanoparticles for arc master (APS 12.3 nm) and larger Y₂O₃ nanoparticles for laser master (APS 50.0 nm). The combination of these parameters is summarized in Table 12. The microstructures of the as-cast ODS stainless steels are shown in FIGS. 17A and 17B.

TABLE 12
Parameters used to produce ODS low carbon steels shown in FIGs. 17A and 17B.
		ODS
	Steel base	master	ODS master alloy
	composition	alloy	composition		Designed final composition
	(wt. %)	type	(wt. %)	DR	(wt. %)
FIG.	316L-1Nb	Arc	Fe1Nb—6.8Y2O3	47	316L-1Nb—0.14Y2O3
17A		master	(Y2O3 APS 12.3 nm)
FIG.	316L-1Nb	Laser	Fe1Nb—6.8Y2O3	47	316L-1Nb—0.14Y2O3
17B		master	(Y2O3 APS 50.0 nm)

Stainless steel 316L is a highly alloyed steel that contains over 33 wt. % of alloying elements. Therefore, when diluting the Fe1Nb-6.8Y₂O₃ master alloy, the DR was deliberately set to a high value of 47, to preventing the excess dilution of the remaining alloying elements such as Cr and Ni. Similar to the nanoparticle dispersion observed in the ODS low carbon steels (FIGS. 16A-16C), the Y₂O₃ nanoparticles are also individually dispersed in the ODS stainless steel, as shown in FIGS. 17A.1-2 and 17B.1-2. The arc master contains smaller Y₂O₃ nanoparticles, while the laser master contains larger ones. After dilution, the different sizes of Y₂O₃ nanoparticles are able to disperse equally well. Based on the particle size analysis, the nanoparticles retained their size from as purchased condition throughout the process.

Hardness of ODS Low Carbon Steel and Stainless Steel

The Vickers hardness of as-cast ODS low carbon steel (LCS) and ODS stainless steel 316L are shown in FIGS. 18A and 18B. For each ODS sample, the hardness of a reference sample that contains no nanoparticles, while manufactured in otherwise identical method, is also shown for comparison.

The LCS 1 reference sample has a hardness of 152 HV. The two ODS variants of LCS 1, one produced by diluting the arc master and the other by laser master, have hardness of 149 and 172 HV, respectively. Despite being designed at the same 0.32% of Y₂O₃ nanoparticles, the ODS LCS 1 produced by laser master has noticeably higher hardness. This is primarily due to the carbon loss during the manufacturing of the master alloys. The matrix of the master alloys contains 1% carbon. The arc master was produced by the electrical arc from a TIG welding torch, as shown in FIG. 11. The Ar shielding gas was blew onto the melting pool from an alumina cone. The lack of enclosure in this experimental setup led to higher level of oxygen contamination and carbon burning. Comparatively, the laser master was produced in a semi-enclosed printing chamber, as shown FIG. 12. In this setup, the Ar shielding gas flew through the chamber from one end and exiting from the other, creating a more stable atmosphere to reduce carbon burning. The LCS 1 reference sample has a hardness of 225 HV. The ODS LCS 2 sample has a hardness of 255 HV. The hardness increment is roughly equal to 7 HV gain per 0.1 wt. % Y₂O₃ for both ODS LCS 1 and LCS 2 produced by laser master.

For the stainless steel samples (FIG. 18B), the addition of 1% Nb by itself provides no tangible hardness increase: 155 HV for 316L-0Nb vs. 158 HV for 316L-1Nb. Drastic hardness increase is found in both ODS samples with 0.14%Y₂O₃. The ODS sample produced by laser master sees a hardness gain of 60 HV (43 HV gain per 0.1% Y₂O₃) with larger Y₂O₃ nanoparticles, while the ODS sample produced by arc master sees a hardness gain of 82 HV (58 HV gain per 0.1% Y₂O₃) with smaller Y₂O₃ nanoparticles. In this case, the master alloy is Fe1Nb-6.8Y₂O₃, containing no combustible alloying elements. Despite Fe₂O₃ inclusions were formed in the arc master (FIG. 14D), they did not affect the dispersion of the Y₂O₃ nanoparticles.

