DEGASSING OF NANO-PHASE SEPARATING POWDERS IN A HYDROGEN CONTAINING ATMOSPHERE

Abstract

Removal of gas-evolving species from powders can be achieved before sintering occurs, reducing or eliminating gas evolution in a dense body that can lead to swelling, or lowering of density.

Description

CLAIM OF PRIORITY

This application is a national stage filing under 35 U.S.C. § 371 of International Patent Application No. PCT/US2023/062660, filed on Feb. 15, 2023, which claims priority to U.S. Provisional Patent Application No. 63/310,444, filed Feb. 15, 2022, each of which is incorporated by reference in its entirety.

GOVERNMENT SPONSORSHIP STATEMENT

This invention was made with government support under 80MSFC19C0050 awarded by the National Aeronautics and Space Administration. The government has certain rights in the invention.

FIELD OF THE INVENTION

The invention relates to phase separating powders and methods of processing.

BACKGROUND

Sintered nanocrystalline materials are often subjected to pressure or other post-sintering processing techniques to achieve higher density materials.

SUMMARY

In one aspect, a method of reducing gas production during metal alloy powder sintering can include providing a mechanically alloyed metal alloy powder, annealing the metal alloy powder at an annealing temperature below a sintering temperature, the annealing of the metal alloy powder being in an atmosphere including hydrogen or the metal alloy powder including an oxygen getter, or both, and sintering the metal alloy powder at a sintering temperature to reduced gas production during the sintering.

In certain circumstances, the method can include pressing the annealed metal alloy powder to form a green body prior to sintering.

In certain circumstances, the method can include pressing the metal alloy powder to form a green body prior to annealing.

In certain circumstances, the metal alloy powder can be a nano-phase separating powder.

In certain circumstances, the metal alloy powder can include an additive.

In certain circumstances, the additive can include an organic material.

In certain circumstances, the metal alloy powder can include nickel, chromium, iron, copper, vanadium, molybdenum, tungsten, silver, zirconium, or combinations thereof.

In certain circumstances, the metal alloy powder can include a ternary nano-phase separating powder.

In certain circumstances, the metal alloy powder can include a quaternary nano-phase separating powder.

In certain circumstances, the oxygen getter can include an alloying element having a strong preference to form an oxide.

In certain circumstances, the oxygen getter can include zirconium, chromium, vanadium, manganese, or combinations thereof.

In certain circumstances, the oxygen getter concentration can be about 1 at % to about 4 at % of the alloy.

In certain circumstances, the annealing temperature can be between about 200° C. and 400° C.

In certain circumstances, sintering can include nano-phase separation sintering.

In certain circumstances, grain size of the metal alloy powder and grain size of the sintered powder is substantially the same.

In another aspect, a metal alloy powder for sintering can include a mechanically alloyed powder having a reduced amount of gas-evolving species compared to the amount of gas-evolving species in the mechanically alloyed powder prior to annealing in a hydrogen atmosphere.

In certain circumstances, the mechanically alloyed powder can be a nano-phase separating powder.

In certain circumstances, the mechanically alloyed powder can include an oxygen getter.

In certain circumstances, the mechanically alloyed powder can include nickel, chromium, iron, copper, vanadium, molybdenum, tungsten, silver, zirconium, or combinations thereof.

In certain circumstances, the mechanically alloyed powder can be a ternary nano-phase separating powder or a quaternary nano-phase separating powder.

In another aspect, a method of forming a sintered alloy can include annealing a nano-phase separating metal alloy powder in the presence of hydrogen, and sintering the nano-phase separating metal alloy powder to form a sintered alloy product including nickel and having a relative density of at least 80%.

In certain circumstances, the sintered alloy product can include a dispersed oxide.

In certain circumstances, the annealing and sintering can be conducted in a single step.

In certain circumstances, the annealing and sintering can be conducted in sequential steps.

In another aspect, a sintered product can include the powder as described herein.

Other aspects, embodiments, and features will be apparent from the following description, the drawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic illustrating nano-phase separation sintering, including phase separation, neck formation, sintering and densification.

FIG. 1B depicts a phase diagram.

FIG. 1C is a schematic illustrating an example of the general process described herein.

FIG. 2A depicts a Ni—Cu phase diagram.

FIG. 2B is a graph illustrating a behavior of the system.

FIG. 3 depicts a phase diagram.

FIG. 4A is a graph showing densification rate.

FIG. 4B is a graph showing change in relative density.

FIG. 5A depicts a Ni—Cr phase diagram.

FIG. 5B depicts a Cu—Cr phase diagram.

FIG. 6A depicts a Ni—Cu phase diagram at 5 at % Cr.

FIG. 6B depicts a Ni—Cu phase diagram at 10 at % Cr.

FIG. 6C depicts a Ni—Cu phase diagram at 15 at % Cr.

FIG. 6D depicts a phase diagram of 60 at % Ni and 15% Cr.

FIG. 6E is a table showing the compositions of stable phases at 550° C.

FIG. 6F is a graph showing the temperature dependence of densification at 5° C./min to 1200° C. and 40° C./min cooling.

FIG. 7A depicts a Ni—Fe phase diagram.

FIG. 7B depicts a Cu—Fe phase diagram.

FIG. 8A depicts a Ni—Cu phase diagram at 5 at % Fe.

FIG. 8B depicts a Ni—Cu phase diagram at 20 at % Fe.

FIG. 8C depicts a Ni—Cu phase diagram at 42 at % Fe.

FIG. 8D depicts a Ni—Cu phase diagram at 42 at % Fe.

FIG. 8E is a table showing the compositions of stable phases at 550° C.

FIG. 8F is a graph showing the temperature dependence of densification at 5° C./min to 945° C. and 40° C./min cooling.

FIG. 8G is a graph showing the change in relative density over time at 5° C./min heating.

FIG. 8H is a graph showing the temperature dependence of densification at 10° C./min to 1000° C. and 40° C./min cooling.

FIG. 8I is a graph showing the change in relative density over time at 5° C./min heating to a final relative density of 0.5864%.

FIG. 8J is a graph showing the temperature dependence of densification at 15° C./min to 1000° C. and 40° C./min cooling.

FIG. 8K is a graph showing the change in relative density over time at 15° C./min heating to a final relative density of 0.5477%.

FIG. 8L is a graph showing the temperature dependence of densification at 20° C./min to 1000° C. and 40° C./min cooling.

FIG. 8M is a graph showing the change in relative density over time at 20° C./min heating to a final relative density of 0.4276%.

FIG. 8N is a graph showing the temperature dependence of densification at 5° C./min to 1100° C. and 40° C./min cooling.

FIG. 8O is a graph showing the change in relative density over temperature at 5° C./min heating.

FIG. 8P is a graph showing the change in relative density over time at 5° C./min heating.

FIG. 8Q is a graph showing the temperature dependence of densification at 10° C./min to 1100° C. and 40° C./min cooling.

