Molybdenum-containing alloys and associated systems and methods

Abstract

Molybdenum-containing alloys and associated systems and methods are generally described. In certain embodiments, secondary and / or three-dimensional elements can be included with the molybdenum to provide beneficial properties during sintering of the molybdenum-containing alloy. The molybdenum-containing alloy, according to certain embodiments, is nanocrystalline. According to certain embodiments, the molybdenum-containing alloy has a high relative density. According to certain embodiments, the molybdenum-containing alloy can be relatively stable. Inventive methods for making the molybdenum-containing alloy are also described herein.

Description

[Technical Field]

[0001] (CROSS-REFERENCE TO RELATED APPLICATIONS) This application claims priority under 35 U.S.C. § 119(e) to U.S. Provisional Application No. 62 / 968,233, filed January 31, 2020, and entitled "Molybdenum-Containing Alloys and Associated Systems and Methods," which is incorporated herein by reference in its entirety for all purposes.

[0002] (Government financial aid) This invention was made with government support under Grant No. 80NSSC19K1055 awarded by the NASA Marshall Space Flight Center. The government has certain rights in this invention.

[0003] (Technical field) Molybdenum-containing alloys and associated systems and methods are generally described. Summary of the Invention [Means for solving the problem]

[0004] (overview) Molybdenum-containing alloys and associated systems and methods are generally described. In certain embodiments, secondary (and optionally tertiary) elements can be included with the molybdenum to provide beneficial properties during sintering of the molybdenum-containing alloy. The molybdenum-containing alloy, according to certain embodiments, is nanocrystalline. According to certain embodiments, the molybdenum-containing alloy has a high relative density. According to certain embodiments, the molybdenum-containing alloy can be relatively stable. Inventive methods for making the molybdenum-containing alloy are also described herein. The subject matter of the present invention in some cases involves interrelated products, alternative solutions to a particular problem, and / or multiple different uses of one or more systems and / or articles.

[0005]

[0003] Certain aspects relate to methods of forming a metal alloy. In some embodiments, the method includes sintering particles comprising molybdenum (Mo) and a second element to produce a metal alloy, wherein Mo is the most abundant element by atomic percentage in the metal alloy, and the metal alloy has a relative density of at least 80%.

[0006] In some embodiments, the method includes sintering particles comprising molybdenum (Mo) and chromium (Cr) to form a metal alloy.

[0007] Also disclosed herein are metal alloys, in some embodiments, the metal alloy comprises molybdenum (Mo) and a second element, wherein Mo is the most abundant element by atomic percentage in the metal alloy, and the metal alloy has a relative density of at least 80%.

[0008] In some embodiments, the metal alloy comprises molybdenum (Mo) and chromium (Cr), with Mo being the most abundant element by atomic percentage in the metal alloy.

[0009] Other advantages and novel features of the present invention will become apparent from the following detailed description of various non-limiting embodiments of the invention when considered in conjunction with the accompanying drawings. In cases where the present specification and any document incorporated by reference includes conflicting and / or inconsistent disclosure, the present specification shall control. The present invention provides, for example, the following items. (Item 1) 1. A method of forming a metal alloy, comprising: sintering particles comprising molybdenum (Mo) and a second element to produce the metal alloy; Mo is the most abundant element by atomic percentage in said metal alloy; The method, wherein the metal alloy has a relative density of at least 80%. (Item 2) 1. A method of forming a metal alloy, comprising: 1. A method comprising sintering particles comprising molybdenum (Mo) and chromium (Cr) to produce the metal alloy. (Item 3) 3. The method of claim 2, wherein Mo is the most abundant element in atomic percentage in the metal alloy. (Item 4) 4. The method according to claim 3, wherein the metal alloy has a relative density of at least 80%. (Item 5) 5. The method according to any one of items 1 and 3 to 4, wherein the second element is chromium. (Item 6) 5. The method according to any one of items 1 and 3 to 4, wherein the second element is palladium (Pd). (Item 7) 7. The method according to any one of items 1 to 6, further comprising a third element. (Item 8) 8. The method of claim 7, wherein the third element is present in the metal alloy in an amount of 0.5 at% to 40 at% of the metal alloy. (Item 9) 9. The method according to any one of items 7 to 8, wherein the second element and the third element exhibit a miscibility gap. (Item 10) 10. The method according to any one of items 7 to 9, wherein the third element is tungsten (W). (Item 11) 10. The method according to any one of items 7 to 9, wherein the third element is tantalum (Ta). (Item 12) 12. The method according to any one of items 1 to 11, wherein the melting point of the metal alloy is at least 2,500°C. (Item 13) 13. The method according to any one of items 1 to 12, wherein the metal alloy has a neutron absorption cross section of 18 barns or less. (Item 14) 14. The method according to any one of items 1 to 13, wherein Mo is present in the metal alloy in an amount of at least 50 at%. (Item 15) 15. The method according to any one of items 1 to 14, wherein the Mo and the second element exhibit a miscibility gap. (Item 16) 16. The method according to any one of items 1 to 15, wherein the metal alloy is nanocrystalline. (Item 17) Item 17. The method of item 16, wherein the nanocrystalline metal alloy has an average grain size of 300 nm or less. (Item 18) 18. The method according to any one of items 1 to 17, wherein the metal alloy is a bulk metal alloy. (Item 19) 19. The method according to any one of items 1 to 18, wherein the metal alloy is substantially stable at a temperature of at least 2,500°C. (Item 20) 20. The method of any one of items 1 to 19, wherein the metal alloy has a first grain size and the sintered material comprising Mo but without the second element has a second grain size, and the first grain size is smaller than the second grain size. (Item 21) 21. The method according to any one of items 1 to 20, wherein the metal alloy is enriched in the second element at the grain boundaries of the metal alloy. (Item 22) A metal alloy comprising molybdenum (Mo) and a second element, Mo is the most abundant element by atomic percentage in said metal alloy; The metal alloy has a relative density of at least 80%. (Item 23) A metal alloy, Contains molybdenum (Mo) and chromium (Cr), A metal alloy, wherein Mo is the most abundant element by atomic percentage in said metal alloy. (Item 24) 24. The metal alloy according to any one of items 22 to 23, wherein the metal alloy is sintered. (Item 25) 25. The metal alloy according to any one of items 23 to 24, wherein Mo is the most abundant element in terms of atomic percentage in the metal alloy. (Item 26) 26. The metal alloy according to any one of items 22 to 25, wherein the metal alloy has a relative density of at least 80%. (Item 27) 27. The metal alloy according to any one of items 22 and 24 to 26, wherein the second element is chromium. (Item 28) 27. The metal alloy according to any one of items 22 and 24 to 26, wherein the second element is palladium (Pd). (Item 29) 29. The metal alloy according to any one of items 22 to 28, further comprising a third element. (Item 30) Item 30. The metal alloy according to item 29, wherein the third element is present in the metal alloy in an amount of 0.5 at% to 40 at% of the metal alloy. (Item 31) 31. The metal alloy according to any one of items 29 to 30, wherein the second element and the third element exhibit a miscibility gap. (Item 32) 32. The metal alloy according to any one of items 29 to 31, wherein the third element is tungsten (W). (Item 33) 32. The metal alloy according to any one of items 29 to 31, wherein the third element is tantalum (Ta). (Item 34) 34. The metal alloy according to any one of items 22 to 33, wherein the melting point of the metal alloy is at least 2,500°C. (Item 35) 35. The metal alloy according to any one of items 22 to 34, wherein the metal alloy has a neutron absorption cross section of 18 barns or less. (Item 36) 36. The metal alloy according to any one of items 22 to 35, wherein Mo is present in the metal alloy in an amount of at least 50 at %. (Item 37) 37. The metal alloy according to any one of items 22 to 36, wherein the Mo and the second element exhibit a miscibility gap. (Item 38) 38. The metal alloy according to any one of items 22 to 37, wherein the metal alloy is nanocrystalline. (Item 39) Item 39. The metal alloy according to item 38, wherein the nanocrystalline metal alloy has an average grain size of 300 nm or less. (Item 40) 40. The metal alloy according to any one of items 22 to 39, wherein the metal alloy is a bulk metal alloy. (Item 41) 41. The metal alloy according to any one of items 22 to 40, wherein the metal alloy is substantially stable at a temperature of at least 2,500°C. (Item 42) 42. The metal alloy according to any one of items 22 to 41, wherein the metal alloy has a first grain size, and the sintered material containing Mo but without the second element has a second grain size, and the first grain size is smaller than the second grain size. (Item 43) 43. The metal alloy according to any one of items 22 to 42, wherein the metal alloy is enriched in the second element at the grain boundaries of the metal alloy. [Brief explanation of the drawings]

[0010] BRIEF DESCRIPTION OF THE DRAWINGS Non-limiting embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, which are schematic and are not intended to be drawn to scale. In the figures, each identical or nearly identical component shown is typically represented by a single numeral. For purposes of clarity, not every component will be labeled in every figure, nor will every component of each embodiment of the invention be shown unless illustration is necessary to enable those skilled in the art to understand the invention.

[0011] [Figure 1] 1A-1C are exemplary schematic diagrams illustrating a sintering process, according to certain embodiments.

[0012] [Figure 2] 2A-2B show SEM images of exemplary molybdenum-chromium alloys according to one set of embodiments.

[0013] [Figure 3] 3A-3B show SEM images of exemplary molybdenum-chromium-tungsten alloys according to one set of embodiments.

[0014] [Figure 4] FIG. 4 is a diagram depicting a process for producing and sintering an alloy exhibiting nano-phase separation sintering, according to some embodiments.

[0015] [Figure 5]FIG. 5 is an SEM image of a powder after it has been mechanically alloyed but before it has been sintered, according to one embodiment.

[0016] [Figure 6] Figure 6 is an SEM image of Mo15Cr sintered up to 1,450°C, according to some embodiments. This sample has a relative density of >98%. The darker phase present is a chromium-rich phase that aided in sintering.

[0017] [Figure 7] 7 is an SEM image of Mo15Cr sintered to 1,200° C. and quenched before reaching full density, according to one embodiment. In this image, it is easier to see where the chromium-rich phase (darker material) has formed necks between grains.

[0018] [Figure 8] 8 is an SEM image of Mo25W15Cr sintered to 1,450° C. and achieving a relative density of greater than 98%, according to one embodiment. Similar to the Mo15Cr sample, the darker phases represent chromium phases formed to facilitate sintering.

[0019] [Figure 9] FIG. 9 shows densification curves for Mo15Cr alloys compared to pure Mo, sintered to 1,450° C. at a heating rate of 10° C. per minute, according to some embodiments.

[0020] [Figure 10] FIG. 10 shows a densification curve for a Mo25W15Cr alloy sintered to 1,450° C. at a rate of 10° C. per minute, according to some embodiments. DETAILED DESCRIPTION OF THE INVENTION

[0021] (Detailed explanation) The present disclosure is generally directed to metal alloys containing molybdenum and methods of making the molybdenum-containing alloys. Some embodiments relate to making the molybdenum-containing alloys via sintering. In some embodiments, secondary and / or three-dimensional elements can be included with the molybdenum to provide beneficial properties during sintering of the molybdenum-containing alloy. In some cases, the molybdenum-containing alloys described herein include additional elements in addition to molybdenum, such as chromium (Cr) and / or tungsten (W). Other elements may also be present. According to some embodiments, the molybdenum-containing alloys described herein can contain at least three elements (e.g., at least three metal elements). However, the presence of all three elements is not strictly required, and in other embodiments, the molybdenum-containing alloys may include only two elements.