Discussion

An electrical arc and a laser beam are two comparable heat sources. Both of them are capable of delivering rapid heating rate and extreme temperature. However, despite the similarity, the microstructure of the ODS master alloy produced using these two methods have very different microstructures. On one hand, the arc masters (FIGS. 13A-13F and 14A-14F) have no observable segregation issue between the oxide nanoparticles and the matrix. The oxide nanoparticles are uniformly dispersed throughout the matrix. On the other hand, the laser masters (FIGS. 15A-15D) have very obvious segregation between the oxide phase and the matrix, in which the oxide phase shows alignment matching the laser raster pattern. The oxide nanoparticles are dispersed in highly particle-dense pseudo-clusters. The difference between the microstructure is due the presence (or the lack thereof) of “global” melting. During the manufacturing of the arc masters, the powder pellet was entirely melted, hence “global”, by the arc and was held at liquid state for 2-3 s before solidification. The global melting significantly improves the dispersion of the nanoparticles, because of the strong Brownian motions. For laser masters, the powder pellet was never entirely melted, hence lacking “global” melting, due to the limitation of the laser spot size. Therefore, the melt pool size was much smaller, limiting the volume in which nanoparticle can travel. In addition, due to the constant scanning motion of the laser, the melt pool was held at liquid state for only a short duration at each location before moving on to the next location, whereas the entire pellet was held in liquid state for 2-3 s for the arc masters. As such, the oxide nanoparticles in the laser masters are confined in pseudo-clusters, as they were unable to disperse beyond the small volume of the melt pool created by the laser. Despite having different quality of nanoparticle dispersion, the dilution experiments found little difference in the final microstructures. Both arc and laser masters are capable of being used to produce ODS steels with uniform nanoparticle dispersion. The nanoparticle size remains largely unchanged in the as-cast ODS steels comparing to in the as-purchased condition.

Due to the difference in experimental setup between the arc melting and SLM process, the master alloys were affected by oxygen contamination differently. The arc masters, despite showing the better nanoparticle dispersion, suffered from carbon burning, which resulted in the lower hardness value after dilution, comparing to the sample with the same designed composition but produced using laser master. If combustible alloying elements are not present in the master, such as the case of Fe1Nb-6.8Y₂O₃, then oxygen does not affect the quality of the arc master. To this end, if the gas protection may be further improved in the arc melting setup, then the arc master would be able to retain its combustible alloying elements while providing better nanoparticle dispersion.

CONCLUSIONS

This work presents a potential scalable manufacturing process for ODS steels by liquid metallurgy. High concentration of oxide nanoparticles is first incorporated and dispersed into a steel matrix by arc melting or SLM, producing the ODS master alloys. The ODS master alloys are then diluted into a steel melt at a high dilution ratio, releasing the oxide nanoparticles. Due to the presence of Nb in the steel melt, the oxide nanoparticles are able to wet with the melt and disperse uniformly. This work validates the theoretical and experimental study: 1. the wettability between the oxide nanoparticle and the melt may be improved to prevent sintering, and 2. the vdW attraction between the nanoparticles may be reduced to promote uniform dispersion. Despite having an initial high cost of producing the ODS master alloys using arc melting or SLM, the final cost is offset by the dilution ratio. When DR is 10, the master alloy only contributes to less than 10% of the final ODS cost. When DR is 50, the master alloy is only less than 2% of the final ODS cost.

Embodiments

Para. 1. A method of producing an oxide-dispersion strengthened (ODS) alloy, wherein the method includes: providing a bulk molten alloy comprising one or more metal elements; adding a plurality of oxide particles that are surface enriched at nanoscale with one or more element that reduces interfacial energy and van der Waal (“vdW’) attractions between the oxide particles and the bulk molten alloy in order to achieve a stable nanoparticle dispersion in the liquid alloy; and casting the ODS alloys.

Para. 2. The method of Para. 1, wherein the metal includes one or more metal elements selected from Al, Mg, Fe, Ag, Cu, Mn, Ni, Ti, Cr, Co, Au, and Zn.