FIG. 8R is a graph showing the change in relative density over temperature at 10° C./min heating.

FIG. 8S is a graph showing the change in relative density over time at 15° C./min.

FIG. 8T is a graph showing the temperature dependence of densification at 20° C./min to 1100° C. and 40° C./min cooling.

FIG. 8U is a graph showing the temperature dependence of densification at 20° C./min to 1100° C. and 40° C./min cooling.

FIG. 8V is a graph showing the change in relative density over time at 20° C./min.

FIG. 9A depicts a Ni—Co phase diagram.

FIG. 9B depicts a Cu—Co phase diagram.

FIG. 10A depicts a Ni—Cu phase diagram at 5 at % Co.

FIG. 10B depicts a Ni—Cu phase diagram at 20 at % Co.

FIG. 10C depicts a Ni—Cu phase diagram at 44 at % Co.

FIG. 11A depicts a Ni—V phase diagram.

FIG. 11B depicts a Cu—V phase diagram.

FIG. 12A depicts a Ni—Cu phase diagram at 6 at % V.

FIG. 12B depicts a Ni—Cu phase diagram at 10 at % V.

FIG. 12C depicts a Ni—Cu phase diagram at 13 at % V.

FIG. 12D depicts a Ni—Cu phase diagram and 13 at % V. At 15 at % Cu.

FIG. 12E is a table showing the stable phases at 1150° C.

FIG. 12F is a graph showing the temperature dependence of the densification rate for 10 at 5° C./min to 1200° C. and 40° C./min cooling.

FIG. 12G is a graph showing the temperature dependences of the change in relative density.

FIG. 13 is a chart showing the diffusion rates and temperature range for metals described herein.

FIG. 14A depicts a phase diagram for Fe—Cu with 40 at % Ni from an Fe database.

FIG. 14B depicts a phase diagram for Fe—Cu with 40 at % Ni from a Ni database.

FIG. 14C depicts a phase diagram for Fe—Cu with 40 at % Ni from a Cu database.

FIG. 14D depicts a phase diagram for Fe—Cu with 40 at % Ni from a high entropy alloy database.

FIG. 15 depicts a Cr—Cu phase diagram at 60 at % Ni.

FIG. 16 depicts a Co—Cu phase diagram at 40 at % Ni.

FIG. 17 depicts a Cu—V phase diagram at 75 at % Ni.

FIGS. 18A, 18B and 18C are graphs showing densification after pretreatment with hydrogen at different heating rates.

FIG. 19 is a graph showing the impact on density of 5 at % and 8 at % Mn to a Ni—Fe—Cu composition.

FIG. 20 is a graph showing the densification behavior of the addition of Mn in a 37.5 at % Ni—37.5 at % Co—20 at % Cu—5 at % Mn composition heated to 1200° C.

FIGS. 21, 22, 23 and 24 are micrographs of the composition of FIG. 20.

FIGS. 25-28 are graphs showing the mechanical properties of the sintered products of 37.5 at % Ni 37.5 at % Fe 20 at % Cu 5 at % Mn.

FIGS. 29A-29B depict phase diagrams.

FIG. 30 is a graph showing sintering of Mo and W based systems alloyed with Cr.

FIG. 31 is a graph showing testing of the Mo25W15Cr samples.

FIG. 32 is a graph showing densification over time.

FIG. 33A is a graph showing densification with temperature.

FIG. 33B is a graph showing change in relative density with time.

FIG. 34 is a schematic showing addition of a getter.

FIG. 35 is a graph showing swelling rate based on Zr content.

DESCRIPTION

Metal powders can be used in the production of metal components, as for example through pressing and sintering, or 3D printing/additive manufacturing. Many metallic powders form gaseous products upon annealing, e.g. CO and CO₂ in the presence of carbon impurities, which can delay or impede the consolidation of the powder and lead to a reduced final density of the sintered microstructure.

Specifically, removing gas-evolving species from powders before sintering occurs is challenging, as gas evolution in a dense body can lead to swelling, or lowering of density. As described herein, a reduction treatment for the powders can be a process step to provide a powder with enhanced sintering capabilities. The general purpose of the reduction treatment is to enable rapid consolidation in powders that exhibit rapid densification through nano-phase separation sintering and tend to otherwise release fugitive gas species during the sintering process. The more specific benefit from removing undesired interstitial impurity elements is the reduction of detrimental degassing products forming upon sintering, which can enable an accelerated consolidation process and an improvement of the density of the final microstructure. Low temperature reduction of magnetite in pure H2 is described, for example, in Spreitzer, D. and Schenk, J. (2019), Reduction of Iron Oxides with Hydrogen—A Review. Steel Research Int., 90:1900108, which is incorporated by reference in its entirety. Importantly, reduction should occur without initiating nanophase separation or initiating grain growth.

For example, reduction of ball-milled and nano-phase separating powders can be conducted in a hydrogen containing atmosphere. Hydrogen is a viable candidate as a forming and reduction agent in many powder metallurgy processes. It exists in abundant concentrations in the atmosphere and is known to interact with metals, as it can easily move along defects within the microstructure and dissolves into metals to some degree.

Processing steps can include the reduction of nano-phase separating powders in a hydrogen containing atmosphere. Prior to the reduction treatment, the elemental powders can be mechanically alloyed, resulting in the formation of a solid solution with a homogeneous distribution of all elements and a refinement of the grain size down to the nanometer scale.

The loose milled powder can be annealed when exposed to a reducing atmosphere at relatively low temperatures, such as between 200° C. and 400° C., for at least 6 hours, at least 8 hours, at least 10 hours, at least 12 hours, at least 14 hours, at least 16 hours, at least 18 hours, at least 20 hours, at least 22 hours, or at least 24 h. For example, powder can be annealed when exposed to a hydrogen atmosphere at relatively low temperatures between 250-350° C. for 24 hours. Reduction can lead to a decrease in mass loss during sintering from about 2% in an as-milled powder to about 1.2-1.8% in an annealed, reduced powder.

After reduction, the powder can be formed into the desired shape as a green body. On the laboratory scale, the powders can be cold-compacted into pellets, but the actual final parts may vary. For example, the annealed metal alloy powder can be pressed to form a green body prior to sintering. In another example, the metal alloy powder can be pressed to form a green body prior to annealing.

The pressed shape can be heated to the desired temperature range, undergoing rapid densification through nano-phase separation at lower temperatures.

Variations on these steps are also envisioned, whereby a reduction treatment can occur on a pressed or formed green body, or a reduction treatment can be conducted in-line with the sintering process itself as a stage of the thermal cycle used in sintering.