[0022] As noted above, the present disclosure includes inventive methods for making molybdenum-containing alloys. For example, certain embodiments are directed to sintering methods in which sintering is accomplished at relatively low temperatures and / or for relatively short time periods. In some embodiments, sintering is carried out with little or no applied pressure during the sintering process. According to some embodiments, as described in more detail below, sintering can be carried out such that undesired grain growth is limited or eliminated (e.g., through selection of materials and / or sintering conditions). Certain embodiments are directed to the recognition that molybdenum-containing materials may be sintered for relatively short times, at relatively low temperatures, and / or with relatively low (or no) applied pressure while maintaining high temperature stability, high relative density, and / or, in some cases, nanocrystalline nature.

[0023] Some of the embodiments described herein can provide advantages over conventional articles, systems, and methods. For example, according to some (but not necessarily all) embodiments, molybdenum-containing metal alloys can have high strength, high hardness, and / or high resistance to grain growth. According to some (but not necessarily all) embodiments, methods for forming metal alloys described herein can utilize relatively small amounts of energy due, for example, to relatively short sintering times, relatively low sintering temperatures, and / or relatively low applied pressures employed.

[0024] In some embodiments, the metal alloy is formed by sintering a plurality of particles. The particles, in some embodiments, may be in the form of a powder. The shape of the particles may be, for example, spherical, cubic, conical, cylindrical, needle-like, irregular, or any other suitable geometric shape. In some embodiments, at least a portion (e.g., at least 50%, at least 75%, at least 90%, or at least 95%) of the particles are single crystalline. In certain embodiments, at least a portion (e.g., at least 50%, at least 75%, at least 90%, or at least 95%) of the particles are polycrystalline.

[0025] The particles from which the metal alloy is formed can have any of a variety of sizes, in some embodiments, at least 50% (or at least 75%, at least 90%, at least 95%, or at least 99%) of the total particle volume is made up of particles having a maximum cross-sectional dimension of less than 1 millimeter (or less than 500 microns, less than 100 microns, or less than 10 microns).

[0026] 1A-1C are exemplary schematic diagrams illustrating a sintering process according to certain embodiments. In FIG. 1A, a plurality of particles 100 are shown in the form of spheres (although, as noted elsewhere, other shapes may be used). As shown in FIG. 1B, the particles 100 may be arranged so that they contact one another. As shown in FIG. 1C, as the particles are heated, they coalesce to form a single solid material 110. During the sintering process, according to certain embodiments, the gaps 105 between the particles 100 (shown in FIG. 1B) may be significantly reduced or eliminated, such that a solid having a high relative density is formed (shown in FIG. 1C).

[0027] According to certain embodiments, the particles from which the alloy is formed contain a relatively large amount of molybdenum (Mo). For example, in some embodiments, Mo is the most abundant element (e.g., the most abundant metal) in the particles by atomic percentage. (Atomic percentage is abbreviated herein as "at.%" or "at %.") According to certain embodiments, Mo is present in the particles in an amount of at least 50 at%, at least 55 at%, at least 60 at%, at least 65 at%, at least 70 at%, at least 80 at%, at least 90 at%, or at least 95 at%. In some embodiments, Mo is present in the particles in an amount up to 96 at%, up to 97 at%, up to 98 at%, up to 99 at%, up to 99.5 at%, or more. Combinations of these ranges are also possible. Other values ​​are also possible.

[0028] According to certain embodiments, at least some of the particles comprise Mo and / or a second element (e.g., a second metal). The phrase "second element" is used herein to describe any element other than Mo. The phrase "second metal" is used herein to describe any metallic element other than Mo. The term "element" is used herein to refer to an element as found in the periodic table. "Metallic elements" are those found in Groups 1-12 of the periodic table excluding hydrogen (H), Al, Ga, In, Tl, and Nh in Group 13 of the periodic table, Sn, Pb, and Fl in Group 14 of the periodic table, Bi and Mc in Group 15 of the periodic table, Po and Lv in Group 16 of the periodic table, the lanthanides, and the actinides.

[0029] In some embodiments, a portion of the particles is composed of Mo while another portion of the particles is composed of a second element (e.g., a second metal such as chromium). In certain embodiments, at least a portion of the particles includes both Mo and a second element (e.g., a second metal such as chromium).

[0030] According to certain embodiments, the second element is selected from the group consisting of chromium (Cr) and palladium (Pd). In some embodiments, both Cr and Pd are present (e.g., when the particles include at least three elements). In other embodiments, only one of Cr and Pd is present. In some embodiments, the second element is Cr.

[0031] According to certain embodiments, the second element and Mo exhibit a miscibility gap. Two elements are considered to exhibit a "miscibility gap" when their phase diagram contains a region where a mixture of the two elements exists as two or more phases. In some embodiments in which the second element and Mo exhibit a miscibility gap, the second element and Mo can exist in the metal alloy between at least two phases.

[0032] In some embodiments, the second element has a melting point lower than that of molybdenum (Mo). As will be understood by those skilled in the art, the melting point of an element refers to the melting point of the element in its pure form. For example, in the case of a metal, the melting point of the metal refers to the melting point of the metal in its pure form.

[0033] In some embodiments, the Mo is at least partially soluble in the second element.

[0034] The second element (e.g., chromium, palladium) may be present in the particles from which the alloy is made in various suitable percentages. According to certain embodiments, the second element is present in the particles in an amount of less than or equal to 40 at.%, less than or equal to 35 at.%, less than or equal to 32 at.%, less than or equal to 30 at.%, less than or equal to 25 at.%, less than or equal to 22 at.%, less than or equal to 20 at.%, less than or equal to 18 at.%, or less than or equal to 16 at.%. In some embodiments, the second element is present in the metal alloy in an amount of at least 0.5 at.%, at least 1 at.%, at least 2 at.%, at least 3 at.%, at least 4 at.%, at least 5 at.%, at least 6 at.%, at least 7 at.%, at least 8 at.%, at least 9 at.%, at least 10 at.%, or more. Combinations of these ranges are also possible. For example, in some embodiments, the second element is present in the metal alloy in an amount of 0.5 at.% to 40 at.% of the metal alloy. In some embodiments, the second element is present in the metal alloy in an amount between 1 at% and 40 at% of the metal alloy. In some embodiments, the second element is present in the metal alloy in an amount between 8 at% and 32 at% of the metal alloy. Other values ​​are also possible.

[0035] In some embodiments, the second element may be an activator element for Mo. Activator elements are those elements that increase the rate of sintering of a material relative to the sintering rate observed in the absence of the activator element under otherwise identical conditions. Activator elements are described in more detail below.

[0036] According to certain embodiments, the second element (eg, for forming an alloy with Mo) can be selected based on one or more of the following conditions: 1. Thermodynamic stability of nanocrystalline grain size, 2. A phase separation region that extends above the sintering temperature; 3. A second (e.g., solute) element with a lower melting temperature, and / or 4. Solubility of Mo in the precipitated second phase.

[0037] According to some embodiments, the second element (e.g., Cr) forms precipitates within the Mo matrix. For example, in some embodiments, the metal alloy comprises a structure consisting of Mo-rich grains and Cr-rich precipitates. In some embodiments, the precipitates of the second element (e.g., Cr) can penetrate the grain boundaries between the Mo grains. In some embodiments, the second metal is chromium. The inventors have recognized and understood, within the context of the present disclosure, that the addition of a second metal, such as chromium, can provide certain benefits when alloyed with molybdenum. While not wishing to be bound by the inventors' theory, it is believed that chromium may segregate to a secondary phase upon heating and, possessing a lower surface energy than molybdenum, may preferentially segregate to the surface of the alloy, forming bridges or "necks" between the molybdenum grains. That is, chromium may form connecting bonds at the grain boundaries of the molybdenum particles within the alloy. In addition, chromium may also enable rapid diffusion of molybdenum through the chromium necks, promoting rapid densification. For example, in Figures 2A-2B, SEM images of an exemplary Mo15Cr alloy are shown capturing different stages of neck formation. In Figure 2A, the Mo15Cr alloy was heated to 850°C and quenched, showing the early stages of neck formation. In Figure 2B, the Mo15Cr sample was heated to 1,200°C and quenched, showing intermediate neck growth and densification. Similarly, Figures 3A-3B show SEM images of an exemplary Mo25W15Cr alloy. In Figure 3A, the Mo25W15Cr was heated to 900°C and quenched, showing the early stages of neck formation. In Figure 3B, the Mo25W15Cr alloy was heated to 1,200°C and quenched, showing intermediate neck growth and densification.

[0038] In some embodiments, the particles from which the metal alloy is formed contain only Mo and the second element (i.e., Mo and the second element without additional metals or other elements). In other embodiments, the particles include Mo, the second element, and a third element. For example, in some embodiments, the particles include a third element (in addition to Mo and the second element). The third element may, in some embodiments, be a metal element. The phrase "third element" is used herein to describe an element that is not Mo and is not the second element. That is, the third element (when present) is different from Mo and the second element. In some embodiments, the metal alloy includes a third metal, in which case the alloy includes Mo, the second metal, and the third metal.

[0039] In some embodiments, the particles from which the alloy is formed (e.g., containing molybdenum, a second metal, and an optional third or additional metal) may contain a relatively large amount of metallic material. In some embodiments, at least 10 at%, at least 20 at%, at least 40 at%, at least 50 at%, at least 70 at%, at least 90 at%, at least 95 at%, at least 99 at%, at least 99.9 at%, or more of the particle material is composed of metal atoms in their metallic form (i.e., in the zero oxidation state). In some embodiments, at least 10 at%, at least 20 at%, at least 40 at%, at least 50 at%, at least 70 at%, at least 90 at%, at least 95 at%, at least 99 at%, at least 99.9 at%, or more of the molybdenum atoms in the particles are in their metallic form. In certain embodiments, at least 10 at%, at least 20 at%, at least 40 at%, at least 50 at%, at least 70 at%, at least 90 at%, at least 95 at%, at least 99 at%, at least 99.9 at%, or more of the atoms of the second element (e.g., second metal) in the particles are in their metallic form. In some embodiments, at least 10 at%, at least 20 at%, at least 40 at%, at least 50 at%, at least 70 at%, at least 90 at%, at least 95 at%, at least 99 at%, at least 99.9 at%, or more of the atoms of the third element (e.g., third metal) in the particles are in their metallic form. In some embodiments, molybdenum atoms can form metallic bonds with other molybdenum atoms and / or other neighboring atoms, such as atoms of the second element (e.g., second metal) and / or third element (e.g., third metal).

[0040] According to certain embodiments, the third element is selected from the group consisting of tungsten (W) and tantalum (Ta). In some embodiments, the third element is W.

[0041] According to some embodiments, the third element (when present) and the second element exhibit a miscibility gap. In some embodiments in which the third element and the second element exhibit a miscibility gap, the third element and the second element can exist in the metal alloy between at least two phases.

[0042] In some embodiments, a third element (e.g., W, Ta) may increase the melting temperature of some of the Mo-based alloys described herein. For example, tungsten has a high melting temperature and forms a solid solution with molybdenum. Therefore, it is believed that the melting temperature of an alloy may be selectively adjusted by increasing the amount of tungsten in the Mo-based alloy. As a non-limiting example, an alloy containing 60 at% molybdenum, 25 at% tungsten, and 15 at% chromium (Mo25W15Cr) may exhibit a melting temperature 100 degrees (°C) higher than pure molybdenum.

[0043] In some embodiments, the third element has a melting point lower than the melting point of molybdenum (Mo).

[0044] The third element (e.g., tungsten) may be present in the particles in various suitable percentages. According to certain embodiments, the third element is present in the particles in an amount less than or equal to 40 at%; less than or equal to 35 at%; less than or equal to 30 at%; less than or equal to 28 at%; less than or equal to 26 at%; or less. In some embodiments, the third element is present in the metal alloy in an amount of at least 0.5 at%; at least 1 at%; at least 2 at%; at least 3 at%; at least 4 at%; at least 5 at%; at least 10 at%; at least 15 at%; at least 20 at%; at least 22 at%; at least 24 at%; or more. Combinations of these ranges are also possible. Other values ​​are also possible.

