HIGH-TEMPERATURE HIGH-STRENGTH ALUMINUM ALLOYS PROCESSED THROUGH THE AMORPHOUS STATE

Abstract

Aluminum alloys having improved strength at 300° C. characterized by formation from an intermediate amorphous state to a final fcc matrix hardened by optimal 25 nm-diameter Ll2 precipitates with an interphase misfit less than about 4% in all three dimensions and Al23Ni6M4 precipitates where M is one or more elements selected from the group consisting of Y and Yb. An appropriate melt of aluminum with selected transition metals (Co, Cu, Fe, Ni, Ti, Y) and Ll2 stabilizers (Sc, Yb) in amounts of about 2 to 12 and 2 to 15 atomic percent, respectively, is processed to achieve an intermediate amorphous state to dissolve Ll2-forming components. The amorphous alloys are then thermo-mechanically devitrified to a final crystalline microstructure. The alloys have good ductility and a short-term tensile strength exceeding about 275 MPa (40 ksi) at 300° C., and are useful for applications such as high-temperature turbine engine components or aircraft structural components.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part utility application of application Ser. No. 10/422,234 filed Apr. 24, 2003 entitled Nanophase Precipitation-Strengthened Al Alloys Processed Through the Amorphous State, which is based upon previously filed provisional applications: Ser. No. 60/375,940 filed Apr. 24, 2002 entitled “Amorphous metal alloy compositions” and Ser. No. 60/450,114 filed Feb. 25, 2003 entitled “Amorphous metal alloy compositions”, all of which are incorporated by reference and for which priority is claimed.

GOVERNMENT INTERESTS

Activities relating to the development of the subject matter of this invention were funded at least in part by United States Government, U.S. Army Aviation & Missile Command Contract No. DAAH01-02-C-R125, and thus may be subject to license rights and other rights in the United States.

BACKGROUND OF THE INVENTION

In a principal aspect, the present invention relates to Al-based alloys processed through an amorphous state, preferably by means of a Rapid Solidification Process (RSP) from molten alloy, and then devitrified to a primarily crystalline microscale fine grain structure by thermo-mechanical processing. To promote glass-forming ability, the Al alloys comprise selected transition metal (TM) and lanthanide rare earth (RE) elements. The final crystalline microstructure has a combination of stable strength at or above about 300° C. and good ductility, characterized by optimal 25 nm-diameter Ll₂ precipitates in an fcc matrix with an interphase misfit typically less than about 4% in all three dimensions, and rod-shaped Al₂₃Ni₆M₄ precipitates.

Improved strength at elevated temperatures has been a continuing goal in Al alloy development for more than three decades. Currently available commercial Al alloys, either manufactured with ingot or powder processing, are not capable of simultaneously achieving high strength and high-temperature stability near 300° C.; such characteristics being particularly important in applications such as fan components in turbine engines. Precipitation hardening introduced by aging is a known method to strengthen Al alloys. Conventional high-strength Al alloys in commercial applications employ Guinier-Preston zones and subsequent precipitation at or below 250° C. Examples of Al alloys processed with relatively high aging temperatures in commercial practice include alloy 2618 (200° C. for 20 hours), 4032 (170-175° C. for 8-12 hours), and 2218 (240° C. for 6 hours) [Metals Handbook-Properties and Selection: Nonferrous Alloys and Special-Purpose Materials, Volume 2, 10^th Edition, ASM International]. At the noted aging temperatures, these alloys have an improved microstructure stability relative to other commercial Al alloys. These Al alloys, when precipitation-hardened, usually possess a room temperature yield strength of about 600 MPa. (85 ksi). However, at temperatures approaching 300° C., the precipitation hardening efficiency in these alloys quickly and significantly degrades as a result of precipitate coarsening and/or dissolution. Due to the unstable strengthening precipitate size distribution at such high temperatures, the yield strength of currently available aluminum alloys at 300° C. is often only 10% of the yield strength at room temperature, and thus renders such alloys unsuitable for high-temperature applications above 150° C. For high-temperature turbine engine components or aircraft structural components, a short-term tensile strength exceeding about 275 MPa (40 ksi) at 300° C. is desired.