The method and materials can have various advantages and present improvements. For example, the low-temperature treatment in hydrogen containing can is suitable to reduce metal powder and produces gaseous H₂O, which avoids the production of CO or CO₂ gas that would be released from the powder during sintering without any prior reduction step. Also, metal powders often contain a small degree of impurities, such as oxygen and carbon, either originating from their production process and/or from processing aids added to support the high-energy milling process that is conducted in preparation for the nano-phase separating sintering process. A reduction treatment can reduce the degree of this undesired impurity contamination, which leads to cleaner final products with an overall improvement of the properties. Besides structural properties, a substantial reduction of the contamination level can facilitate an easier and an accelerated sintering process of nano-phase separating powders with an improvement in the final density and thus mechanical properties of the sintered part.

Nano-phase separating powders can exhibit additional benefits because of the numerous grain boundaries that can provide diffusion pathways for hydrogen from the environmental gas into the powder. This can increase the reduction kinetics and can reduce the overall process time and cost. As the powder has an internal grain structure in the order of a few nanometers, bulk diffusion distances are reduced by several orders of magnitude, which makes ball-milled powders very suitable candidates for reduction in a hydrogen containing gas.

Another contributing factor is that diffusion of H₂ is relatively fast in nanocrystalline powders due to the large fraction of grain boundaries. H₂ is advantageous in terms of reduction productivity compared to CO containing atmospheres due to this faster diffusion. A low-temperature treatment in a hydrogen containing environment can enable a reduction of impurities within the powder while retaining a nanocrystalline grain size and a supersaturated solid solution.

A reduction process of metallic powders can be used in many unrelated applications and industrial processes. Therefore, the amount of nano-phase separating powder reduced in a hydrogen containing atmosphere can easily be scaled up from laboratory (in the order of a few grams) to industrial scale (several kilograms), which enables expandability and applicability of the process on industrial scale.

In a commercial application, ball-milled nano-phase separating powders enable a rapid densification to full density at relatively low process temperatures without application of external pressure during the sintering process, which is suitable for larger components with complex shape geometries produced in additive manufacturing processes. A reduction process of the loose powder in a hydrogen containing atmosphere performed on an industrial scale just before the sintering procedure can facilitate both an improved process and final integrity of the printed component. This can be achieved by a reduction of potential fugitive gaseous species and a smaller fraction of remaining oxide compounds, which results in an acceleration of the sintering process in nano-phase separating powders and thus a lowered input of resources. A reduction of the nano-phase separating powder in a hydrogen containing atmosphere as an intermediate step after ball milling of the powders and prior to sintering can easily be integrated into an industrial processing route of additive manufacturing. As the degree of hydrogen in the atmosphere can vary, the reduction temperature can be lowered and retain the nanocrystalline grain size of the nano-phase separating powder.

The ternary Ni alloys exhibit excellent rapid densification behavior to high density near 1100° C. This is lower than for traditional sintering methods of nickel-base alloys, including activated sintering and liquid phase sintering. Acceleration of the sintering process saves resources in terms of temperature and time, beneficial for productivity. The densification of these alloys to high density does neither require processing at high temperatures nor an isothermal annealing, which reduces the energetic input needed for the full consolidation of the powders. Faster sintering is also beneficial for a higher throughput and a general efficiency of the process. The sintering process does not require the application of an additional pressure, which can enable the scalability of the component thickness as well as a larger range of geometry complexities.

The ternary Ni alloys can withstand high homologous temperatures, while accelerated densification of the metal powders can be promoted. Some methods that promote rapid densification result in the production of a secondary, low melting temperature phase that prevents the alloy from being used at elevated homologous temperatures. In the nano-phase separating Ni alloys, the low-temperature precipitation of a Cu-rich phase will not prevent use of these alloys at higher temperatures, as the alloys are designed to redissolve secondary phases at higher temperature. The overall melting temperature of the product remains thus relatively high.

The hydrogen low-temperature annealing treatment reduces very high fractions of interstitial impurities, such as oxygen or carbon, resulting from powder contamination or from adding high-energy ball-milling processing aids. The reduction treatment of the loose powder prior to sintering reduces undesired elements from the alloy, which can reduce the fraction of undesired oxide phases and improve the overall purity of the alloy, which is often at question of components produced via powder metallurgy routes. The rapid densification via nano-phase separation requires processing only in the solid state and shape change of the sintering components remains very small. The shape accuracy of the sintered components is one major challenge in additive manufacturing techniques, which suffers from gravitational slumping by forming a liquid phase during sintering. Nano-phase separating Ni alloys promote high specific tolerances for final part geometries.

The mechanical properties of commercial Ni-base alloys benefits from the precipitation of a second phase. In ternary or quaternary nano-phase separating alloys, strengthening effects from a reduction of the final grain size, solid solution strengthening through the addition of the ternary component as well as by the presence of a secondary phase can be expected.

Any production step of the ternary nano-phase separating Ni powders is industrially scalable. Mechanical alloying via ball-milling is a commonly used industrial method, that allows for a transition from the laboratory (grams scale) to industrially (kilograms or tons scale) relevant powder quantities. The low-temperature reduction treatment in a hydrogen containing atmosphere is also scalable and depends mainly on the scale of the furnace as well as the degree of gas circulation. The ternary and quaternary Ni systems are the only Ni alloys that benefit from densification through nano-phase separation sintering.

Derivation of Gas Production During H₂ Treatment of Ball-Milled Powder

Chemical reactions involved include:

$H_{2}k1\to \leftarrow k2H_2^{*}{H}_2^{*}+\frac{1}{4}{\mathrm{Fe}}_3{O}_4^{*}k3\to \leftarrow k4\frac{3}{4}{\mathrm{Fe}}^{*}+H_2{O}^{*}{H}_2{O}^{*}k5\to \leftarrow k6H_{2}O\frac{d\left[H_2^{*}\right]}{\mathrm{dt}}=k1*p_{H_2}-k2\left[H_2^{*}\right]-k{3\left[H_2^{*}\right]\left[{\mathrm{Fe}}_3{O}_4^{*}\right]}^{\frac{1}{4}}+k4*{\left[{\mathrm{Fe}}^{*}\right]}^{\frac{3}{4}}*{\left[H_2{O}^{*}\right]}^2\frac{d\left[H_2{O}^{*}\right]}{\mathrm{dt}}=-k5*\left[H_2{O}^{*}\right]+k6*p_{H_{2}O}+k{3\left[H_2^{*}\right]\left[{\mathrm{Fe}}_3{O}_4^{*}\right]}^{\frac{1}{4}}-k{4\left[{\mathrm{Fe}}^{*}\right]}^{\frac{3}{4}}\left[H_2{O}^{*}\right]\frac{{\mathrm{dp}}_{H_{2}O}}{\mathrm{dt}}=k5\left[H_2{O}^{*}\right]-k6*p_{H_{2}O}\mathrm{QSSA}:k3k1,k2:\mathrm{all}\mathrm{surface}\mathrm{sites}\mathrm{are}\mathrm{occupied}:\frac{d\left[H_2^{*}\right]}{\mathrm{dt}}=0=k1*p_{H_2}-\left(k2+k{3\left[{\mathrm{Fe}}_3{O}_4^{*}\right]}^{\frac{1}{4}}\right)\left[H_2^{*}\right]+k{4\left[{\mathrm{Fe}}^{*}\right]}^{\frac{3}{4}}\left[H_2{O}^{*}\right]\left[H_2^{*}\right]=\frac{k1*p_{H_2}}{\left[k2+k{3\left[{\mathrm{Fe}}_3{O}_4^{*}\right]}^{\frac{1}{4}}\right]}+\frac{k{4\left[{\mathrm{Fe}}^{*}\right]}^{\frac{3}{4}}\left[H_2{O}^{*}\right]}{k2+k{3\left[{\mathrm{Fe}}_3{O}_4^{*}\right]}^{\frac{1}{4}}}(\mathrm{pre}-\mathrm{equilibrium}\mathrm{condition}:k2k3,k4~0)\left[H_2^{*}\right]=\frac{k1}{k2}{p}_{H_2}=K_{\mathrm{eq}}*p_{H_2}\mathrm{QSSA}:\left(\mathrm{every}\mathrm{produced}H2O\mathrm{goes}\mathrm{to}\mathrm{the}\mathrm{gas}\mathrm{phase}\right):\frac{d\left[H_2{O}^{*}\right]}{\mathrm{dt}}=0\left(k4~0\right)\frac{d\left[H_2{O}^{*}\right]}{\mathrm{dt}}=0=k6*p_{H_{2}O}-k5\left[H_2{O}^{*}\right]+k3*K_{\mathrm{eq}}*p_{H_2}*{\left[{\mathrm{Fe}}_3{O}_4^{*}\right]}^{\frac{1}{4}}-k{4\left[{\mathrm{Fe}}^{*}\right]}^{\frac{3}{4}}\left[H_2{O}^{*}\right]\left[H_2{O}^{*}\right]=\frac{k_6}{k_5}*p_{H_{2}O}+\frac{k_3}{k_5}*K_{\mathrm{eq}}*p_{H_2}*{\left[{\mathrm{Fe}}_3{O}_4^{*}\right]}^{\frac{1}{4}}\frac{{\mathrm{dp}}_{H_{2}O}}{\mathrm{dt}}=k5\left(\frac{k_6}{k_5}*p_{H_{2}O}+\frac{k_3}{k_5}*K_{\mathrm{eq}}*p_{H_2}*{\left[{\mathrm{Fe}}_3{O}_4^{*}\right]}^{\frac{1}{4}}\right)-k6*p_{H_{2}O}\frac{{\mathrm{dp}}_{H_{2}O}}{\mathrm{dt}}=+k_3*K_{\mathrm{eq}}*p_{H_2}*{\left[{\mathrm{Fe}}_3{O}_4^{*}\right]}^{\frac{1}{4}}-p_{H_2}+p_{H_{2}O}=1\mathrm{or}{p}_{H_2}=1-p_{H_{2}O}\frac{{\mathrm{dp}}_{H_{2}O}}{\mathrm{dt}}=k_3*K_{\mathrm{eq}}*\left(1-p_{H_{2}O}\right)*{\left[{\mathrm{Fe}}_3{O}_4^{*}\right]}^{\frac{1}{4}}\mathrm{with}{k}_3=v_0{e}^{-\frac{Q_{\mathrm{reaction}}}{\mathrm{RT}}}$

In some circumstances, a gas reduction treatment in a hydrogen containing atmosphere alters the microstructure of the ball-milled nanocrystalline powders or causes the process of nanophase separation to take place in the Ni alloys. In this case, gas released on the surface or interior of the powder green bodies reduces the degree of consolidation or slows down densification at the expense of the final density/properties or the input of the required resources.

As alternative to a degassing treatment, a set of alloying components that can be added with the purpose to act as oxygen getter and enable nano-phase separation sintering. The alloying element as selective oxygen getter is characterized to have a strong preference to interact with powder contamination and ball-milling process additives and form oxides that retain thermodynamic stability beyond the sintering process window. The thermodynamic stability is characterized by a strong negative heat of formation between the oxygen getting element and oxygen. The formation of strong oxide compounds enables accelerated low temperature nano-phase separation without a competition between densification and degassing of the powders.

Oxygen getter components of ball-milled nano-phase separation sintering Ni alloys are described herein. Nano-phase separating Ni powders are characterized by an accelerated onset of densification through the precipitation of a second phase at interparticle necks that enables to reach full density at low temperatures. For alloys with all constituents with oxides unstable in the presence of carbon at a certain sintering temperature, fugitive gaseous species form. By adding a component with stability beyond the maximum process temperature, nano-phase separation can operate without interfering with swelling of the microstructure caused by gas production. These getter elements are characterized by a strong preference to form oxide compounds, which is displayed in a Richardson-Ellingham diagram by high negative heat of formation between the getter and oxygen. Elements typically serving as oxygen getter for nano-phase separating Ni alloys are: Zr, Cr, V, or Mn, or combinations thereof.

Adding an element that forms a (high-temperature) stable compound with oxygen can open a processing window for Ni powder alloys that are prone to reduction upon sintering and enables the production of components made of Ni alloys via additive manufacturing techniques. This concept applies very well to Ni alloys that have been designed to experience accelerated low-temperature densification through nano-phase separation. Those powders can be processed by high-energy ball milling with appropriate processing additives in preparation to sintering. Getter elements can be added particularly to those alloys that are prone to forming gaseous CO₂, CO, H₂O that cannot be removed by an annealing treatment prior to the onset of nano-phase separation.

Application of the oxygen getter method can prevent an interplay between degassing and densification in nano-phase separating Ni alloys, which can lead to a denser final microstructure with fewer trapped gas pores and thus an improvement to the overall mechanical properties as well as reduced processing time and need for energy resources (time, temperature). Blending with alloying constituents that form high temperature stable compounds with oxygen can improve certain mechanical properties such as strength and hardness through oxide-dispersed strengthening.

A method of reducing gas production during metal alloy powder sintering can include providing a mechanically alloyed metal alloy powder, annealing the metal alloy powder at an annealing temperature below a sintering temperature, the annealing of the metal alloy powder being in an atmosphere including hydrogen or the metal alloy powder including an oxygen getter, or both, and sintering the metal alloy powder at a sintering temperature to reduced gas production during the sintering.

The hydrogen can be a pure hydrogen or hydrogen mixed with an inert gas, for example, argon or helium.

In certain circumstances, the metal alloy powder can be a mechanically alloyed powder. The mechanical alloying (e.g., ball milling) can be performed at a relatively low temperature. For example, in some embodiments, the mechanical alloying (e.g., ball milling) is performed while the particles are at a temperature of less than or equal to 150° C., less than or equal to 100° C., less than or equal to 75° C., less than or equal to 50° C., less than or equal to 40° C., less than or equal to 35° C., less than or equal to 30° C., less than or equal to 25° C., or less than or equal to 20° C. In some embodiments, the mechanical working (e.g., ball milling) is performed while the particles are at a temperature of at least 0° C. In some embodiments, the mechanical alloying (e.g., ball milling) can be performed at a temperature of the surrounding, ambient environment.