Para. 3. The method of Para. 1, wherein the bulk molten alloy is steel.

Para. 4. The method of any one of Paras. 1-3, wherein the oxide particles are surface enriched at nanoscale with various elements, for example Nb in liquid steel, which are capable of reducing interfacial energy and vdW attraction between the oxide particles and the bulk molten alloy.

Para. 5. The method of any one of Paras. 1-4, wherein the oxide particle surface include a higher concentration of the element that reduces interfacial energy and vdW attraction within the first a few (e.g. 5) atomic layers in the molten alloy.

Para. 6. The method of any one of Paras. 1-5, wherein the oxide particles include at least one selected from Al₂O₃, Y₂O₃, ZrO₂, MgAl₂O₄, yttria stabilized zirconia and MgO, and any suitable oxides for different metal/alloy systems.

Para. 7. The method of any one of Paras. 1-6, wherein the oxide particles are microparticles or nanoparticles.

Para. 8. A bulk metal matrix comprising a metal matrix and a plurality of uniformly dispersed oxide particles that are surface enriched at nanoscale with an element that reduces interfacial energy and vdW attraction between the oxide particles and the bulk metal in molten metal.

Para. 9. A bulk metal matrix formed by any one of the methods of Paras. 1-7.

Para. 10. A method of producing an oxide-dispersion strengthened (ODS) metal, including providing a bulk molten metal including one or more metal; adding an alloy including a metal and a plurality of oxide particles uniformly dispersed therein; and casting the ODS metal.

Para. 11. The method of Para. 10, wherein the alloy includes one or metal found in the bulk molten metal.

Para. 12. The method of Para. 10 or 11, wherein the metal includes one or more alloys selected from Al, Mg, Fe, Ag, Cu, Mn, Ni, Ti, Cr, Co, Au, and Zn.

Para. 13. The method of any one of Paras. 10-12, wherein the bulk molten metal is steel.

Para. 14. The method of any one of Paras. 10-13, wherein the alloy includes various elements such as Nb, Cr to reduce interfacial energy and vdW attraction of oxide nanoparticles in molten metals.

Para. 15. A method of manufacturing ODS master alloy with high oxide nanoparticle loading for dilution to final ODA alloys including: rapidly heating one or more metal of the alloy with the oxide nanoparticles for a period of time that less than the amount of time to sinter a majority of the oxide nanoparticles.

Para. 16. The method of Para. 15, wherein the metal of the alloy with the oxide nanoparticles are rapidly heated using arc melting or laser melting.

While certain embodiments have been illustrated and described, it should be understood that changes and modifications may be made therein in accordance with ordinary skill in the art without departing from the technology in its broader aspects as defined in the following claims.

The embodiments, illustratively described herein may suitably be practiced in the absence of any element or elements, limitation or limitations, not specifically disclosed herein. Thus, for example, the terms “comprising,” “including,” “containing,” etc. shall be read expansively and without limitation. Additionally, the terms and expressions employed herein have been used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the claimed technology. Additionally, the phrase “consisting essentially of” will be understood to include those elements specifically recited and those additional elements that do not materially affect the basic and novel characteristics of the claimed technology. The phrase “consisting of” excludes any element not specified.

While the disclosure has been described with reference to the specific embodiments thereof, it should be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the true spirit and scope of the disclosure as defined by the appended claim(s). In addition, many modifications may be made to adapt a particular situation, material, composition of matter, method, operation or operations, to the objective, spirit and scope of the disclosure. All such modifications are intended to be within the scope of the claim(s) appended hereto. In particular, while certain methods may have been described with reference to particular operations performed in a particular order, it will be understood that these operations may be combined, sub-divided, or re-ordered to form an equivalent method without departing from the teachings of the disclosure. Accordingly, unless specifically indicated herein, the order and grouping of the operations are not a limitation of the disclosure.

In addition, where features or aspects of the disclosure are described in terms of Markush groups, those skilled in the art will recognize that the disclosure is also thereby described in terms of any individual member or subgroup of members of the Markush group.