[0045] According to certain embodiments, the total amount of all metal elements (e.g., secondary elements, tertiary elements, and any additional optional elements) in the particles other than Mo constitutes less than 50 at% of the particle, less than 40 at% or less, less than 35 at% or less, less than 32 at% or less, less than 30 at% or less, less than 25 at% or less, less than 22 at% or less, less than 20 at% or less, less than 18 at% or less, or less than 16 at%. In some embodiments, the total amount of all elements (e.g., secondary elements, optional tertiary elements, and any additional optional elements) in the particles other than Mo constitutes at least 0.5 at% of the particle, at least 1 at%, at least 2 at%, at least 3 at%, at least 4 at%, at least 5 at%, at least 8 at%, at least 10 at%, at least 12 at%, at least 14 at%, or more. Combinations of these ranges are also possible. Other values ​​are also possible.

[0046] In some embodiments, the total amount of chromium (Cr), palladium (Pd), tungsten (W), and tantalum (Ta) present in the particles is less than 50 at% of the particles, less than 40 at% or less, less than 35 at% or less, less than 32 at% or less, less than 30 at% or less, less than 25 at% or less, less than 22 at% or less, less than 20 at% or less, less than 18 at% or less, or less than 16 at%. In some embodiments, the total amount of chromium (Cr), palladium (Pd), tungsten (W), and tantalum (Ta) present in the particles is at least 0.5 at%, at least 1 at%, at least 2 at%, at least 3 at%, at least 4 at%, at least 5 at%, at least 8 at%, at least 10 at%, at least 12 at%, at least 14 at%, or more. Combinations of these ranges are also possible. For example, in some embodiments, the total amount of chromium (Cr), palladium (Pd), tungsten (W), and tantalum (Ta) present in the particle is between 0.5 at% and 50 at% of the particle, and in some of these embodiments, the remaining at least 90 at% (or at least 95 at%, at least 98 at%, at least 99 at%, or at least 99.9 at%) of the particle is made from molybdenum.

[0047] Those skilled in the art will understand that to determine the total amount of chromium (Cr), palladium (Pd), tungsten (W), and tantalum (Ta) present in a given set of particles, the atomic percentages of each of these elements will be added together. For example, if the particles contain 60 at% Mo, 15 at% Cr, and 25 at% W, the total amount of chromium (Cr), palladium (Pd), tungsten (W), and tantalum (Ta) present will be 40 at% (i.e., 15 at% from Cr, 25 at% from W, and 0 at% for all other elements in the list). Those skilled in the art will also understand that when performing this calculation, not all of the elements in the above list will necessarily be present in the particles. In the exemplary calculation described above, for example, palladium and tantalum are not present in the particles.

[0048] In some embodiments, the combined amount of chromium (Cr) and tungsten (W) present in the particles is less than 50 at% of the particle, less than 40 at% or less, less than 35 at% or less, less than 32 at% or less, less than 30 at% or less, less than 25 at% or less, less than 22 at% or less, less than 20 at% or less, less than 18 at% or less, or less than 16 at%. In some embodiments, the combined amount of chromium (Cr) and tungsten (W) present in the particles is at least 0.5 at%, at least 1 at%, at least 2 at%, at least 3 at%, at least 4 at%, at least 5 at%, at least 8 at%, at least 10 at%, at least 12 at%, at least 14 at%, or more. Combinations of these ranges are also possible. For example, in some embodiments, the combined amount of chromium (Cr) and tungsten (W) present in the particles is between 0.5 at% and 50 at% of the particle. In some of these embodiments, the remainder of the particles, at least 90 at% (or at least 95 at%, at least 98 at%, at least 99 at%, or at least 99.9 at%), is made from molybdenum.

[0049] In some embodiments, the particles comprise Mo, Cr, and W. In some embodiments, Mo is present in the particles in an amount of at least 50 at% (e.g., 50 at% to 99 at%), Cr is present in the particles in an amount of 0.5 at% to 30 at%, and W is present in the particles in an amount of 0.5 at% to 30 at%. In some embodiments, W is present in the particles in an amount of 20 at% to 30 at%, Cr is present in the particles in an amount of 10 at% to 20 at%, and the remaining at least 90 at% (or at least 95 at%, at least 98 at%, at least 99 at%, or at least 99.9 at%) of the particles is Mo. In some embodiments, Mo is present in the particles in an amount of 50 at% to 70 at%, W is present in the particles in an amount of 20 at% to 30 at%, and Cr is present in the particles in an amount of 10 at% to 20 at%.

[0050] The particles to be sintered may be nanocrystalline particles, according to some embodiments. The nanocrystalline particles may include grains with a particle size of less than or equal to 1,000 nm, less than or equal to 900 nm, less than or equal to 800 nm, less than or equal to 700 nm, less than or equal to 600 nm, less than or equal to 500 nm, less than or equal to 400 nm, less than or equal to 300 nm, less than or equal to 200 nm, less than or equal to 150 nm, less than or equal to 125 nm, less than or equal to 100 nm, less than or equal to 50 nm, less than or equal to 40 nm, less than or equal to 30 nm, less than or equal to 20 nm, or less than or equal to 10 nm. According to some embodiments, at least a portion of the nanocrystalline particles have a particle size of less than or equal to 10 nm. In some embodiments, at least a portion of the nanocrystalline particles have a particle size greater than or equal to 5 nm and less than or equal to 25 nm, hi some embodiments, at least a portion of the nanocrystalline particles have a particle size greater than or equal to 10 nm and less than or equal to 20 nm.

[0051] In some embodiments, at least a portion of the nanocrystalline particles comprise Mo, a second element (e.g., a second metal such as chromium), and / or a third element (e.g., a third metal such as tungsten). In some embodiments, some of the nanocrystalline particles are comprised of Mo, while another portion of the nanocrystalline particles are comprised of the second element, and another portion of the nanocrystalline particles are comprised of the third element. In some embodiments, at least a portion of the nanocrystalline particles comprise both Mo and the second element. In some embodiments, at least a portion of the nanocrystalline particles comprise both Mo and a third element. In some embodiments, at least a portion of the nanocrystalline particles comprise Mo, the second element, and the third element.

[0052] In some embodiments, Mo is the most abundant element, by atomic percentage, in at least some of the nanocrystalline particles. In some embodiments, Mo is the most abundant metal, by atomic percentage, in at least some of the nanocrystalline particles. In some embodiments, Mo is the most abundant metal element, by atomic percentage, in at least some of the nanocrystalline particles. In some embodiments, at least some of the particles contain Mo in an amount of at least 50 at%, at least 55 at%, at least 60 at%, at least 70 at%, at least 80 at%, at least 90 at%, or at least 95 at%. In some embodiments, at least some of the particles contain Mo in an amount up to 96 at%, up to 97 at%, up to 98 at%, or more. Combinations of these ranges are also possible. Other values ​​are also possible.

[0053] According to some embodiments, at least some of the particles are formed by mechanically processing a powder containing Mo and a second element. For example, some embodiments include creating the particles at least in part by mechanically processing a powder containing a plurality of Mo particles and a plurality of second element particles (e.g., particles containing Cr). Some embodiments include creating the particles at least in part by mechanically processing particles containing both Mo and the second element.

[0054] According to some embodiments, at least some of the particles are formed by mechanically processing a powder including Mo, a second element (e.g., chromium), and a third element (e.g., tungsten). For example, some embodiments include creating particles (e.g., nanocrystalline particles) at least in part by mechanically processing a powder including a plurality of Mo particles, a plurality of second element particles (e.g., particles including Cr), and a plurality of third element particles (e.g., particles including W). Some embodiments include creating particles (e.g., nanocrystalline particles) at least in part by mechanically processing particles including both Mo and the second element, both Mo and the third element, both the second element and the third element, and / or all of Mo, the second element, and the third element.

[0055] In embodiments utilizing mechanical processing, any suitable method of mechanical processing may be employed to mechanically process the powder and form particles. According to certain embodiments, at least a portion of the particles are formed by ball milling a powder containing Mo and a second element (and / or, when present, a third element). The ball milling process may be, for example, a high-energy ball milling process. In a non-limiting exemplary ball milling process, tungsten carbide or steel milling vials may be employed, with a ball-to-powder ratio of 2:1 to 20:1 (e.g., 5:1 to 12:1, such as 10:1) and an ethanol process control agent content of 0.01 to 3 mg / g of powder. According to certain other embodiments, the mechanical processing is carried out in the absence of a process control agent. Other types of mechanical processing, including, but not limited to, shaker milling and planetary milling, may also be employed. In some embodiments, the mechanical processing (e.g., via ball milling or another process) may be carried out under conditions sufficient to produce particles containing a supersaturated phase (e.g., nanocrystalline particles). The supersaturated phase is described in more detail below.

[0056] According to certain embodiments, mechanical processing (e.g., ball milling) is performed at a relatively low temperature. For example, in some embodiments, mechanical processing (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, mechanical processing (e.g., ball milling) is performed while the particles are at a temperature of at least 0°C. In some embodiments, mechanical processing (e.g., ball milling) is performed at the temperature of the surrounding ambient environment.

[0057] In certain embodiments, mechanical processing (e.g., ball milling) may be performed for a time 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, mechanical processing (e.g., ball milling) may be performed for a time less than or equal to 18 hours. In some embodiments, mechanical processing (e.g., ball milling) may be performed for a time between 6 and 18 hours. In some cases, if the mechanical processing time is too long, Mo and / or the second element (and / or the third element, if present) may be contaminated by the material used to perform the mechanical processing (e.g., the milling vial material). The amount of the second element (and / or the third element, if present) dissolved in Mo may increase with increasing mechanical processing (e.g., milling) time, in some cases. In some embodiments, after the mechanical processing step (e.g., ball milling step), a phase rich in the second element material may be present.

[0058] According to certain embodiments, Mo and the second element (and / or the third element, if present) are present in the particles in a non-equilibrium phase. According to certain embodiments, the particles may include a non-equilibrium phase in which the second element (and / or the third element, if present) is dissolved in Mo. In some embodiments, the non-equilibrium phase includes a solid solution. According to some embodiments, the non-equilibrium phase may be a supersaturated phase including the second element (and / or the third element, if present) dissolved in Mo. As used herein, a "supersaturated phase" refers to a phase in which a substance is dissolved in another substance in an amount that exceeds its solubility limit. In some embodiments, the supersaturated phase may include an activator element and / or a stabilizer element that are forcibly dissolved in Mo in an amount that exceeds the amount of the activator element that would otherwise be dissolved in the equilibrium phase of Mo. For example, in one set of embodiments, the supersaturated phase is a phase including an activator element that is forcibly dissolved in Mo in an amount that exceeds the amount of the activator element that would otherwise be dissolved in the equilibrium Mo phase.

[0059] In some embodiments, the supersaturated phase may be the only phase present after the mechanical processing (eg, ball milling) process.

[0060] According to certain embodiments, the non-equilibrium phase may undergo decomposition during sintering of the particles (which sintering is described in more detail below). Sintering of the particles may cause the formation of a third element-rich phase at at least one of the particle's surface and / or grain boundaries. In some such embodiments, Mo is soluble in the second and / or third element-rich phase. The formation of the second and / or third element-rich phase may be the result of decomposition of the non-equilibrium phase during sintering. The second and / or third element-rich phase, according to certain embodiments, may act as a fast diffusion path for Mo, improving the sintering rate and accelerating the rate of sintering of the particles. According to some embodiments, decomposition of the non-equilibrium phase during sintering of the particles accelerates the rate of sintering of the particles.