In certain embodiments, the mechanical alloying (e.g., ball milling) may be conducted for a time of greater than or equal to 6 hours (e.g., greater than or equal to 8 hours, greater than or equal to 10 hours, greater than or equal to 12 hours, or greater than or equal to 15 hours). In certain embodiments, the mechanical alloying (e.g., ball milling) may be conducted for a time of less than or equal to 18 hours. In some embodiments, the mechanical alloying (e.g., ball milling) may be conducted for a time of 6 hour to 18 hours.

In certain circumstances, the metal alloy powder can be a nano-phase separating powder.

In certain circumstances, the metal alloy powder can include an additive, for example, an organic material. The additive can be an impurity in the metal alloy powder. In certain embodiments, the additive can be introduced to the powder during a processing step as a lubricant or other processing aid. The additive can be, for example, an alcohol such as ethanol.

In certain circumstances, the metal alloy powder can include nickel, chromium, iron, copper, vanadium, molybdenum, tungsten, silver, zirconium, or combinations thereof. In certain circumstances, the oxygen getter can include an alloying element having a strong preference to form an oxide. Examples of the oxygen getter can include zirconium, chromium, vanadium, manganese, or combinations thereof. In certain circumstances, the metal alloy powder can include a binary nano-phase separating powder, a ternary nano-phase separating powder or a quaternary nano-phase separating powder.

The alloys described herein can be the alloys may be at least one of Ni—Cu, Ni—Fe, Ni—Ag, Mo—Cr, Mo—W—Cr, Ni—Cu—Fe, Ni—Cu—Co, Ni—Cu—Cr, Ni—Cu—V, Ni—Cu—Ag, Ni—Cu—Mo, Ni—Cu—W, Ni—Cu—Mn, Ni—Cu—Zr, Ni—Cu—Fe—Mn, Ni—Cu—Co—Mn, Ni—Cu—Cr—Mn, Ni—Cu—V—Mn, Ni—Cu—Ag—Mn, Ni—Cu—Mo—Mn, Ni—Cu—W—Mn, Ni—Cu—Fe—Zr, Ni—Cu—Co—Zr, Ni—Cu—Cr—Zr, Ni—Cu—V—Zr, Ni—Cu—Ag—Zr, Ni—Cu—Mo—Zr, Ni—Cu—W—Zr, Ni—Ag—Mn, Ni—Ag—V, Ni—Ag—Cr, Ni—Ag—W, Ni—Ag—Mo, Ni—Ag—Fe, or Ni—Ag—Zr.

In certain circumstances, the oxygen getter concentration can be about 1 at %, about 2 at %, about 3 at %, about 4 at %, or about 5 at % of the alloy.

In certain circumstances, sintering can include nano-phase separation sintering.

In certain circumstances, grain size of the metal alloy powder and grain size of the sintered powder is substantially the same. In certain circumstances, the grain size can be a nanoscale feature or nanophase. The nanoscale feature or nanophase can be 100 nm, 90 nm, 80 nm, 70 nm, 60 nm, 50 nm, 40 nm, 30 nm, 20 nm, 10 nm, 5 nm or smaller. The nanophase, nanoscale feature, or nanocrystal can refer to the size of a crystal (or a “grain”) being less than or equal to about 1000 nm—e.g., 500 nm, 200 nm, 100 nm, 50 nm, 20 nm, 10 nm, 5 nm, 2 nm, etc. For example, the grain size may be between 1000 nm and about 2 nm—e.g., about 500 nm and about 2 nm, about 200 nm and about 2 nm, about 100 nm and about 2 nm, about 50 nm and about 2 nm, about 30 nm and about 2 nm, about 20 and about 2 nm, about 10 nm and about 2 nm. In some embodiments, the size may refer to the largest dimension of the grain. The size of the grains referred herein may be determined as an “average” and may be measured by any suitable techniques. The dimensions may refer the diameter, length, width, height, depending on the geometry of the grain. In some instances (and as provided below), a stable nanocrystalline material may also refer to a material comprising an amorphous phase.

In another aspect, a metal alloy powder for sintering can include a mechanically alloyed powder having a reduced amount of gas-evolving species compared to the amount of gas-evolving species in the mechanically alloyed powder prior to annealing in a hydrogen atmosphere.

In another aspect, a method of forming a sintered alloy can include annealing a nano-phase separating metal alloy powder in the presence of hydrogen, and sintering the nano-phase separating metal alloy powder to form a sintered alloy product including nickel and having a relative density of at least 80%. The annealing and sintering can be conducted in a single step or in sequential steps.

In certain circumstances, the sintered alloy product can include a dispersed oxide. The dispersed oxide can be an oxide of an oxygen getter, for example, zirconium, chromium, vanadium, manganese, or combinations thereof.

The dispersed oxide can be a dispersed phase of a nanoparticle. The nanoparticle can have a size of less than or equal to about 1000 nm—e.g., 500 nm, 200 nm, 100 nm, 50 nm, 20 nm, 10 nm, 5 nm, 2 nm, or 1 nm.

In certain circumstances, the metal alloy powder can be engineered to have a phase separation temperature at which diffusion of the third metal occurs to phase separate as a nanoscale phase. Moreover, it can be preferred that the nanoscale phase can redissolve at a transition temperature higher than the phase separation temperature.

According to certain embodiments, the mechanical alloying (e.g., ball milling) is performed at a relatively low temperature. For example, in some embodiments, the mechanical alloying (e.g., ball milling) is performed while the particles are at a temperature of less than or equal to 150° C., less than or equal to 100° C., less than or equal to 75° C., less than or equal to 50° C., less than or equal to 40° C., less than or equal to 35° C., less than or equal to 30° C., less than or equal to 25° C., or less than or equal to 20° C. In some embodiments, the mechanical working (e.g., ball milling) is performed while the particles are at a temperature of at least 0° C. In some embodiments, the mechanical alloying (e.g., ball milling) can be performed at a temperature of the surrounding, ambient environment.

In certain embodiments, the mechanical alloying (e.g., ball milling) may be conducted for a time of greater than or equal to 6 hours (e.g., greater than or equal to 8 hours, greater than or equal to 10 hours, greater than or equal to 12 hours, or greater than or equal to 15 hours). In certain embodiments, the mechanical alloying (e.g., ball milling) may be conducted for a time of less than or equal to 18 hours. In some embodiments, the mechanical alloying (e.g., ball milling) may be conducted for a time of 6 hour to 18 hours.

In some embodiments, the mechanical alloying (e.g., ball milling) can be conducted in an inert atmosphere, for example, an argon atmosphere.