In order to achieve a combination of high strength and usable high-temperature properties in Al alloys, researchers have investigated a variety of intermetallic precipitation dispersions. The Al-based Ll₂ phase is one of the best-known precipitates to achieve a good combination of high strength and high toughness of ambient temperatures. There are reportedly only seven elements stabilizing the Ll₂ phase: Er, Lu, Np, Sc, Tm, U, and Yb [Knipling, K. E. et al. Z. Metallkd 97:246-265]. Since crystalline Al has very limited solubility for these Ll₂ stabilizers, it is difficult to produce a fine dispersion through crystalline solid-state heat treatments. Alternatively, with RSP from the liquid state, it is possible to either (1) directly produce a fine crystalline structure, or (2) produce partially amorphous Al alloys. Nonetheless, crystalline RSP Al alloys have not been able to meet the high-temperature strength requirements due to the difficulty of producing small, stable particles at adequate volume fraction. The focus on amorphous RSP Al alloys has been primarily on face centered cubic (fcc)-Al nanocrystals to enhance the ambient strength [Kim, Inoue, Masumoto, Mater Trans JIM 1990; 31: 747]. Upon devitrification, Al nanocrystals of up to 30% volume fraction can be dispersed within the amorphous matrix. However, this nanoscale ultra-fine grain stricture is undesirable because at high temperatures, typically ≧0.4-0.5 T_m where T_m is the material's absolute melting temperature, the contribution of grain boundary strengthening is minimal and the refined grain structures promote rapid diffusional creep. In addition, it has been reported that ultra-fine grain sizes may be undesirable when, considering formability and fracture toughness [Hornbogen, Starke, Acta metall. mater. 1993; 41: 1].

In sum, previous development on Al-based materials with high strength at elevated temperatures failed to meet the property objective of 275 MPa at 300° C.

SUMMARY OF THE INVENTION

The present invention is directed to a new class of Al alloys characterized by formation from an intermediate amorphous state to a final fcc-Al matrix hardened by a combination of Ll₂ precipitates and Al₂₃Ni₆M₄ precipitates in order to establish the Al-based analogue of Ni-base superalloys and achieve high-temperature strength with usable ductility.

An appropriate melt of Al with selected TM and RE is first processed to achieve an intermediate amorphous state to dissolve Ll₂-forming components. The preferred method to achieve a primarily (above 70% in volume) amorphous state is RSP from the molten alloy by process techniques such as powder atomization, melt spinning, and spray casting. The RSP process should have a cooling rate of at least about 10³° C./sec, preferably at least 10⁴° C./sec. Other methods to achieve amorphous microstructure through a solid-state process, such as mechanical milling, may also be used. The intermediate amorphous alloys are then thermo-mechanically devitrified to a final primarily (above 70% in volume) fcc/Ll₂/Al₂₃Ni₆M₄ crystalline microstructure with at least about 70% fcc phase in volume, at least about 0.10% Ll₂ phase, and at least about 10% Al₂₃Ni₆M₄ phase in volume.

The selection of alloying elements is based on (1) good glass-forming ability with RSP, (2) long-term strength at or above 300° C., and (3) composition tolerance for a robust design. For glass-forming ability, elements with strong short-range ordering effects, and slow long-range diffusing kinetics in molten Al are employed. For long-term strength at or above 300° C., the alloy of the present invention employs 25 nm-diameter Ll₂ particles which are reported to provide optimal creep resistance [E. A. Marquis, Microstructural Evolution and Strengthening Mechanisms in Al—Sc and Al—Mg—Sc Alloys, Ph.D. thesis, Northwestern University, 2002.]. For a robust design, the present invention employs Al₂₃Ni₆M₄, where M is one or more elements selected from the group consisting of Y and Yb. When there is deficiency of the Ll₂-formers, the incoherent D0₁₁-Al₃Ni phase is expected to precipitate, leading to low ductility. Al₂₃Ni₆M₄ is more solute-rich that the Al₃X phase and will consume less Al for a given amount of solute, giving rise to a higher amount of fcc matrix which in turn increases the ductility.