[0061] Some, but not all, embodiments include cold pressing the plurality of particles for at least a portion of the time prior to sintering. According to some embodiments, it has been discovered that metal alloys including Mo and a second element (e.g., Mo and Cr) and / or metal alloys including Mo, a second element, and a third element (e.g., Mo, Cr, and W) can be compressed so that high relative densities are achieved without the need for simultaneous heating. In some embodiments, cold pressing includes compressing the plurality of particles at a force greater than or equal to 300 MPa, greater than or equal to 400 MPa, greater than or equal to 500 MPa, greater than or equal to 750 MPa, greater than or equal to 1,000 MPa, or higher. In some embodiments, cold pressing includes compressing the plurality of particles at a force of up to or greater than 1,400 MPa. Combinations of these ranges are also possible (e.g., greater than or equal to 300 MPa and less than or equal to 1,400 MPa). Other ranges are also possible.

[0062] According to some embodiments, cold pressing is carried out at a relatively low temperature. For example, in some embodiments, cold pressing is carried out 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, cold pressing is carried out at the temperature of the surrounding ambient environment.

[0063] As noted above, certain embodiments involve sintering a plurality of particles to form a metal alloy. Those skilled in the art are familiar with the process of sintering, which involves applying heat to materials (e.g., particles) that are to be sintered so that the materials become a single solid mass.

[0064] According to certain embodiments, sintering can be carried out when the metal particles are at relatively low temperatures and / or for relatively short time periods while maintaining the ability to form metal alloys having high relative density, small grain size, and / or equiaxed grains.

[0065] According to certain embodiments, sintering the plurality of particles involves heating the particles to a sintering temperature of less than or equal to 2,200°C, less than or equal to 2,000°C, less than or equal to 1,900°C, less than or equal to 1,800°C, less than or equal to 1,700°C, less than or equal to 1,600°C, less than or equal to 1,500°C, less than or equal to 1,400°C, less than or equal to 1,300°C, less than or equal to 1,200°C, less than or equal to 1,100°C, less than or equal to 1,000°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 greater than or equal to 750°C, greater than or equal to 850°C, greater than or equal to 1,000°C, greater than or equal to 1,200°C, greater than or equal to 1,450°C, or greater than or equal to 1,600°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 greater than or equal to 750°C and less than or equal to 2,200°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.

[0066] According to certain embodiments, sintering the plurality of particles involves maintaining the particles within a sintering temperature range 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 greater than or equal to 600° C. and less than or equal to 1,100° C. for a sintering duration greater than or equal to 6 hours and less than or equal to 24 hours.

[0067] According to some embodiments, sintering involves heating the particles to a first sintering temperature that is lower than the second sintering temperature required to sinter Mo in the absence of the second element. To determine whether such a condition is met, one skilled in the art would compare the temperature required to achieve sintering in a sample containing Mo and the second element with the temperature required to achieve sintering in a sample containing Mo without the second element but that is otherwise the same as the sample containing Mo and the second element. In some embodiments, the first sintering temperature can be at least 25°C, at least 50°C, at least 100°C, or at least 200°C lower than the second sintering temperature.

[0068] According to certain embodiments, a non-equilibrium phase present in the particle (e.g., any of the non-equilibrium phases described above or elsewhere herein) undergoes decomposition during sintering. In some such embodiments, the decomposition of the non-equilibrium phase accelerates the rate of sintering of the particle.

[0069] In some embodiments, sintering further includes forming a second phase at at least one of the surfaces and grain boundaries of the particles during sintering. In some such embodiments, the second phase is enriched in the second element. The term "enriched," with respect to the content of an element in a phase, refers to a content of the element in the phase of at least 50 at% (e.g., at least 60 at%, at least 70 at%, at least 80 at%, at least 90 at%, at least 99 at%, or higher). The term "phase" is generally used herein to refer to a state of matter. For example, a phase can refer to a phase shown on a phase diagram. Generally, when multiple phases are present, they are distinguishable from one another even when both are solid phases.

[0070] Sintering may be carried out in a variety of suitable environments. In some embodiments, the particles are in an inert atmosphere during the sintering process. The use of an inert atmosphere can be useful, for example, when reactive metals are employed in the particles. For example, Mo and Cr react (separately and / or together) with oxygen.

[0071] In some embodiments, sintering is carried out in an atmosphere where at least 90 vol.%, at least 95 vol.%, at least 99 vol.%, or substantially all of the atmosphere is composed of an inert gas, which can be or include, for example, helium, argon, xenon, neon, krypton, a combination of two or more of these, or other inert gases.

[0072] In certain embodiments, an oxygen scavenger (e.g., a getter) may be included in the sintering environment. The use of an oxygen scavenger can reduce the extent to which the metal is oxidized during the sintering process, which can be advantageous, according to certain embodiments. In some embodiments, the sintering environment can be controlled so that oxygen is present in amounts less than 1 vol.%, less than 0.1 vol.%, less than 100 parts per million (ppm), less than 10 ppm, or less than 1 ppm.

[0073] In some embodiments, sintering is carried out in an atmosphere containing a gas that would react with oxygen gas (i.e., O) when exposed to oxygen gas under sintering conditions. In some embodiments, sintering is carried out in an atmosphere including hydrogen gas (H). In some embodiments, the combination of hydrogen gas and an inert gas constitutes at least 90 vol.%, at least 95 vol.%, at least 99 vol.%, or substantially all of the atmosphere in which sintering is carried out. In some embodiments, the combination of hydrogen gas and argon gas constitutes at least 90 vol.%, at least 95 vol.%, at least 99 vol.%, or substantially all of the atmosphere in which sintering is carried out.

[0074] According to certain embodiments, sintering occurs essentially without externally applied stress. For example, in some embodiments, the maximum external pressure applied to the nanocrystalline particles for at least 20%, at least 50%, at least 75%, at least 90%, or at least 98% of the time during which sintering is performed is less than or equal to 2 MPa, less than or equal to 1 MPa, less than or equal to 0.5 MPa, or less than or equal to 0.1 MPa. The maximum external pressure applied to the nanocrystalline particles refers to the maximum pressure applied as a result of the application of a force external to the nanocrystalline particles and excludes pressure caused by gravity that arises between the nanocrystalline particles and the surface on which they are positioned during the sintering process. Some of the sintering processes described herein can enable the production of relatively high-density sintered ultrafine and nanocrystalline materials even in the absence or substantial absence of external pressure applied during the sintering process. According to certain embodiments, sintering can be a pressureless sintering process.

[0075] According to some embodiments, at least one activator element may be present during the sintering process. The activator element may increase the sintering rate of Mo. According to some embodiments, the activator element may provide a high diffusion path for Mo atoms. For example, in some embodiments, the activator element atoms may surround Mo atoms, providing a relatively high transport diffusion path for Mo atoms, thereby reducing the activation energy for Mo diffusion. In some embodiments, this technique is referred to as activated sintering. In some embodiments, the activator element may reduce the temperature required to sinter the nanocrystalline particles relative to the temperature that would be required to sinter the nanocrystalline particles in the absence of the activator element but under otherwise the same conditions. Thus, according to some embodiments, sintering may involve a first sintering temperature, which may be lower than a second sintering temperature required to sinter Mo in the absence of a third element. To determine the sintering temperature required to sinter Mo in the absence of a third element, a sample of Mo material that does not contain the third element but is otherwise identical to the nanocrystalline particle material would be prepared. The minimum temperature required to sinter a sample without the third element would then be determined. In some embodiments, the presence of the second element reduces the sintering temperature by at least 25°C, at least 50°C, at least 100°C, at least 200°C, or more.

[0076] According to some embodiments, at least one stabilizer element may be present during the sintering process. The stabilizer element may be any element capable of reducing the amount of grain growth that occurs relative to the amount that would occur in the absence of the stabilizer element but under otherwise the same conditions. In some embodiments, the stabilizer element reduces grain growth by reducing the grain boundary energy of the material being sintered and / or by reducing the driving force for grain growth. According to some embodiments, the stabilizer element may exhibit a positive heat of mixing with the material being sintered. The stabilizer element may stabilize nanocrystalline Mo by segregating at the grain boundaries. This segregation may reduce the grain boundary energy and / or reduce the driving force for grain growth in the alloy.

[0077] In some embodiments, the stabilizer element may also be an activator element. The use of a single element as both a stabilizer element and an activator element, according to certain embodiments, has the added benefit of eliminating the need to consider interactions between the activator and the stabilizer. In some embodiments, the element that may be utilized as both an activator element and a stabilizer element may be a metal element, which may be any of the metal elements described above.

[0078] According to certain embodiments, when one element cannot act as both a stabilizer and an activator, two elements may be employed. The interaction between the two elements may be considered, according to some embodiments, to ensure that the roles of activator and stabilizer are properly fulfilled. For example, when the activator and stabilizer form intermetallic compounds, the elements may, in some cases, be prevented from fulfilling their designated roles. As a result, combinations of activators and stabilizers with the ability to form intermetallic compounds at the expected sintering temperature should be avoided, at least in some cases. The potential for the formation of intermetallic compounds between two elements may be analyzed using a phase diagram.

[0079] According to one set of embodiments, molybdenum and chromium particles (e.g., 10, 20, or 30 at% Cr, remainder molybdenum) can be mechanically alloyed via ball milling, cold pressed, and subsequently annealed (e.g., in a thermomechanical analyzer for several hours). In some embodiments, the Mo-Cr alloy system exhibits nanocrystalline grain size stabilization by Cr segregation to Mo grain boundaries and by the formation of Cr-rich precipitates that pin the grain boundaries and further prevent grain growth.

[0080] According to certain embodiments, powders of the elements Mo, Cr, and W are mixed and milled to achieve supersaturation and particle size reduction to the nanometer scale. In some embodiments, annealing of the compacted powder leads to the development of a nanoduplex structure consisting of Mo-rich grains and Cr-rich precipitates.

[0081] As noted above, certain embodiments relate to inventive metal alloys, which, according to certain embodiments, comprise molybdenum and at least one other metal.

[0082] According to certain embodiments, the metal alloy includes a relatively large amount of molybdenum (Mo). For example, in some embodiments, Mo is the most abundant element (e.g., the most abundant metal) in the metal alloy by atomic percentage. According to certain embodiments, Mo is present in the metal alloy in an amount of at least 50 at%, at least 55 at%, at least 60 at%, at least 65 at%, at least 70 at%, at least 80 at%, at least 90 at%, or at least 95 at%. In some embodiments, Mo is present in the metal alloy in an amount up to 96 at%, up to 97 at%, up to 98 at%, up to 99 at%, up to 99.5 at%, or more. Combinations of these ranges are also possible. Other values ​​are also possible.

[0083] The metal alloys described herein can include a second element, for example, the metal alloys described herein can include a second metal.

[0084] According to certain embodiments, the second element is selected from the group consisting of chromium (Cr) and palladium (Pd). In some embodiments, both Cr and Pd are present (e.g., when the alloy includes at least three elements). In other embodiments, only one of Cr and Pd is present. In some embodiments, the second element is Cr.

[0085] In some embodiments, Mo is at least partially soluble in the second element, e.g., in some embodiments, Mo and the second element are in solid solution.

[0086] The second element (e.g., chromium, palladium) may be present in the metal alloy in various suitable percentages. According to certain embodiments, the second element is present in the metal alloy in an amount of less than or equal to 40 at%; less than or equal to 35 at%; less than or equal to 32 at%; less than or equal to 30 at%; less than or equal to 25 at%; less than or equal to 22 at%; less than or equal to 20 at%; less than or equal to 18 at%; or less than or equal to 16 at%. In some embodiments, the second element is present in the metal alloy in an amount of at least 0.5 at%; at least 1 at%; at least 2 at%; at least 3 at%; at least 4 at%; at least 5 at%; at least 6 at%; at least 7 at%; at least 8 at%; at least 9 at%; at least 10 at%; or more. Combinations of these ranges are also possible. For example, in some embodiments, the second element is present in the metal alloy in an amount of 0.5 at% to 40 at% of the metal alloy. In some embodiments, the second element is present in the metal alloy in an amount between 1 at% and 40 at% of the metal alloy. In some embodiments, the second element is present in the metal alloy in an amount between 8 at% and 32 at% of the metal alloy. Other values ​​are also possible.