In certain circumstances, the nanoscale feature or nanophase can be 100 nm, 90 nm, 80 nm, 70 nm, 60 nm, 50 nm, 40 nm, 30 nm, 20 nm, 10 nm, 5 nm or smaller. The nanophase, nanoscale feature, or nanocrystal can refer to the size of a crystal (or a “grain”) being less than or equal to about 1000 nm—e.g., 500 nm, 200 nm, 100 nm, 50 nm, 20 nm, 10 nm, 5 nm, 2 nm, etc. For example, the grain size may be between 1000 nm and about 2 nm—e.g., about 500 nm and about 2 nm, about 200 nm and about 2 nm, about 100 nm and about 2 nm, about 50 nm and about 2 nm, about 30 nm and about 2 nm, about 20 and about 2 nm, about 10 nm and about 2 nm. In some embodiments, the size may refer to the largest dimension of the grain. The size of the grains referred herein may be determined as an “average” and may be measured by any suitable techniques. The dimensions may refer the diameter, length, width, height, depending on the geometry of the grain. In some instances (and as provided below), a stable nanocrystalline material may also refer to a material comprising an amorphous phase.

The alloy can be composed of three, four or more metals. Each metal can have a concentration of about 25 at % (atomic percent) to about 95 at % of the alloy. When two metal components of a ternary alloy each has a concentration of about 25 at % to about 95 at % of the alloy, the third metal component can have a concentration of about 5 at % to about 50 at % of the alloy. For example, each metal concentration can be at least 5 at %, at least 10 at %, at least 15 at %, at least 20 at %, at least 25 at %, at least 30 at %, at least 35 at %, at least 40 at %, at least 45 at %, at least 50 at %, at least 55 at %, at least 60 at %, at least 65 at %, at least 70 at %, at least 75 at %, at least 80 at %, at least 85 at %, at least 90 at %, or at least 95 at %.

In another aspect, a method of nano-phase separation sintering of a powder can include providing a mechanically alloyed powder including a first metal element and a second metal forming a metal alloy and a third metal having a miscibility gap with the metal alloy, and sintering the mechanically alloyed powder to form a sintered product.

In another aspect, a metal alloy powder for sintering can include a mechanically alloyed powder including a first metal element and a second metal forming a metal alloy and a third metal having a miscibility gap with the metal alloy.

In certain circumstances, the sintered product achieves at least 80%, at least 85%, at least 90%, at least 95%, at least 97%, or at least 98% density. These densities can be achieved at low temperatures without the need for an applied pressure during the sintering. In certain circumstances, sintering can include nano-phase separation sintering.

According to certain embodiments, sintering the plurality of particles involves heating the particles to a sintering temperature of less than or equal to 2200° C., less than or equal to 2000° C., less than or equal to 1900° C., less than or equal to 1800° C., less than or equal to 1700° C., less than or equal to 1600° C., less than or equal to 1500° C., less than or equal to 1400° C., less than or equal to 1300° C., less than or equal to 1200° C., less than or equal to 1100° C., less than or equal to 1000° C., less than or equal to 900° C., less than or equal to 850° C., less than or equal to 800° C., or less than or equal to 750° C. According to certain embodiments, sintering the plurality of particles involves heating the particles to a sintering temperature of greater than or equal to 750° C., greater than or equal to 850° C., greater than or equal to 1000° C., greater than or equal to 1200° C., greater than or equal to 1450° C., or greater than or equal to 1600° C. Combinations of these ranges are also possible. For example, in some embodiments, sintering the plurality of particles involves heating the particles to a sintering temperature that is greater than or equal to 750° C. and less than or equal to 2200° C. In some embodiments, the temperature of the sintered material is within these ranges for at least 10%, at least 25%, at least 50%, at least 75%, at least 90%, or at least 99% of the sintering time.

According to certain embodiments, sintering the plurality of particles involves maintaining the particles within the range of sintering temperatures for less than 72 hours, less than 48 hours, less than or equal to 24 hours, less than or equal to 12 hours, less than or equal to 6 hours, less than or equal to 4 hours, less than or equal to 3 hours, less than or equal to 2 hours, or less than or equal to 1 hour (and/or, in some embodiments, for at least 10 minutes, at least 20 minutes, at least 30 minutes, at least 50 minutes, at least 3 hours, or at least 6 hours). Combinations of these ranges are also possible. For example, in some embodiments, sintering the plurality of particles involves heating the particles to a first sintering temperature that is greater than or equal to 600° C. and less than or equal to 1100° C. for a sintering duration greater than or equal to 6 hours and less than or equal to 24 hours.

In certain circumstances, the rate of heating can be 2° C./min, 3° C./min, 4° C./min, 5° C./min, 10° C./min, 15° C./min, or 20° C./min.

In certain circumstances, grain size of the metal alloy powder and grain size of the sintered powder can be substantially the same. The grain size can be a nano-scale grain size.

A sintered product can include the powder as described herein.

Referring to FIG. 1A, generalized requirements for nano-phase separation sintering are illustrated, including phase separation, neck formation, sintering and densification. The characteristics of the alloy are important to perform these steps. For example, nanocrystallinity can ensure rapid diffusion to the particle surface. Moreover, supersaturation can allow a desired second phase to form at necks. Mechanical alloying can facilitate both of these important factors.

Referring to FIG. 1B, a phase diagram suggests the interplay between concentration, temperature, phase separation, reduced surface energy and high solubility that least to the phase separation enhanced sintering described herein.

FIG. 1C illustrates an example of the general process example with Fe.

Ternary and higher order Ni—Cu alloy systems are of particular interest. Requirements for the selection of the ternary element include the following. The ternary element should possess a high positive heat of mixing with Cu. This can increase the transition temperature from miscibility gap to the solid solution phase field. The ternary element should also ideally form a solid solution with Ni. This property can favor an interdiffusion as required for nano-phase separation sintering.

FIG. 2A shows a Ni—Cu phase diagram. FIG. 2B illustrates a behavior of the system. For example, increased mobility leads to decreased undercooling and increased undercooling leads to decreased mobility.

For example, a 70 at % Ni, 15 at % Cu and 15 at % Co alloy was studied. The phase diagram for the system is shown in FIG. 3. The alloy was heated at different rates (5, 10, 15° C./min) up to 1000° C. then cooled 40° C./min. FIG. 4A shows the densification rate at 10K/min to 1000° C., then 40K/min cooling. FIG. 4B shows the change in relative density.

A Cr alloy was also studied. FIG. 5A is a Ni—Cr phase diagram. FIG. 5B is a Cu—Cr phase diagram. Other phase diagrams at other Cr concentrations can be found at FIGS. 6A-6C. FIG. 6A is a Ni—Cu phase diagram at 5 at % Cr. FIG. 6B is a Ni—Cu phase diagram at 10 at % Cr. FIG. 6C is a Ni—Cu phase diagram at 15 at % Cr.