Thus, it is an object of the invention to provide a new class of high-temperature high-strength Ll₂-phase strengthened Al alloys processed through the amorphous state, preferably with RSP, and then subsequently devitrified with thermo-mechanical, processes.

A further object of the invention is to combine selected TM and RE to provide good glass forming ability during RSP such as powder atomization or melt spinning to form an amorphous Al alloy, dissolving the Ll₂-stabilizers before the devitrification process.

Another object of the invention is to provide aluminum alloys with usable strength at or above about 300° C. by selecting Ll₂-stabilizers which reduce the interphase lattice misfit in all three dimensions to promote a finer dispersion.

Another object of the invention is to employ Al₂₃Ni₆M₄ precipitates to provide composition tolerance and maintain reasonable alloy ductility.

These and a other objects, advantages and features will be set forth in the detailed description which follows.

BRIEF DESCRIPTION OF THE DRAWING

In the detailed description which follows, reference will be made to the drawing comprised of the following figures:

FIG. 1 is an X-ray diffractogram of the alloy of Example 1 as melt-spun with positions of fcc pure aluminum reflections indicating a fully amorphous state;

FIG. 2 is an X-ray diffractogram of the alloy of Example 1 after devitrification at 550° C. for 24 hours, with positions of reflections of pure fcc Al, Al₃Yb, and Al₂₃Ni₆Yb₄ phases, indicating the desired phases: fcc+Ll₂;

FIG. 3 is a Scanning Electron Microscope (SEM) secondary electron image of devitrified alloy of Example 1 indicating phase constituents fcc+Ll₂Al₂₃Ni₆Yb₄; and

FIG. 4 is an SEM secondary electron image of devitrified alloy of Example 2 indicating phase constituents fcc+Ll₂+Al₂₃Ni₆Yb₄.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

General Summary

In general, the subject matter of the invention comprises an Al alloy in crystalline form having higher or greater strength particularly at elevated temperatures, i.e. greater than about 300° C. The Al alloy is; made by compounding a mixture of Al with selected TM and RE in amorphous state followed by devitrification to a mixed crystalline state comprising fcc, Ll₂, and Al₂₃Ni₆M₄ phases wherein the ratios of the crystalline states are within certain preferred ranges. Preferably the resultant alloy has at least about 70% by volume fcc phase, at least about 10% by volume Ll₂ phase, and at least about 10% by volume Al₂₃Ni₆M₄ phase where M is selected from the group consisting of Y, Yb and a combination of Y and Yb with limited residual amorphous or quasi-crystalline phase material.

The choice of starting materials may vary, as may the compounding processes, the glass formation processes and the devitrification processes. In the amorphous state, there may be some crystalline material contained therein, but preferably no more than about 30% by volume. The particle size of alloys passing through a fully or almost fully glassy state is much finer than that of alloys without passing through the glassy state or only passing through a partially glassy state with Ll₂ already present in the as-spun condition. Thus forming the mixture in the amorphous intermediate state constitutes a very important aspect of the invention.

The alloy materials, in addition to Al, include one or more TM taken or selected from the group of Cu, Ni, Co, Ti, Fe, Y, and Sc, and one or more RE selected or taken from the group of Er, Tm, Yb, and Lu. TM metals are utilized in the range of about 2 to 12 at %, and RE materials are utilized in the range of about 2 to 15 at %.

The processes for mixing or forming the starting materials in the amorphous state are not necessarily limiting. Thus, it is contemplated that solid state processing, liquid or melt processing as well as gas phase processing may be utilized, though liquid phase processing is preferred. The completeness of the amorphous state is at least about 70% by volume and preferably greater.