[0087] In some embodiments, a metal alloy (e.g., containing molybdenum, a second metal, and an optional third or additional metal) may contain a relatively large amount of metallic material. In some embodiments, at least 50 at%, at least 70 at%, at least 90 at%, at least 95 at%, at least 99 at%, at least 99.9 at%, or more of the metal alloy is composed of metal atoms in their metallic form (i.e., in the zero oxidation state). In some embodiments, at least 50 at%, at least 70 at%, at least 90 at%, at least 95 at%, at least 99 at%, at least 99.9 at%, or more of the molybdenum atoms in the metal alloy are in their metallic form. In some embodiments, at least 50 at%, at least 70 at%, at least 90 at%, at least 95 at%, at least 99 at%, at least 99.9 at%, or more of the second element (e.g., second metal) atoms in the metal alloy are in their metallic form. In some embodiments, at least 50 at%, at least 70 at%, at least 90 at%, at least 95 at%, at least 99 at%, at least 99.9 at%, or more of the atoms of the third element (e.g., third metal) in the metal alloy are in their metallic form. In some embodiments, molybdenum atoms can form metallic bonds with other molybdenum atoms and / or other neighboring atoms, such as atoms of the second element (e.g., second metal) and / or third element (e.g., third metal).

[0088] In some embodiments, the metal alloy includes only Mo and a second element (i.e., Mo and a second element without additional metals or other elements). In other embodiments, the metal alloy includes Mo, a second element, and a third element. For example, in some embodiments, the metal alloy includes a third element (in addition to Mo and the second element). The third element, in some embodiments, can be a metal element. In some embodiments, the metal alloy includes a third metal, in which case the alloy includes Mo, the second metal, and the third metal.

[0089] According to certain embodiments, the third element is selected from the group consisting of tungsten (W) and tantalum (Ta). In some embodiments, the third element is W.

[0090] The third element (e.g., tungsten) may be present in the metal alloy in various suitable percentages. According to certain embodiments, the third element is present in the metal alloy in an amount less than or equal to 40 at%; less than or equal to 35 at%; less than or equal to 30 at%; less than or equal to 28 at%; or less than or equal to 26 at%. In some embodiments, the third element is present in the metal alloy in an amount of at least 0.5 at%; at least 1 at%; at least 2 at%; at least 3 at%; at least 4 at%; at least 5 at%; at least 6 at%; at least 7 at%; at least 8 at%; at least 9 at%; at least 10 at% or more. Combinations of these ranges are also possible. Other values ​​are also possible.

[0091] According to certain embodiments, the combined amount of all non-Mo metal elements (e.g., the second element, the third element, and any additional optional elements) in the metal alloy constitutes less than 50 at.%, less than or equal to 40 at.%, less than or equal to 35 at.%, less than or equal to 32 at.%, less than or equal to 30 at.%, less than or equal to 25 at.%, less than or equal to 22 at.%, less than or equal to 20 at.%, less than or equal to 18 at.%, or less than or equal to 16 at.% of the metal alloy. In some embodiments, the combined amount of all non-Mo elements (e.g., the second element, the optional third element, and any additional optional elements) in the metal alloy constitutes at least 0.5 at.%, at least 1 at.%, at least 2 at.%, at least 3 at.%, at least 4 at.%, at least 5 at.%, at least 8 at.%, at least 10 at.%, at least 12 at.%, at least 14 at.%, or more. Combinations of these ranges are also possible. Other values ​​are also possible.

[0092] In some embodiments, the combined amount of chromium (Cr), palladium (Pd), tungsten (W), and tantalum (Ta) present in the metal alloy is less than 50 at%, less than or equal to 40 at%, less than or equal to 35 at%, less than or equal to 32 at%, less than or equal to 30 at%, less than or equal to 25 at%, less than or equal to 22 at%, less than or equal to 20 at%, less than or equal to 18 at%, or less than or equal to 16 at% of the metal alloy. In some embodiments, the combined amount of chromium (Cr), palladium (Pd), tungsten (W), and tantalum (Ta) present in the metal alloy is at least 0.5 at%, at least 1 at%, at least 2 at%, at least 3 at%, at least 4 at%, at least 5 at%, at least 8 at%, at least 10 at%, at least 12 at%, at least 14 at%, or more. Combinations of these ranges are also possible. For example, in some embodiments, the combined amount of chromium (Cr), palladium (Pd), tungsten (W), and tantalum (Ta) present in the metal alloy is between 0.5 at% and 50 at% of the metal alloy, and in some of these embodiments, the remaining at least 90 at% (or at least 95 at%, at least 98 at%, at least 99 at%, or at least 99.9 at%) of the metal alloy is molybdenum.

[0093] Those skilled in the art will understand that to determine the total amount of chromium (Cr), palladium (Pd), tungsten (W), and tantalum (Ta) present in a given metal alloy, one would sum the atomic percentages of each of these elements. For example, if a metal alloy contains 60 at% Mo, 15 at% Cr, and 25 at% W, the total amount of chromium (Cr), palladium (Pd), tungsten (W), and tantalum (Ta) present would be 40 at% (i.e., 15 at% from Cr, 25 at% from W, and 0 at% with respect to all other elements in the list). Those skilled in the art will also understand that when performing this calculation, not all of the elements in the above list will necessarily be present in the metal alloy. In the exemplary calculation described above, for example, palladium and tantalum are not present in the Mo-W-Cr alloy.

[0094] In some embodiments, the combined amount of chromium (Cr) and tungsten (W) present in the metal alloy is less than 50 at% of the metal alloy, less than or equal to 40 at%, less than or equal to 35 at%, less than or equal to 32 at%, less than or equal to 30 at%, less than or equal to 25 at%, less than or equal to 22 at%, less than or equal to 20 at%, less than or equal to 18 at%, or less than or equal to 16 at%. In some embodiments, the combined amount of chromium (Cr) and tungsten (W) present in the metal alloy is at least 0.5 at%, at least 1 at%, at least 2 at%, at least 3 at%, at least 4 at%, at least 5 at%, at least 8 at%, at least 10 at%, at least 12 at%, at least 14 at%, or more. Combinations of these ranges are also possible. For example, in some embodiments, the combined amount of chromium (Cr) and tungsten (W) present in the metal alloy is between 0.5 at% and 50 at% of the metal alloy. In some of these embodiments, the remaining at least 90 at% (or at least 95 at%, at least 98 at%, at least 99 at%, or at least 99.9 at%) of the metal alloy is molybdenum.

[0095] In some embodiments, the metal alloy comprises Mo, Cr, and W. In some embodiments, Mo is present in the metal alloy in an amount of at least 50 at% (e.g., 50 at% to 99 at%), Cr is present in the metal alloy in an amount of 0.5 at% to 30 at%, and W is present in the metal alloy in an amount of 0.5 at% to 30 at%. In some embodiments, W is present in the metal alloy in an amount of 20 at% to 30 at%, Cr is present in the metal alloy in an amount of 10 at% to 20 at%, and the remaining at least 90 at% (or at least 95 at%, at least 98 at%, at least 99 at%, or at least 99.9 at%) of the metal alloy is Mo. In some embodiments, Mo is present in the metal alloy in an amount of 50 at% to 70 at%, W is present in the metal alloy in an amount of 20 at% to 30 at%, and Cr is present in the metal alloy in an amount of 10 at% to 20 at%.

[0096] The molybdenum-containing metal alloy, according to certain embodiments, is a nanocrystalline metal alloy. Nanocrystalline metals have certain advantages over their microcrystalline counterparts due to the large volume fraction of grain boundaries. As an example, nanocrystalline alloys generally have significantly higher tensile strength.

[0097] Nanocrystalline materials generally refer to materials containing at least some grains with a grain size of less than or equal to 1,000 nm. In some embodiments, nanocrystalline materials contain grains with a grain size of less than or equal to 900 nm, less than or equal to 800 nm, less than or equal to 700 nm, less than or equal to 600 nm, less than or equal to 500 nm, less than or equal to 400 nm, less than or equal to 300 nm, less than or equal to 200 nm, less than or equal to 100 nm, less than or equal to 50 nm, less than or equal to 20 nm, less than or equal to 10 nm, or less than or equal to 5 nm. In some embodiments, nanocrystalline materials contain grains with a grain size of at least 1 nm or at least 5 nm. Thus, in the case of metal alloys, nanocrystalline metal alloys are metal alloys containing grains with a grain size of less than or equal to 1,000 nm. In some embodiments, the nanocrystalline metal alloy comprises grains with a size of less than or equal to 900 nm, less than or equal to 800 nm, less than or equal to 700 nm, less than or equal to 600 nm, less than or equal to 500 nm, less than or equal to 400 nm, less than or equal to 300 nm, less than or equal to 200 nm, less than or equal to 150 nm, less than or equal to 125 nm, less than or equal to 100 nm, less than or equal to 50 nm, less than or equal to 20 nm, or less than or equal to 10 nm. In some embodiments, the nanocrystalline metal alloy comprises grains with a size of at least 1 nm, at least 2 nm, or at least 5 nm. Other values ​​are also possible.

[0098] The "particle size" of a grain generally refers to the grain's largest dimension. The largest dimension may be the grain's diameter, length, width, or height, depending on its geometric shape. According to certain embodiments, the grains may be spherical, cubic, conical, cylindrical, needle-like, or any other suitable geometric shape.

[0099] According to one embodiment, a relatively large percentage of the volume of the metal alloy is made up of small grains. For example, in some embodiments, at least 50%, at least 75%, at least 90%, at least 95%, at least 99%, or substantially all of the volume of the metal alloy is composed of grains having a grain size of less than or equal to 1,000 nm, less than or equal to 900 nm, less than or equal to 800 nm, less than or equal to 700 nm, less than or equal to 600 nm, less than or equal to 500 nm, less than or equal to 400 nm, less than or equal to 300 nm, less than or equal to 200 nm, less than or equal to 150 nm, less than or equal to 125 nm, less than or equal to 100 nm, less than or equal to 50 nm, less than or equal to 20 nm, or less than or equal to 10 nm (and / or, in some embodiments, as small as 5 nm, as small as 2 nm, or as small as 1 nm). Other values ​​are also possible.

[0100] According to some embodiments, the metal alloy may have a relatively small average grain size. The "average grain size" of a material (e.g., a metal alloy) refers to the number average grain size of the grains in the material. According to some embodiments, the metal alloy (e.g., bulk and / or nanocrystalline metal alloy) has an average grain size of less than or equal to 1,000 nm, less than or equal to 900 nm, less than or equal to 800 nm, less than or equal to 700 nm, less than or equal to 600 nm, less than or equal to 500 nm, less than or equal to 400 nm, less than or equal to 300 nm, less than or equal to 200 nm, less than or equal to 150 nm, less than or equal to 125 nm, less than or equal to 100 nm, less than or equal to 50 nm, less than or equal to 20 nm, or less than or equal to 10 nm. In some embodiments, the metal alloy has an average grain size as small as 25 nm, as small as 10 nm, as small as 5 nm, as small as 2 nm, as small as 1 nm, or smaller. Combinations of these ranges are also possible. Other values ​​are also possible.