An alloy of 60 at % Ni, 25 at % Cu and 15 at % Cr was studied. A phase diagram of 60 at % Ni and 15% Cr is shown in FIG. 6D. FIG. 6E is a table showing the compositions of stable phases at 550° C. FIG. 6F shows the temperature dependence of densification at 5° C./min to 1200° C. and 40° C./min cooling.

An Fe alloy was studied. FIG. 7A is a Ni—Fe phase diagram. FIG. 7B is a Cu—Fe phase diagram. Other phase diagrams at other Fe concentrations can be found at FIGS. 8A-8C. FIG. 8A is a Ni—Cu phase diagram at 5 at % Fe. FIG. 8B is a Ni—Cu phase diagram at 20 at % Fe. FIG. 8C is a Ni—Cu phase diagram at 42 at % Fe.

FIG. 8D is a Ni—Cu phase diagram at 42 at % Fe. FIG. 8E is a table showing the compositions of stable phases at 550° C. One set of experiments were conducted at 15 at % Cu. FIG. 8F shows the temperature dependence of densification at 5° C./min to 945° C. and 40° C./min cooling. FIG. 8G shows the change in relative density over time at 5° C./min heating. FIG. 8H shows the temperature dependence of densification at 10° C./min to 1000° C. and 40° C./min cooling. FIG. 8I shows the change in relative density over time at 5° C./min heating to a final relative density of 0.5864%. FIG. 8J shows the temperature dependence of densification at 15° C./min to 1000° C. and 40° C./min cooling. FIG. 8K shows the change in relative density over time at 15° C./min heating to a final relative density of 0.5477%. FIG. 8L shows the temperature dependence of densification at 20° C./min to 1000° C. and 40° C./min cooling. FIG. 8M shows the change in relative density over time at 20° C./min heating to a final relative density of 0.4276%.

Another set of experiments were conducted at 15 at % Cu. FIG. 8N shows the temperature dependence of densification at 5° C./min to 1100° C. and 40° C./min cooling. FIG. 8O shows the change in relative density over temperature at 5° C./min heating. FIG. 8P shows the change in relative density over time at 5° C./min heating. FIG. 8Q shows the temperature dependence of densification at 10° C./min to 1100° C. and 40° C./min cooling. FIG. 8R shows the change in relative density over temperature at 10° C./min heating. FIG. 8S shows the change in relative density over time at 15° C./min. FIG. 8T shows the temperature dependence of densification at 20° C./min to 1100° C. and 40° C./min cooling. FIG. 8U shows the temperature dependence of densification at 20° C./min to 1100° C. and 40° C./min cooling. FIG. 8V shows the change in relative density over time at 20° C./min.

A Co alloy was studied. FIG. 9A is a Ni—Co phase diagram. FIG. 9B is a Cu—Co phase diagram. Other phase diagrams at other Co concentrations can be found at FIGS. 10A-10C. FIG. 10A is a Ni—Cu phase diagram at 5 at % Co. FIG. 10B is a Ni—Cu phase diagram at 20 at % Co. FIG. 10C is a Ni—Cu phase diagram at 44 at % Co.

A V alloy was studied. FIG. 11A is a Ni—V phase diagram. FIG. 11B is a Cu—V phase diagram. Other phase diagrams at other V concentrations can be found at FIGS. 12A-12C. FIG. 12A is a Ni—Cu phase diagram at 6 at % V. FIG. 12B is a Ni—Cu phase diagram at 10 at % V. FIG. 12C is a Ni—Cu phase diagram at 13 at % V. FIG. 12D is a Ni—Cu phase diagram and 13 at % V. At 15 at % Cu, FIG. 12E shows the stable phases at 1150° C. FIG. 12F shows the temperature dependence of the densification rate for 10 at 5° C./min to 1200° C. and 40° C./min cooling. FIG. 12G shows the temperature dependences of the change in relative density.

Based on the experiments described herein, Ni self-diffusion impacts the limit of accelerated densification.

FIG. 13 is a chart showing the diffusion rates and temperature range for metals described herein.

The phase diagram information can vary depending on the source of the data. FIG. 14A is a phase diagram for Fe—Cu with 40 at % Ni from an Fe database. FIG. 14B is a phase diagram for Fe—Cu with 40 at % Ni from a Ni database. FIG. 14C is a phase diagram for Fe—Cu with 40 at % Ni from a Cu database. FIG. 14D is a phase diagram for Fe—Cu with 40 at % Ni from a high entropy alloy database.

Other phase diagrams can be created. FIG. 15 is a Cr—Cu phase diagram at 60 at % Ni. FIG. 16 is a Co—Cu phase diagram at 40 at % Ni. FIG. 17 is a Cu—V phase diagram at 75 at % Ni.

Pretreatment of an alloy in a reducing atmosphere can improve the sintering performance of the alloy. For example, pretreatment with hydrogen gas at 300° C. for 24 hours can reduce swelling. FIGS. 18A, 18B and 18C show densification after pretreatment with hydrogen at different heating rates.

As noted above, and oxygen scavenger can be included in an alloy. FIG. 19 shows the impact on density of 5 at % and 8 at % Mn to a Ni—Fe—Cu composition.

In another example, FIG. 20 shows the densification behavior of the addition of Mn in a 37.5 at % Ni—37.5 at % Co—20 at % Cu—5 at % Mn composition heated to 1200° C. Microstructures are shown in the images of FIGS. 21, 22, 23 and 24.

The mechanical properties of the sintered products described herein have been tested. Results of mechanical testing of 37.5 at % Ni 37.5 at % Fe 20 at % Cu 5 at % Mn are shown in FIGS. 25-28.

Referring to FIGS. 29A-29B, Cr meets the thermodynamic criteria for rapid sintering of both W and Mo based alloys. Initial testing demonstrated accelerated sintering in Mo and W based systems alloyed with Cr, for example W15Cr and Mo15Cr, as shown in FIG. 30. Initial testing of the Mo25W15Cr samples for sintering exhibited >98% relative density in a sample heated to 1450° C., as shown in FIG. 31.

In another example, Ag/Zr 16/1, 16/4, 20/4, 24/4 powders were characterized by SEM and XRD to have distributions of particles between 1 and 20 microns with average particle sizes falling between 6 and 8 microns, aspect ratio of 1.5. A secondary phase of Ag was present with increased content at the solubility limit. For these powders, sintering in reducing atmosphere (containing 3% H₂) vs pure Ar atmosphere was critical for reducing oxidation and promoting sintering and densification as shown in FIG. 32 for Ni 16 at %, Ag 4 at %, Zr.

The binary system Ni—Ag (Ni-20Ag) shows two swelling events at a heating rate of 3° C./min, shown in FIG. 33A. Heat treating of Ni—Ag powders at 300 C shows great improvement in swelling. Ni-20Ag can have up to a 91% density when heat treated prior to sintering. See, FIG. 33B.