Development Technique

Precipitation-hardened Al alloys are difficult to develop for high strength due to limited solubility of alloying elements. Al alloys with high fractions of precipitate that cannot be completely solution-treated have very coarse particles that tend to limit strength, corrosion resistance and toughness. In contrast, the Al alloys of the present invention exhibit high strength, good ductility, and high-temperature stability at or above 300° C.

By carefully selecting an appropriate Al alloy composition, processing techniques can achieve a fully amorphous state after rapid cooling. Furthermore, this glass can then be thermo-mechanically processed such that the glass devitrifies into a crystalline fcc matrix with nanophase precipitates. By passing through the glass state, the equilibrium solidification that would produce coarse precipitates is avoided. Certain TM such as Fe, Co, Ni and Cu promote short-range ordering in liquid Al, which leads to low partial molar volume, low thermal expansion, and high viscosity that are beneficial to glass-forming ability. RE elements such as Ce, Gd, Yb, and Er with large atomic size exhibit low diffusivity in Al and thus retard crystal nucleation. Therefore, Al-TM-RE comprise a class of glass-forming system for Al alloys of the present invention.

The elements Er, Lu, Tm and Yb are reported as the only RE Ll₂-stabilizers. Among these four RE elements, Yb has the smallest lattice parameter and relatively low-cost. Er has the lowest cost. To evaluate the effect on glass-forming ability of these alloying additions, a reduced glass transition temperature (T_rg) model was developed. In the Al-TM-RE system, this model predicts that Er has no beneficial effect to T_rg. As a consequence, alloys of the invention utilize Yb as the preferred Ll₂-stabilizer rather than Er, Tm, and Lu.

Sc is the oily TM element that can form a stable Ll₂ with Al. Compared to RE Ll₂ formers, Sc can form Ll₂ with a smaller lattice parameter, reducing the misfit between Ll₂ and Al matrix. However, Sc is by far the most expensive of the Ll₂-stabilizers and therefore embodiments of the invention seek to limit. Sc as much as possible. Efforts have been made to search for other TM to substitute for Sc. A preliminary requirement for such substitution is solubility. Ti has a substantial solubility in Al₃Sc. In addition, Ti has the lowest diffusion coefficient in solid Al among TMs. Adding Ti to Al₃Sc thus reduces the coarsening rate of Ll₂ precipitates. Moreover, addition of Ti decreases the lattice parameter of Al₃(Sc,Ti) and hence minimizes the lattice misfit with Al. Thus, alloys of the invention incorporate Yb and Sc as base Ll₂ formers but are not limited to these elements. TM such as Ti, V, Zr, etc., which will result in low misfit and thus retard coarsening are considered useful.

For a robust design, the present invention employs Al₂₃Ni₆M₄, where M is one or more elements selected from the group consisting of Y and Yb. To introduce both Al₂₃Ni₆M₄ and Ll₂ in the design, thermodynamic equilibrium calculations were performed using the thermodynamic database and calculation package Thermo-Calc® [Sundman, B. B. Jansson, and J. O. Andersson. 1985. Calphad 9: 153-190]. Thermodynamic calculations predict that Y has certain solubility in Ll₂, which expands the Ll₂ lattice spacing, increasing the misfit. Therefore, a design criterion should be set to limit the partitioning of Y in Ll₂. In addition, other phases such as Al₃Ni, Al₃Y and Al₉CO₂ should be avoided.

Al-base alloys will have good ductility when the amount of fcc is equal to or over about 70%. Thus, the total amount of Al₂₃Ni₆M₄ and Ll₂ is fixed to less than about 30%. At the desired phase constitution, Co content is set by [x_Ni+x_Co]/x_Y=6/4 because Co has a small solubility in Al₂₃Ni₆M₄ by substituting for Ni. After examining the effect of Co addition based on thermodynamic calculations, an optimum was found around 0.6 at % Co, at which partitioning of Y in Ll₂ is almost zero. If Co addition is significantly more than 0.6 at %, Al₉CO₂ and Al₃Y may precipitate.

Experimental Results

The present invention alloys, through, computational design of multi-component Al-TM-RE systems incorporate, desired processing properties-glass forming ability and the desired microstructure—a fine dispersion of Ll₂ after devitrification in the Al matrix.