[0101] According to an embodiment, at least one cross-section of the metal alloy intersecting the geometric center of the metal alloy has a small volume average cross-sectional grain size. The "volume average cross-sectional grain size" of a given cross-section of the metal alloy can be determined by taking a cross-section of the object, tracing the perimeter of each grain in an image of the cross-section of the object (which can be a magnified image such as an image obtained from a transmission electron microscope), and finding the circle equivalent diameter D of each traced grain cross-section. i The "equivalent circle diameter" of a grain cross section is determined by calculating the area (A = πr 2 The volume average cross-sectional diameter (G CS,avg ) is calculated as follows:

number

[0102] According to certain embodiments, at least one cross section of the metal alloy intersecting the geometric center of the metal alloy has a volume average cross-sectional grain size that is less than or equal to 1,000 nm, less than or equal to 900 nm, less than or equal to 800 nm, less than or equal to 700 nm, less than or equal to 600 nm, less than or equal to 500 nm, less than or equal to 400 nm, less than or equal to 300 nm, less than or equal to 200 nm, less than or equal to 150 nm, less than or equal to 125 nm, less than or equal to 100 nm, less than or equal to 50 nm, less than or equal to 20 nm, or less than or equal to 10 nm. In some embodiments, at least one cross-section of the metal alloy intersecting the geometric center of the metal alloy has a volume average cross-sectional grain size as small as 25 nm, as small as 10 nm, as small as 5 nm, as small as 2 nm, as small as 1 nm, or smaller. Combinations of these ranges are also possible. Other values ​​are also possible.

[0103] According to an embodiment, at least one cross-section of the metal alloy (optionally intersecting a geometric center of the metal alloy) has a volume average cross-sectional grain size of less than or equal to 1,000 nm, less than or equal to 900 nm, less than or equal to 800 nm, less than or equal to 700 nm, less than or equal to 600 nm, less than or equal to 500 nm, less than or equal to 400 nm, less than or equal to 300 nm, less than or equal to 200 nm, less than or equal to 150 nm, less than or equal to 125 nm, less than or equal to 100 nm, less than or equal to 50 nm, less than or equal to 20 nm, less than or equal to 10 nm (and / or as small as 25 nm, as small as 10 nm, as small as 5 nm, as small as 2 nm, as small as 1 nm, or smaller); At least a second cross-section of the metal alloy that is orthogonal to the first cross-section (optionally intersecting the geometric center of the metal alloy) has a volume average cross-sectional grain size that is less than or equal to 1,000 nm, less than or equal to 900 nm, less than or equal to 800 nm, less than or equal to 700 nm, less than or equal to 600 nm, less than or equal to 500 nm, less than or equal to 400 nm, less than or equal to 300 nm, less than or equal to 200 nm, less than or equal to 150 nm, less than or equal to 125 nm, less than or equal to 100 nm, less than or equal to 50 nm, less than or equal to 20 nm, less than or equal to 10 nm (and / or as small as 25 nm, as small as 10 nm, as small as 5 nm, as small as 2 nm, as small as 1 nm, or smaller). Other values ​​are also possible.

[0104] According to an embodiment, at least one cross section of the metal alloy (optionally intersecting the geometric center of the metal alloy) is less than or equal to 1,000 nm, less than or equal to 900 nm, less than or equal to 800 nm, less than or equal to 700 nm, less than or equal to 600 nm, less than or equal to 500 nm, less than or equal to 400 nm, less than or equal to 300 nm, less than or equal to 200 nm, less than or equal to 150 nm, and wherein at least a second cross-section of the metal alloy perpendicular to the first cross-section (optionally intersecting a geometric center of the metal alloy or otherwise) has a volume-average cross-sectional grain size of 1,and a volume average cross-sectional grain size of less than or equal to 1000 nm, less than or equal to 900 nm, less than or equal to 800 nm, less than or equal to 700 nm, less than or equal to 600 nm, less than or equal to 500 nm, less than or equal to 400 nm, less than or equal to 300 nm, less than or equal to 200 nm, less than or equal to 150 nm, less than or equal to 125 nm, less than or equal to 100 nm, less than or equal to 50 nm, less than or equal to 20 nm, or less than or equal to 10 nm (and / or as small as 25 nm, as small as 10 nm, as small as 5 nm, as small as 2 nm, as small as 1 nm, or smaller), perpendicular to the first cross-section and perpendicular to the second cross-section (optionally, depending on the geometry of the metal alloy). At least a third cross-section of the metal alloy (as intersected by the geometric center) has a volume average cross-sectional grain size of less than or equal to 1,000 nm, less than or equal to 900 nm, less than or equal to 800 nm, less than or equal to 700 nm, less than or equal to 600 nm, less than or equal to 500 nm, less than or equal to 400 nm, less than or equal to 300 nm, less than or equal to 200 nm, less than or equal to 150 nm, less than or equal to 125 nm, less than or equal to 100 nm, less than or equal to 50 nm, less than or equal to 20 nm, or less than or equal to 10 nm (and / or as small as 25 nm, as small as 10 nm, as small as 5 nm, as small as 2 nm, as small as 1 nm, or smaller).

[0105] In some embodiments, the metal alloy comprises grains that are relatively equiaxed. In certain embodiments, at least a portion of the grains in the metal alloy have an aspect ratio of less than or equal to 2, less than or equal to 1.8, less than or equal to 1.6, less than or equal to 1.4, less than or equal to 1.3, less than or equal to 1.2, or less than or equal to 1.1 (and in some embodiments, up to 1). The aspect ratio of a grain is calculated by dividing the largest cross-sectional dimension of the grain intersecting the geometric center of the grain by the largest dimension of the grain that is perpendicular to the largest cross-sectional dimension of the grain. The aspect ratio of a grain is expressed as a single number, with 1 corresponding to an equiaxed grain. In some embodiments, the number average aspect ratio of the grains in the metal alloy is less than or equal to 2, less than or equal to 1.8, less than or equal to 1.6, less than or equal to 1.4, less than or equal to 1.3, less than or equal to 1.2, or less than or equal to 1.1 (and in some embodiments, up to 1).

[0106] Without wishing to be bound by any particular theory, it is believed that relatively equiaxed grains may be present when a metal alloy is produced in the absence (or substantial absence) of applied pressure (e.g., via a pressureless or substantially pressureless sintering process).

[0107] In certain embodiments, the metal alloy has a relatively low cross-sectional average grain aspect ratio. In some embodiments, the cross-sectional average grain aspect ratio in the metal alloy is less than or equal to 2, less than or equal to 1.8, less than or equal to 1.6, less than or equal to 1.4, less than or equal to 1.3, less than or equal to 1.2, or less than or equal to 1.1 (and in some embodiments, up to 1). The "cross-sectional average grain aspect ratio" of a metal alloy is considered to fall within a particular range if at least one cross-section of the metal alloy intersecting the geometric center of the metal alloy is composed of grain cross sections with an average aspect ratio that falls within the range. For example, the cross-sectional average grain aspect ratio of a metal alloy would be less than 2 if the metal alloy includes at least one cross-section intersecting the geometric center of the metal alloy, the cross-section being composed of grain cross sections with an average aspect ratio of less than 2. To determine the average aspect ratio of the grain cross sections from which a cross section of a metal alloy is constructed (also referred to herein as "average aspect ratio of grain cross sections"), a cross section of the metal alloy is obtained, the perimeter of each grain in an image of the cross section of the metal alloy (which may be a magnified image such as an image obtained from a transmission electron microscope) is traced, and the aspect ratio of each traced grain cross section is calculated. The aspect ratio of a grain cross section is calculated by dividing the maximum cross-sectional dimension of the grain cross section (which intersects the geometric center of the grain cross section) by the largest dimension of the grain cross section that is perpendicular to the maximum cross-sectional dimension of the grain cross section. The aspect ratio of a grain cross section is expressed as a single number, with 1 corresponding to an equiaxed grain cross section. The average aspect ratio (AR) of the grain cross sections from which a cross section of a metal alloy is constructed is avg ) is calculated as the number average of

number

[0108] According to certain embodiments, a metal alloy having a cross-sectional average grain aspect ratio that falls within a particular range (e.g., any of the ranges described elsewhere herein) has a first cross-section that intersects the geometric center of the metal alloy and has an average aspect ratio of grain cross-sections that falls within the range, and at least a second cross-section that is orthogonal to the first cross-section and intersects the geometric center of the metal alloy and has an average aspect ratio of grain cross-sections that falls within the range. For example, according to certain embodiments, a metal alloy having a cross-sectional average grain aspect ratio of less than 2 includes a cross-section that intersects the geometric center of the metal alloy and has an average aspect ratio of grain cross-sections that is less than 2, and at least a second cross-section that is orthogonal to the first cross-section and intersects the geometric center of the metal alloy and has an average aspect ratio of grain cross-sections that is less than 2.

[0109] According to certain embodiments, a metal alloy having a cross-sectional average grain aspect ratio that falls within a particular range (e.g., any of the ranges described elsewhere herein) has a first cross-section that intersects the geometric center of the metal alloy and has an average aspect ratio of the grain cross-section that falls within the range, a second cross-section that is orthogonal to the first cross-section, intersects the geometric center of the metal alloy and has an average aspect ratio of the grain cross-section that falls within the range, and at least a third cross-section that is orthogonal to the first and second cross-sections, intersects the geometric center of the metal alloy and has an average aspect ratio of the grain cross-section that falls within the range. For example, according to certain embodiments, a metal alloy having a cross-sectional average grain aspect ratio of less than 2 includes a first cross-section intersecting a geometric center of the metal alloy, the first cross-section having an average aspect ratio of the grain cross-sections of less than 2; a second cross-section orthogonal to the first cross-section, intersecting the geometric center of the metal alloy, and having an average aspect ratio of the grain cross-sections of less than 2; and at least a third cross-section orthogonal to the first and second cross-sections, intersecting the geometric center of the metal alloy, and having an average aspect ratio of the grain cross-sections of less than 2.

[0110] According to certain embodiments, grains within the metal alloy can be both relatively small and relatively equiaxed. For example, according to certain embodiments, at least one cross-section (and in some embodiments, at least a second cross-section orthogonal to the first cross-section and / or at least a third cross-section orthogonal to the first and second cross-sections) can have a volume-average cross-sectional grain size and an average aspect ratio of the grain cross-sections that fall within any of the ranges outlined above or elsewhere herein.

[0111] Some of the metal alloys described herein can have high relative densities. In some such embodiments, the metal alloys have high relative densities while maintaining their nanocrystalline character.

[0112] The term "relative density" refers to the ratio of the experimentally measured density of a metal alloy to the maximum theoretical density of the metal alloy. rel ) is expressed as a percentage and is calculated as follows:

number

[0113] In some embodiments, the metal alloy (e.g., sintered metal alloy, nanocrystalline metal alloy, and / or bulk metal alloy) has a relative density of at least 80%, at least 85%, at least 90%, at least 92%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% (and / or in some embodiments, up to 99.8%, up to 99.9%, or more). In some embodiments, the nanocrystalline alloy has a relative density of 100%. Other values ​​are also possible.

[0114] According to certain embodiments, the metal alloy is fully dense. As used herein, the term "fully dense" (or "full density") refers to a material with a relative density of at least 98%. According to certain embodiments, the relative density of a metal alloy can affect other material properties of the metal alloy. Thus, by controlling the relative density of a metal alloy, other material properties of the metal alloy can also be controlled.

[0115] According to certain embodiments, the metal alloys described herein may be substantially stable at relatively high temperatures. A metal alloy is considered "substantially stable" at a particular temperature if it comprises at least one cross-section intersecting the geometric center of the alloy, where the volume average cross-sectional grain size (as described above) of the cross-section does not increase by more than 20% (relative to the original volume average cross-sectional grain size) when the metal alloy is heated to that temperature in an argon atmosphere for 24 hours. One skilled in the art would be able to determine whether a metal alloy is substantially stable at a particular temperature by taking a cross-section of the article, determining the volume average cross-sectional grain size of the cross-section at 25°C, heating the cross-section to a particular temperature in an argon atmosphere for 24 hours, allowing the cross-section to cool back to 25°C, and determining the volume average cross-sectional grain size of the cross-section after heating. A metal alloy would be considered substantially stable if the volume average cross-sectional grain size of the cross-section after the heating step is less than 120% of the volume average cross-sectional grain size of the cross-section prior to the heating step. According to certain embodiments, a metal alloy that is substantially stable at a particular temperature comprises at least one cross-section intersecting the geometric center of the metal alloy, wherein the volume average cross-sectional grain size of the cross-section does not increase by more than 15%, more than 10%, more than 5%, or more than 2% (relative to the original volume average grain size) when the body is heated to that temperature in an argon atmosphere for 24 hours.