In another example, FIG. 34 shows addition of a getter, in this case, Zr, to a Ni—Ag alloy. The addition of Zr to the Ni—Ag alloy system almost eliminates second swelling event and drastically reduces a first swelling event. APT performed on 16/4 Ag/Zr show 99.5% of Zr is present in ZrO clusters dispersed throughout grains and at boundaries. Zr is acting as a getter within the system—stray oxygen from PCA is now trapped in more stable ZrO instead of forming CO. Lack of grain boundary segregation means grains may not be stabilized in NC regime. The remaining O presence points to need for reducing contaminants, or increasing Zr or other “getter” element in system. Processing parameters like heating rate (3-50 K/min) and compaction pressure (100-1250 MPa) seem to play minor roles in the performance of the getter. See, FIG. 35.

Experimental Methods

This work focuses on the sintering of a model nanocrystalline material produced through a relatively standard route with standard inputs, so as to focus on the physics of the kinetic competition outlined above. Ni is selected as the base metal for its technological relevance, and because it occupies a unique position: for microcrystalline materials debinding easily precedes sintering, but for nanocrystalline materials the onset of sintering is at the low end of the debinding range, so that the conflation of organic burnout and sintering is expected to be nearly unavoidable.

Nanocrystalline Ni—Fe alloy powders were produced through high-energy ball milling. Nickel powder (Alfa Aesar, 99.9% purity, 3-7 μm particle size) was milled on a SPEX 8000D Mixer/Mill and with a ball-to-powder ratio of 10:1 (5 g powder batch) with hardened steel vials and media. High-energy ball-milling was conducted in a glovebox maintained under an ultra-high purity Ar atmosphere to limit atmospheric oxygen contamination. To balance fracturing and cold-welding of the powder particles during high-energy ball milling, approximately 5 weight percent (wt %) ethanol (C₂H₆O) was added as a process control agent (PCA). Note that this is the only organic species used in the present work, and is therefore the primary source of carbon in the system.

The powder was milled for 20 h and then imaged using a Merlin Zeiss high-resolution scanning electron microscope (HR-SEM) in secondary electron (SE) mode to estimate the as-milled powder size of 51 (±23) μm. The as-milled grain size of about 23 nm was determined on a Panalytical X'Pert PRO using Cu-Kα radiation with a wavelength 2=1.5418 Å and a step size of 0.0167° operated at 45 kV and 40 mA for voltage and current, respectively. The average grain size was determined from the peak broadening corrected by the lattice strain using a classical Williamson-Hall analysis and instrumental contributions using a NIST LaB6 as reference. Although pure Ni is the input to this process, the wear of the steel media/vials for these milling conditions leads to the production of Ni—Fe solid solution alloy powders. The final composition of powders was determined using an energy-dispersive x-ray (EDX) spectrometry detector in the SEM, under the same conditions as described before. The accumulation of Fe contamination as function of ball-milling time. All of the powders reported in this paper were produced under the same 20 h milling conditions and have an iron content of 9.75 atomic percent (at %) (±0.41), which was generally confirmed by wavelength dispersive spectroscopy measurements. Wavelength dispersive spectroscopy (WDS) analysis was based on four separate measurements, which were all comparable. The analysis was performed on a JEOL JSM7900F Scanning electron microscope with an Oxford Wave WDS spectrometer (voltage of 20 kV, probe current of 1.25 nA). For each element analyzed, the Kα line was used with standard references for all elements that were expected to be part of the alloy through wear of the initial Ni powder with the grinding media. The experiments on the pressed compacts reported in the following include powders from three independent but otherwise identical ball-milling batches. X-ray diffraction confirmed that the as-milled powders were FCC solid solutions.

The ball-milled powders were pressed into cylindrical specimens with dimensions of approximately 6 and 1.5 mm for diameter and height, respectively, to an initial relative density in the range of—55.8-58.9% using a uniaxial hydraulic press (model YLJ-15 L from MTI Corporation) with a pressure of—400-450 MPa acting on the pellets, and no additional added binder phases. The initial relative densities of the pressed specimens were determined from the ratio of initial and theoretical density (8.80 g/cm³, accounting for the Fe content) based on mass and dimension measurements. TMA data are presented by converting a raw change in length into a relative density assuming an isotropic shape change, which was generally confirmed after each experiment with caliper measurements.

Details of one or more embodiments are set forth in the accompanying drawings and description. Other features, objects, and advantages will be apparent from the description, drawings, and claims. Although a number of embodiments of the invention have been described, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. It should also be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified representation of various features and basic principles of the invention.

Claims

1. A method of reducing gas production during metal alloy powder sintering comprising:

providing a mechanically alloyed metal alloy powder;

annealing the metal alloy powder at an annealing temperature below a sintering temperature, the annealing of the metal alloy powder being in an atmosphere including hydrogen or the metal alloy powder including an oxygen getter, or both; and

sintering the metal alloy powder at a sintering temperature to reduced gas production during the sintering.

2. The method of claim 1, further comprising pressing the annealed metal alloy powder to form a green body prior to sintering.

3. The method of claim 1, further comprising pressing the annealed metal alloy powder to form a green body prior to annealing.

4. The method of claim 1, wherein the metal alloy powder is a nano-phase separating powder.

5. The method of claim 1, wherein the metal alloy powder includes an additive.

6. The method of claim 5, wherein the additive includes an organic material.

7. The method of claim 1, wherein the metal alloy powder includes nickel.

8. The method of claim 1, wherein the metal alloy powder includes chromium.

9. The method of claim 1, wherein the metal alloy powder includes iron.

10. The method of claim 1, wherein the metal alloy powder includes copper.

11. The method of claim 1, wherein the metal alloy powder includes vanadium.

12. The method of claim 1, wherein the metal alloy powder includes molybdenum.

13. The method of claim 1, wherein the metal alloy powder includes tungsten.

14. The method of claim 1, wherein the metal alloy powder includes silver.

15. The method of claim 1, wherein the metal alloy powder includes zirconium.

16. The method of claim 1, wherein the metal alloy powder includes a ternary nano-phase separating powder.

17. The method of claim 1, wherein the oxygen getter includes an alloying element having a strong preference to form an oxide.

18. The method of claim 17, wherein the oxygen getter includes zirconium, chromium, vanadium, manganese or combinations thereof.

19-22. (canceled)

23. A metal alloy powder for sintering comprising:

a mechanically alloyed powder having a reduced amount of gas-evolving species compared to the amount of gas-evolving species in the mechanically alloyed powder prior to annealing in a hydrogen atmosphere.

24-28. (canceled)

29. A method of forming a sintered alloy, comprising:

annealing a nano-phase separating metal alloy powder in the presence of hydrogen; and

sintering the nano-phase separating metal alloy powder to form a sintered alloy product including nickel and having a relative density of at least 80%.

30-32. (canceled)