Example 1

Prototypes of preferred embodiments can be made by arc-melting, melt spinning or wedge casting. Through melt spinning, ribbons of Al-3.46Ni-2.78Y-0.72Co-0.42Yb-0.63Sc-0.42Zr-0.21Ti (at %) were made. Melt-spun ribbons are approximately 3-4 mm wide and 30-40μ in thickness. The ribbons were characterized using micro-hardness) x-ray diffraction, and SEM analysis. The x-ray diffraction pattern (FIG. 1) of the as-spun ribbon indicates a partial amorphous microstructure without intermetallic precipitates. After devitrification at 550° C. for 24 hours, x-ray diffraction (FIG. 2) shows precipitation of Al₂₃Ni₆Yb₄ and peaks of Ll₂. It is noted that the peaks of Ll₂ are shifted compared to Ll₂-Al₃Yb, indicating; decrease of lattice parameters due to dissolution of Sc, Ti, and Zr in Al₃Yb. Such decrease of the Ll₂ lattice parameter will reduce the misfit. FIG. 3 shows an SEM image of the devitrified specimens confirming the phase constituents fcc+Ll₂+Al₂₃Ni₆Yb₄. The matrix is fcc-Al, the large sized grey phase material is Al₂₃Ni₆Yb₄, and the small white particles are Ll₂ phase particles. The Ll₂ particles remain smaller than ˜50 nm in diameter, while the rod-shaped Al₂₃Ni₆Yb₄ phase material is less than 1μ in length. The small Ll₂ particles will provide optimal creep resistance at or above 300° C. and the Al₂₃Ni₆Yb₄ material is present to avoid detrimental compounds and improve the ductility at the high temperature.

Example 2

Ribbons of Al-3Ni-2.42Y-0.62Co-0.6Yb-0.6Sc-0.6Zr-0.6Ti (at %) were made using the protocol of Example 1. The ribbons were characterized using micro-hardness, x-ray diffraction, and SEM analysis. FIG. 4 shows an SEM image of the devitrified specimens confirming the phase constituents fcc+Ll₂+Al₂₃Ni₆Yb₄. The matrix is fcc-Al, the large sized grey phase material is Al₂₃Ni₆Yb₄, and the small white particles are Ll₂ phase material. The Ll₂ particles remain smaller than ˜50 nm in diameter, while the rod-shaped Al₂₃Ni₆Yb₄ phase material is less than 1μ in length. The small Ll₂ particles will provide optimal creep resistance at or above 300° C. and the Al₂₃Ni₆Yb₄ material is present to avoid detrimental compounds and improve the ductility.

Example 3

Scale-up processing of the alloy in Example 1 was engineered. Amorphous powder produced by high-pressure He atomization can be used as a raw material to produce an amorphous bulk by consolidation at high temperatures. The amorphous alloy powder is produced by gas atomization, followed by sieving, precompaction, canning and sealing into a Cu tube, carried out in a well-controlled atmosphere with an oxide or moisture concentration below 1 ppm. Powder of the alloy in Example 1 was successfully atomized and extruded. The extrusion is a thermo-mechanical process where the glass devitrifies into a crystalline fcc matrix with nanophase precipitates.

Example 4

Powder of the alloy in Example 2 was successfully atomized and extruded using the protocol of Example 3. The extrusion is a thermo-mechanical process where the glass devitrifies into a crystalline fcc matrix with nanophase precipitates.

Variations of the described aluminum alloy as well as the process for manufacture thereof and the product created by the process arc available to provide the expected functionality of high short-term and long-term strength at temperatures above about 300° C. Thus the invention is to be limited only by the following claims and equivalents thereof.