[0116] In some embodiments, the metal alloy is substantially stable at at least one temperature greater than or equal to 100 degrees Celsius (°C). In an embodiment, the metal alloy is substantially stable at at least one temperature greater than or equal to 700°C, greater than or equal to 800°C, greater than or equal to 900°C, greater than or equal to 1,000°C, greater than or equal to 1,100°C, greater than or equal to 1,200°C, greater than or equal to 1,300°C, greater than or equal to 1,400°C, greater than or equal to 1,500°C, greater than or equal to 1,600°C, greater than or equal to 1,700°C, greater than or equal to 1,800°C, greater than or equal to 1,900°C, greater than or equal to 2,000°C, greater than or equal to 2,100°C, greater than or equal to 2,200°C, greater than or equal to 2,300°C, greater than or equal to 2,400°C, or greater than or equal to 2,500°C. Other ranges are also possible.

[0117] Certain metal alloys described herein are sintered metal alloys. Exemplary sintering methods that can be used to produce metal alloys according to the present disclosure are described above.

[0118] Some of the metal alloys described herein are stable against grain growth.

[0119] The metal alloy, according to certain embodiments, can be a bulk metal alloy (e.g., a bulk nanocrystalline metal alloy). A "bulk metal alloy" is a metal alloy that is not in the form of a thin film. In certain embodiments, the bulk metal alloy has a smallest dimension of at least 1 micron. In some embodiments, the bulk metal alloy has a smallest dimension of at least 5 microns, at least 10 microns, at least 25 microns, at least 50 microns, at least 100 microns, at least 500 microns, at least 1 millimeter, at least 1 centimeter, at least 10 centimeters, at least 100 centimeters, or at least 1 meter. Other values ​​are also possible. According to certain embodiments, the metal alloy is not in the form of a coating.

[0120] In some embodiments, the metal alloy has a thickness of at least 0.01 mm. 3 , at least 0.1 mm 3 , at least 1 mm 3 , at least 5 mm 3 , at least 10 mm 3 , at least 0.1 cm 3 , at least 0.5 cm 3 , at least 1 cm 3 , at least 10 cm 3 , at least 100cm 3 , or at least 1 m 3 Other values ​​are also possible.

[0121] According to certain embodiments, the metal alloy comprises multiple phases. For example, in some embodiments, the metal alloy is a dual-phase metal alloy. In some cases, the metal alloy comprises a first solid phase rich in Mo and a second solid phase rich in a second metal. In other embodiments, the metal alloy is a single-phase metal alloy.

[0122] Some embodiments relate to molybdenum-based metal alloys with thermally stable nanocrystalline microstructures. The alloys can be prepared from metal powders by mechanical alloying and then consolidated at high temperatures into fully dense materials while retaining their nanoscale grain size. According to some embodiments, dense nanocrystalline alloys are significantly stronger than similar alloys that are not nanocrystalline.

[0123] According to certain embodiments, the alloys are based on molybdenum (Mo) and typically contain various compositions of chromium (Cr) and / or tungsten (W). They are prepared, according to some embodiments, by high-energy ball milling of elemental powders, which results in mechanical alloying (creating the alloy) and grain refinement (forming a nanocrystalline structure). In some embodiments, the alloy powder is then cold-pressed and annealed in an inert atmosphere without any applied pressure. According to certain embodiments, the addition of Cr is believed to stabilize the grain boundaries so that the nanocrystalline structure is maintained during the annealing process. Also, according to some embodiments, the addition of Cr is believed to help accelerate the sintering (densification) process by forming a second phase during annealing. In some embodiments, alloys containing Mo and Cr can achieve full densification at temperatures around 1,450°C, which is lower than most conventional sintering methods for producing molybdenum-based alloys. This may also provide the advantage of allowing the use of conventional equipment when producing alloys according to the methods described herein and may reduce the energy required to produce parts.

[0124] Some, but not necessarily all, of the embodiments described herein may have one or more advantages and / or improvements over existing methods, devices, and / or materials. According to some embodiments, the methods described herein enable the creation of fully dense bulk nanocrystalline parts with potentially complex shapes in a scalable manner. Alternative methods, such as severe plastic deformation (SPD) of dense coarse-grained materials, are generally considered not scalable and are generally limited to simple part shapes. Additionally, some of the methods described herein enable sintering of powders without applied pressure during heating, which significantly simplifies the processing route.

[0125] Certain articles, systems, and / or methods described herein can have any of a variety of commercial applications and / or can be particularly economically attractive. For example, certain of the alloys described herein can be made using much less energy (due to low-temperature and low-pressure processing) than would be required for other types of molybdenum-containing alloys. Also, according to certain embodiments, bulk metal parts (e.g., nanocrystalline metal parts) can replace any structural metal part in commercial applications because they can offer significantly improved mechanical properties. The molybdenum alloys described herein, according to some embodiments, can replace conventional molybdenum alloy parts in the construction, automotive, aerospace, and nuclear industries, and the like. In some embodiments, they can be used to reduce weight when their increased strength is not required. For example, according to certain embodiments, thinner panels can provide the same engineering properties as thicker ones made from conventional alloys. In some embodiments, the alloys described herein can be used to provide both increased strength and weight reduction.

[0126] Some of the alloys described herein may also be advantageous in high-temperature structural materials, such as in nuclear thermal propulsion. In some embodiments, the alloys may possess a sufficiently high melting temperature so that they can operate at high temperatures (e.g., temperatures of at least about 2,500°C) for at least short periods of time (e.g., at least 1 minute, at least 10 minutes, or more). In addition, certain embodiments of the alloys described herein may have low neutron absorption cross sections, making them particularly suitable for use in nuclear reactors.

[0127] The following examples are intended to illustrate certain embodiments of the present invention, but do not exemplify the full scope of the invention.

[0128] (Example) This example describes enhanced sintering of Mo-based alloys. In certain embodiments, this set of alloys can function as structural materials in nuclear thermal propulsion. Some of the alloys described herein can be sintered at low temperatures, rapidly, and / or without the need for applied pressure during the sintering process. In some such embodiments, the alloy can also have a sufficiently high melting temperature such that it can operate at temperatures up to 2,500°C (e.g., for at least short periods of time) and / or have an acceptable neutron absorption cross section for use in nuclear reactors.

[0129] Molybdenum is a viable candidate for structural materials in various nuclear reactor applications. Molybdenum has a relatively low neutron absorption cross section, meaning that neutrons released during nuclear reactions remain contained within the reactor. These neutrons then react with the nuclear fuel, sustaining the nuclear reactions that allow the reactor to function. Furthermore, molybdenum has a high melting temperature, which allows it to remain structurally stable within the high-temperature environment of a nuclear reactor. Finally, it has a high thermal conductivity, which is useful for transferring heat to the working fluid. These factors make molybdenum alloys promising candidates for use in nuclear reactor designs, including nuclear thermal propulsion systems.

[0130] However, pure molybdenum is generally not a suitable material for producing the complex components required for advanced nuclear technologies, such as nuclear thermal propulsion. Due to its high melting temperature, it is generally difficult to produce parts from molybdenum. Sintering pure molybdenum also generally requires high applied pressure to achieve full density, which limits the complexity of components that can be produced from it. Therefore, it would be useful to design molybdenum alloys to facilitate pressureless sintering at lower temperatures. Furthermore, while molybdenum does have a high melting temperature, nuclear thermal propulsion requires operation at temperatures slightly above the practical operating range of pure molybdenum. Therefore, alloying molybdenum to increase its melting temperature would further increase its usefulness.

[0131] In some embodiments, molybdenum alloys can be designed to achieve rapid sintering (which may involve nanophase separation sintering). This example discloses a molybdenum alloy that undergoes rapid, low-temperature, pressureless sintering via nanophase separation sintering. In this example, the alloying element selected to promote nanophase separation sintering in molybdenum is chromium. Chromium has been observed to segregate into secondary phases upon heating. Additionally, chromium has a lower surface energy than molybdenum and therefore preferentially segregates to the surfaces of powder particles, forming necks between them. Additionally, chromium allows for rapid diffusion of molybdenum through the chromium necks, promoting rapid densification. These steps in the sintering process can be seen in Figure 4.

[0132] To create the alloy, pure molybdenum and chromium powders were combined using mechanical alloying. Most tests employed a ratio of approximately 15 at% chromium and the remainder molybdenum (or Mo15Cr). This produced the metallic powder (approximately 1 micron in diameter), shown in Figure 5, which was supersaturated (the molybdenum and chromium were uniformly dispersed) and nanocrystalline with a particle size of approximately 10 nm. This powder was then pressed into the shape of a part (or green body), ensuring that the powder particles were in contact with each other. When the green body was raised to a higher temperature, chromium atoms diffused to the surfaces of the powder particles. These atoms segregated from the powder particles and formed a solid chromium phase on the surface of the powder particles. This segregated phase formed between the particles created necks between them, as seen in Figure 7. These necks acted as rapid diffusion paths between the particles, allowing material to flow through the system and further fill the voids between the particles. This allowed the green body to be sintered and reach a relative density of >98%. The resulting alloys underwent the onset of sintering at temperatures as low as 750°C and reached full density at temperatures as low as about 1,450°C, as seen in the microstructures in Figure 6 and the densification curves in Figure 9.

[0133] Tungsten was included in the alloy to create a molybdenum-based alloy with a higher melting temperature. Tungsten has a high melting temperature and forms a solid solution with molybdenum. Therefore, it is believed that the melting temperature of the alloy can be adjusted by increasing the amount of tungsten, a co-alloyed material. For example, an alloy of 60 at% molybdenum, 25 at% tungsten, and 15 at% chromium (Mo25W15Cr) should exhibit a melting temperature 100 degrees higher than pure molybdenum. Similar to the tested Mo15Cr alloy, a relative density of over 98% was achieved in a sample heated to only 1,450°C without any applied pressure, as seen in the microstructure in Figure 8 and the densification curve in Figure 10.

[0134] The final melting temperature of the material remained high because the chromium phase formed to accelerate sintering redissolved in the bulk molybdenum or molybdenum-tungsten material instead of melting at a lower temperature. The addition of chromium did affect the melting temperature, but more tungsten addition could be used to compensate for this.

[0135] The following processing steps were used to prepare the alloy. 1. Elemental molybdenum, chromium, and tungsten powders were mechanically alloyed via ball milling. 2. The powder was formed into the desired shape (in the laboratory, the powder was pressed into pellets, but the actual final part may have other shapes and sizes). 3. The pressed powder was placed in a furnace with a controlled atmosphere. In this set of examples, argon was used, but other gases may also work. 4. The pressed powder was heated to the desired temperature range. The initial stage of sintering was observed around 750-800°C, and the final density was achieved at a temperature around 1,450°C.

[0136] The alloys described in this example may provide one or more of the following advantages: The alloys described in this example achieved full densification at temperatures near 1,450°C, which is lower than most conventional sintering methods for producing molybdenum-based alloys. This is beneficial because it allows more conventional equipment to be used in producing these alloys. This also reduces the energy required to produce the part.

[0137] The alloys described in this example were not held at their sintering temperature for too long. This is beneficial for reducing the amount of energy required to produce the parts. Also, faster sintering of parts is beneficial for having a higher throughput of parts that can be used or sold by the entities that make them.