As will be understood by one skilled in the art, for any and all purposes, particularly in terms of providing a written description, all ranges disclosed herein also encompass any and all possible subranges and combinations of subranges thereof. Any listed range may be easily recognized as sufficiently describing and enabling the same range being broken down into at least equal halves, thirds, quarters, fifths, tenths, etc. As a non-limiting example, each range discussed herein may be readily broken down into a lower third, middle third and upper third, etc. As will also be understood by one skilled in the art all language such as “up to,” “at least,” “greater than,” “less than,” and the like, include the number recited and refer to ranges which may be subsequently broken down into subranges as discussed above. Finally, as will be understood by one skilled in the art, a range includes each individual member.

Other embodiments are set forth in the following claims.

Claims

1. A method of producing an oxide-dispersion strengthened (ODS) alloy, the method comprising:

providing a master alloy powder comprising a metal or metal alloy and particles of a metal oxide;

adding the master alloy powder to a molten diluent alloy to form an oxide-dispersion strengthened (ODS) alloy; and

allowing the ODS to solidify;

wherein: the molten diluent alloy comprises a molten metal or metal alloy; a wetting-enhancing metal (or an alloying element) is added to the molten diluent alloy either prior to, during, or after adding of the master alloy to the molten diluent alloy; and the wetting-enhancing alloy reduces an interfacial energy and van der Waals attraction between the particles of the metal oxide and the molten alloy diluent to achieve a stable nanoparticulate ODS solid.

2. The method of claim 1, wherein the allowing the ODS to solidify comprises casting the ODS alloy.

3. The method of claim 1, wherein the metal or metal alloy of the master alloy powder and the molten diluent alloy are individually Al, Mg, Fe, Ag, Cu, Mn, Ni, Ti, Cr, Co, Au, or Zn.

4. The method of claim 1, wherein the metal or metal alloy of the master alloy powder and the molten diluent alloy comprise Fe, Mg, Mn, Co, or a mixture of any two or more thereof.

5. The method of claim 1, wherein the metal or metal alloy of the master alloy powder and the molten diluent alloy are the same.

6. The method of claim 1, wherein the metal or metal alloy of the master alloy powder and the molten diluent alloy both comprise a steel.

7. The method of claim 1, wherein the particles of metal oxide particles comprise Al2O3, Y2O3, ZrO2, MgAl2O4, yttria stabilized zirconia, MgO, or a mixture of any two or more thereof.

8. The method of claim 1, wherein the particles of the metal oxide particles are microparticles or nanoparticles.

9. The method of claim 8, wherein the particles of the metal oxide particles are nanoparticles.

10. The method of claim 1, wherein the wetting-enhancing metal comprises Nb, Cr, or a mixture thereof.

11. A bulk metal matrix formed by the method of claim 1.

12. A bulk metal matrix comprising a metal matrix and a plurality of uniformly dispersed metal oxide particles, wherein the oxide particles comprise at their surface a wetting-enhancing metal (or an alloying element) that reduces the interfacial energy and vdW attraction between the oxide particles and the bulk metal in molten metal.

13. The bulk metal matrix of claim 12, wherein the metal matrix comprises Al, Mg, Fe, Ag, Cu, Mn, Ni, Ti, Cr, Co, Au, Zn, or a mixture of any two or more thereof.

14. The bulk metal matrix of claim 12, wherein the metal matrix comprises a steel.

15. The bulk metal matrix of claim 12, wherein the metal oxide particles comprise Al2O3, Y2O3, ZrO2, MgAl2O4, yttria stabilized zirconia, MgO, or a mixture of any two or more thereof.

16. The bulk metal matrix of claim 12, wherein the wetting-enhancing metal comprises Nb, Cr, or a mixture thereof.

17. A method of manufacturing an ODS master alloy with high oxide nanoparticle loading for dilution to final ODA alloys comprising rapidly heating one or more metal of the alloy with the oxide nanoparticles for a period of time that less than the amount of time to sinter a majority of the oxide nanoparticles.

18. The method of claim 17, wherein the metal of the alloy with the oxide nanoparticles are rapidly heated using arc melting or laser melting.