Claims

1. An aluminum alloy characterized by high strength iii the temperature range greater than about 300° C. comprising, in combination:

an alloy mixture in primarily crystalline form having at least about 70% by volume fcc phase, at least about 110% by volume Ll2 precipitate phase, and at least about 10% by volume Al23Ni6M4 precipitate phase where M is one or more elements selected from the group consisting of Y and Yb, said alloy consisting essentially of one or more transition metals selected from the group consisting of about 2 to 12 atomic percent Co, Cu, Fe, Ni, Ti, and Y; and one or more elements comprising said Ll2 phase selected from the group consisting of about 2 to 15 atomic percent Sc and Yb; optionally of transition metals selected from the group consisting of Cr, Li, Mn, V, and Zn, and the balance Al and incidental elements and impurities; said Ll2 phase in the form of a precipitate particle dispersion having a particle diameter of less than about 80 nm.

2. The alloy of claim 1 having a tensile yield strength of at least about 275 MPa at 300° C.

3. An aluminum alloy characterized by high strength at a temperature greater than about 300° C. made by a process comprising the steps of:

(a) formulating a melt comprised of Al; at least one transition metal selected from the group consisting of Co, Cu, Fe, Ni, Ti and Y; at least one element selected from the Ll2-stabilizing element group consisting of Sc and Yb; optionally of transition metals selected from the group consisting of Cr, Li, Mn, V, and Zn;

(b) converting the melt to at least about 70% by volume amorphous material; and

(c) devitrifying, at least in part, the amorphous material to a mixture of Ll2 crystalline rare earth precipitate phase material in a particle dispersion wherein the particle size is less than about 80 nm, Al23Ni6M4 precipitate phase where M is one or more elements selected from the group consisting of Y and Yb, and fcc phase material.

4. The alloy product by the process of claim 3 wherein the transition metal is provided in an amount of about 2 to 12 atomic percent.

5. The alloy product by the process of claim 3 wherein the Ll2-stabilizing element is provided in an amount of about 2 to 10 atomic percent.

6. The alloy product by the process of claim 3 wherein devitrifying the amorphous material comprises forming at least about 70% by volume fcc phase.

7. The alloy product by the process of claim 3 wherein devitrifying the amorphous material comprises forming at least 10% by volume Ll2 phase in partial form.

8. The alloy product by the process of claim 3 wherein devitrifying the amorphous material comprises forming at least 10% by volume Al23Ni6M4 phase in partial form, where M is one or more elements selected; from the group consisting of Y and Yb.

9. The alloy product by the process of claim 3 wherein converting the melt to amorphous material comprises at least one step selected from the group consisting of gas powder atomization, water powder atomization and melt spinning.

10. The alloy product by the process of claim 3 wherein, devitrification comprises at least one step selected from the group consisting of hot isostatic pressing, thermal aging, and extrusion.

11. The product by the process of claim 3 wherein converting the melt comprises rapid solidification processing.

12. An aluminum alloy consisting essentially of about 2-12 atomic percent of at least one transition element selected from the group consisting of Co, Cu, Fe, Ni, Ti, and Y; about 2-15 atomic percent of at least one element selected from the group consisting, of Yb and Sc; optionally of transition metals selected from the group consisting of Cr, Li, Mn, V, and Zn; and the balance Al and incidental elements and impurities characterized by greater than about 70% crystalline microstructure with a dispersion of Ll2 phase particles greater than 10% by volume and Al23Ni6M4 phase particles greater than 10% by volume, where M is one or more elements selected from the group consisting of Y and Yb, in a matrix of greater than 70% by volume fcc phase generated by a rapid solidification process from a substantially amorphous vitrified phase, said particle diameter of said Ll2 particles in the range of about 1.0 to 80 nm.

13. The alloy of claim 12 wherein the nominal particle diameter of the Ll2 particles is about 25 nm.

14. The alloy of claim 12, consisting essentially of about 0.7 atomic % Co, 3.5 atomic % Ni, and at least one element selected from the group consisting of Sc and Yb.

15. The alloy of claim 12 consisting essentially of about 0.7 atomic % Co, about 3.5 atomic % Ni, and Sc, Ti, Y, Yb, and Zr, cumulatively in the range of about 4 to 8 atomic %.