[0138] The alloys described in this example can be sintered without the application of external pressure. This means that items can be made from the alloys using simpler tools than similarly possible alloys. Furthermore, more complex geometries can be made from these alloys, making them useful for creating new objects with very specific functions.

[0139] The alloys described in this example can withstand high temperatures while exhibiting rapid sintering. Some methods of promoting densification in materials result in the creation of secondary, low-melting-temperature phases that prevent the resulting material from being used at higher temperatures. In contrast, in the alloys described in this example, the secondary phases that form remelt back into the base alloy, and the melting temperature remains high.

[0140] For the alloy described in this example, the material remained solid during the entire sintering process, and therefore, shape change during the process was very limited. This is important when producing parts with specific tolerances on the final part geometry. Some accelerated sintering techniques result in part deformation during the sintering process before full density (specifically, those where liquid is formed) is reached.

[0141] For the alloys described in this example, the melting temperature and neutron absorption properties can be tailored for the desired operating conditions of the final product. Tungsten alone does not have the neutron absorption properties necessary to be used as a structural material in nuclear reactors (specifically, meant to contain the reaction). Molybdenum alone cannot withstand the high temperatures of some advanced reactor designs. Using both of these elements to create an enhanced combination of high melting temperature and acceptable neutron absorption capacity will facilitate the production of novel nuclear systems.

[0142] For the alloys described in this example, powder production is industrially scalable. Mechanical alloying through ball milling is a common industrial method that is easy to scale from laboratory quantities (a few grams) to industrially relevant quantities (several kilograms). Other methods of accelerating sintering, such as producing nano-sized powders, are generally difficult to scale to commercially viable levels.

[0143] These are also believed to be the only molybdenum-based alloys designed to exhibit nanophase-separated sintering. The molybdenum-based alloys described herein have a variety of potential commercial applications. For example, in some cases, molybdenum-based alloys (e.g., also with chromium and / or tungsten) can be used in nuclear thermal propulsion. Nuclear thermal propulsion systems generally require structures capable of operating at temperatures of approximately 2,500°C while exhibiting sufficiently low neutron absorption cross sections. Furthermore, molybdenum-based alloys can be advantageous in producing the complex geometries required to maximize surface area and create through channels for more efficient heat transfer to the propellant. Some of the molybdenum-based alloys described herein co-optimize these properties in a way not believed possible with any other alloy.

[0144] Some of the molybdenum-based alloys described herein can be incorporated into nuclear thermal propulsion systems (e.g., for deep space missions and potentially manned missions to Mars). Some of the molybdenum-based alloys described herein facilitate the extrusion of components through conventional manufacturing techniques. Some of the molybdenum-based alloys described herein may also provide a route for 3D printing novel components with complex geometries (e.g., for more specialized spacecraft). Some of the molybdenum-based alloys described herein may also be utilized for high-temperature applications in novel nuclear reactors (e.g., fission and / or fusion reactors).

[0145] While several embodiments of the present invention have been described and illustrated herein, those skilled in the art will readily envision various other means and / or structures for performing the functions and / or obtaining one or more of the results and / or advantages described herein, and each such variation and / or modification is deemed to be within the scope of the present invention. More generally, those skilled in the art will readily appreciate that all parameters, dimensions, materials, and configurations described herein are meant to be exemplary, and that the actual parameters, dimensions, materials, and / or configurations will depend on the specific application or applications for which the teachings of the present invention are used. Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments of the invention described herein. Accordingly, it is to be understood that the foregoing embodiments are presented by way of example only and that, within the scope of the appended claims and their equivalents, the invention may be practiced otherwise than as specifically described and claimed. The present invention is directed to each individual feature, system, article, material, and / or method described herein. In addition, any combination of two or more such features, systems, articles, materials, and / or methods is included within the scope of the present invention, if such features, systems, articles, materials, and / or methods are not mutually inconsistent.

[0146] The indefinite articles "a" and "an," as used herein in the specification and claims, unless clearly indicated to the contrary, should be understood to mean "at least one."

[0147] The term "and / or," as used herein in the specification and claims, should be understood to mean "either or both" of the elements so conjoined, i.e., elements present conjunctively in some cases and disjunctively in other cases. Unless clearly indicated to the contrary, other elements may optionally be present other than the elements specifically identified by the "and / or" clause, whether related or unrelated to those elements specifically identified. Thus, as a non-limiting example, a reference to "A and / or B," when used in conjunction with open-ended language such as "comprising," can refer in one embodiment to A without B (optionally including elements other than B); in another embodiment to B without A (optionally including elements other than A); in yet another embodiment to both A and B (optionally including other elements), etc.

[0148] As used herein in the specification and claims, "or" should be understood to have the same meaning as "and / or" as defined above. For example, when separating items in a list, "or" or "and / or" shall be interpreted as inclusive, i.e., the inclusion of at least one, but also more than one of several elements or lists of elements, optionally including additional unlisted items. Only terms clearly indicated to the contrary, such as "only one of" or "exactly one of," or, when used in the claims, "consisting of," will refer to the inclusion of exactly one element of several elements or lists of elements. In general, the term "or" as used herein shall only be interpreted as indicating exclusive alternatives (i.e., "one or the other, but not both") when preceded by terms of exclusivity, such as "either," "one of," "only one of," or "exactly one of." "Consisting essentially of," when used in the claims, shall have its ordinary meaning as used in the field of patent law.

[0149] As used herein in the specification and claims, the phrase "at least one" in reference to a list of one or more elements should be understood to mean at least one element selected from any one or more of the elements in the list of elements, but not necessarily including at least one of each and every element specifically listed in the list of elements, and not excluding any combinations of elements in the list of elements. This definition also allows for the optional presence of elements other than those specifically identified in the list of elements to which the phrase "at least one" refers, whether related or unrelated to those specifically identified elements. Thus, as a non-limiting example, "at least one of A and B" (or, equivalently, "at least one of A or B," or, equivalently, "at least one of A and / or B") can refer to, in one embodiment, at least one A, optionally including more than one A, with no B present (optionally including elements other than B); in another embodiment, at least one B, optionally including more than one B, with no A present (optionally including elements other than A); in yet another embodiment, at least one A, optionally including more than one A, and at least one B (optionally including other elements), optionally including more than one B, etc.

[0150] In the claims, as well as in the above specification, all transitional phrases such as "comprising," "including," "carrying," "having," "containing," "involving," "holding," and the like, are to be understood to be open-ended, i.e., meaning "including, but not limited to." Only the transitional phrases "consisting of" and "consisting essentially of" shall be closed or semi-closed transitional phrases, respectively, as set forth in the U.S. Patent Office Manual of Patent Examining Procedures, Section 2111.03.

Claims

1. 1. A method of forming a metal alloy, comprising: sintering nanocrystalline particles comprising molybdenum (Mo) and chromium (Cr) to produce the metal alloy; Mo is present in the metal alloy in an amount of at least 55 at%; said sintering results in said metal alloy having a relative density of at least 98%; a maximum external pressure applied to the nanocrystalline particles is less than or equal to 2 MPa for at least 20% of the time that the sintering is carried out; the nanocrystalline particles have a total particle volume; at least 75% of the total particle volume is made up of nanocrystalline particles having a maximum cross-sectional dimension between 1 micrometer and 1 millimeter; the nanocrystalline particles comprise grains having a size of at least 1 nm and no greater than 100 nanometers; the sintering comprises heating the nanocrystalline particles to a temperature of 1600°C or less for at least 99% of the sintering time; The method, wherein the sintering time is 24 hours or less.

2. 1. A method of forming a metal alloy, comprising: sintering nanocrystalline particles comprising molybdenum (Mo) and chromium (Cr) to produce the metal alloy; Mo is present in the metal alloy in an amount of at least 55 at%; a maximum external pressure applied to the nanocrystalline particles is less than or equal to 2 MPa for at least 20% of the time that the sintering is carried out; the nanocrystalline particles have a total particle volume; at least 75% of the total particle volume is made up of nanocrystalline particles having a maximum cross-sectional dimension between 1 micrometer and 1 millimeter; said sintering results in said metal alloy having a relative density of at least 98%; The method, wherein the nanocrystalline particles comprise grains having a size of at least 1 nm and no greater than 100 nanometers.

3. 1. A method of forming a metal alloy, comprising: sintering nanocrystalline particles comprising molybdenum (Mo) and chromium (Cr) to produce the metal alloy; said sintering results in said metal alloy having a relative density of at least 98%; a maximum external pressure applied to the nanocrystalline particles is less than or equal to 2 MPa for at least 20% of the time that the sintering is carried out; non-equilibrium phases undergo decomposition during the sintering of the nanocrystalline particles; The method, wherein the nanocrystalline particles comprise grains having a size of at least 1 nm and no greater than 100 nanometers.

4. 1. A method of forming a metal alloy, comprising: sintering nanocrystalline particles comprising molybdenum (Mo) and chromium (Cr) to produce the metal alloy; Mo is present in the metal alloy in an amount of at least 55 at%; said sintering results in said metal alloy having a relative density of at least 98%; a maximum external pressure applied to the nanocrystalline particles is less than or equal to 2 MPa for at least 20% of the time that the sintering is carried out; During the sintering, the Cr forms necks between Mo particles and / or grains; The method, wherein the nanocrystalline particles comprise grains having a size of at least 1 nm and no greater than 100 nanometers.

5. 4. The method of claim 3, wherein Mo is the most abundant element in atomic percent in the metal alloy.

6. The method of any one of claims 1 to 5, further comprising a third element.

7. The method of claim 6, wherein the third element is present in the metal alloy in an amount between 0.5 at% and 40 at% of the metal alloy.

8. The method of any one of claims 6 to 7, wherein the Cr and the third element exhibit a miscibility gap.

9. The method according to any one of claims 6 to 8, wherein the third element is tungsten (W).

10. The method according to any one of claims 6 to 8, wherein the third element is tantalum (Ta).

11. The method according to any one of claims 1 to 10, wherein the melting point of the metal alloy is at least 2,500°C.

12. 12. The method of any one of claims 3 and 5 to 11, wherein Mo is present in the metal alloy in an amount of at least 50 at%.

13. The method of any one of claims 1 to 12, wherein the metal alloy is nanocrystalline.

14. 14. The method of claim 13, wherein the nanocrystalline metal alloy has an average grain size of 300 nm or less.

15. The method of any one of claims 1 to 14, wherein the metal alloy is a bulk metal alloy.

16. The method of any one of claims 1 to 15, wherein the metal alloy is substantially stable at a temperature of at least 2,500°C.

17. 17. The method of any one of claims 1 to 16, wherein the metal alloy has a first grain size and the Cr-free, Mo-containing sintered material has a second grain size, the first grain size being smaller than the second grain size.

18. The method of any one of claims 1 to 17, wherein the metal alloy is enriched in Cr at the grain boundaries of the metal alloy.

19. The method of any one of claims 1 to 18, wherein the metal alloy comprises grains having a size of 1000 nm or greater.

20. The method of any one of claims 1 to 19, wherein the Cr is present in the metal alloy in an amount of 0.5 at% to 40 at% of the metal alloy.

21. 21. The method of any one of claims 1 to 20, wherein at least 90% of the total particle volume is made up of nanocrystalline particles having a maximum cross-sectional dimension of between 1 micrometer and 1 millimeter.

22. The method of any one of claims 1 to 21, wherein the Mo and the Cr are present in the nanocrystalline particles in a non-equilibrium phase.

23. 23. The method of claim 22, wherein the non-equilibrium phase is a supersaturated phase comprising the Cr dissolved in the Mo.

24. 24. The method of any one of claims 1 to 23, further comprising mechanically working the nanocrystalline particles and / or precursor particles prior to sintering such that at least some of the nanocrystalline particles contain both the Mo and the Cr.

25. 25. The method of any one of claims 1 or 5 to 24, wherein the sintering time is 12 hours or less.

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