NANOPHASE DISPERSION STRENGTHENED LOW CTE ALLOY

Abstract

A metal matrix composite material of a low coefficient of thermal expansion (CTE) alloy strengthened by nanophase dispersed particles. The low CTE alloy can be an iron-nickel alloy or an iron-nickel-cobalt alloy. The nanophase particles can be a refractory oxide, carbide or nitride. Also disclosed is a method of making a metal matrix composite material in which the nanophase particles are combined with the low CTE alloy to form a metal matrix composite material having the nanophase particles dispersed therein.

Description

RELATED APPLICATION

This application is based upon prior filed provisional patent applications Ser. No. 60/992,403 filed Dec. 5, 2007, and 61/013,340 filed Dec. 13, 2007, the entire subject matter of which are incorporated by reference herein.

ORIGIN OF INVENTION

The invention described herein was made by an employee of the United States Government, and may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION

Structural materials with low coefficients of thermal expansion (CTE) find wide application in space structures that require high pointing accuracy and dimensional stability in the presence of dynamic thermal disturbances. Historically, Invar 36 (36 weight percent nickel, 64 weight percent iron) has been the material of choice, notwithstanding its low strength, low specific stiffness and high density. These limitations of low strength, low specific stiffness and high density were highlighted most recently in trade studies concerning its use in the James Webb Space Telescope Integrated Science Instrument Module (JWST ISIM) and mirror back plane.

BRIEF SUMMARY OF THE INVENTION

The advantages of the invention have been achieved by providing, according to a first aspect of the invention, a metal matrix composite material comprising a low coefficient of thermal expansion (CTE) alloy strengthened by nanophase dispersed particles.

According to a second aspect of the invention, there is provided a metal matrix composite material comprising a low coefficient of thermal expansion (CTE) alloy selected from the group consisting of iron-nickel and iron-nickel-cobalt alloys strengthened by nanophase dispersed particles.

According to a third aspect of the invention, there is provided a method of making a metal matrix composite material, the method comprising the steps of:

forming nanophase particles;

combining the nanophase particles with a low coefficient of thermal expansion (CTE) alloy to form a metal matrix composite material having the nanophase particles dispersed therein.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a metal matrix composite material which includes a low CTE alloy strengthened by nanophase particles dispersed within the metal matrix composite material. It is believed that a number of suitable low CTE alloys would benefit from

The present inventor believes that low CTE alloys, particularly the iron—36 weight percent nickel alloy (Invar 36) can be strengthened with a high volume fraction of small, finely dispersed, inert, non-deformable particles and still maintain a secant coefficient of thermal expansion (also called an average coefficient of thermal expansion) less than or equal to 1.5 ppm/K over the temperature range 4K to 330K.

The nanophase particles could comprise about 10 to 45 volume percent of the metal matrix composite material but most preferably is about 30 volume percent. The nanophase particles should be inert meaning they do not dissolve into the metal matrix as an atomic scale solute that would degrade thermal expansion properties. Moreover, the nanophase particles should be refractory with a standard molar enthalpy of formation less than −300 kJ mol⁻¹ so that the atoms within the nanophase particles are bound tightly together and are therefore not easily deformed or cut by a passing dislocation.

The present invention addresses the shortcomings of the prior art by introducing an inert nanophase dispersion into the low CTE alloy, preferably the iron—36 weight percent nickel alloy (Invar 36), matrix forming an ex situ composite resulting in a lower density, higher strength metal matrix composite material with higher specific stiffness. The ex situ composite refers to being added to the metal matrix from outside as compared to an in situ composite formed by adding solutes that then nucleate and grow in a precipitation reaction within the metal matrix. The dispersion of inert nanophase particles will reinforce the metal matrix through Orowan strengthening which is a strengthening mechanism resulting from dispersion hardening. Importantly, there will also be a substantial increase in the microyield with this metal matrix composite. A Hall-Petch effect will contribute to this as well as the Orowan Strengthening. The Hall-Petch effect predicts that as the grain size decreases the yield strength increases.

The nanophase particles should have a grain size distribution of between 10 nanometers (nm) and 200 nm but generally less than about 100 nm. Any aggregates or agglomerations of nanophase particles should be less than about 200 nm. The thrust of the present invention is based on the increase in shear stress required to force a dislocation through a field of non-deformable particles. Based on the theoretical description as described by Orowan, the increase in shear stress is directly proportional to the square root of the volume fraction of particles and inversely proportional to the particle diameter. So it would be desirable to have a high volume fraction of small particles. Regarding the size of the nanophase particles, it should be noted that whatever technology is used to produce the nanophase particles, there will most likely be a size distribution of the nanophase particles. Further, with respect to the nanophase particles in the metal matrix, there will be a point where the nanophase particles can be too small and the dislocations can cut through them resulting in a reduction in strength. What is too small a size of nanophase particle will depend on the chemistry of the nanophase particle. However, it is believed that the size distribution above should be sufficient for the present invention.

As noted above, the nanophase particles should be inert. The nanophase particles, besides being inert, may be made from any material that can be formed to a grainsize of about 100 nm or less and that will contribute to Orowan strengthening of the metal matrix composite. Thus, the nanophase particles may be refractory oxides, carbides, nitrides or mixtures thereof.

Suitable oxides could include, for example, Al₂O₃, TiO₂, Y₂O₃, HfO₂, ZrO₂, SiO₂, Ta₂O₅ and ZrSiO₄ while suitable nitrides could include, for example, TaN, TiN, ZrN, AlN, Si₃N₄, VN, CrN, NbN, and HfN. If carbides are to be used as the nanophase particles, it is necessary that they contain no free carbon which could react with the low CTE alloy matrix to adversely affect the low expansion properties of the low CTE alloy. Suitable carbides could include, for example, TiC, ZrC, HfC, VC, NbC, TaC, Cr₃C₂, Mo₂C, WC, and SiC.

The metal matrix composite material will be made by a process in which the nanophase particles will be synthesized separately as a powder or as a ceramic preform by conventional processes that are then combined with (i.e., introduced into the matrix of) the low CTE alloy, preferably the iron-36 weight percent nickel alloy (Invar 36) during a secondary process such as powder metallurgy, mechanical alloying or liquid metal infiltration.

The powder metallurgy process includes mixing powders of the nanophase particles with powders of the low CTE alloy to form a compact and then heating to a temperature sufficient to cause sintering of the low CTE alloy. As an example, if 30 volume percent of Al₂O₃ were to be added to the iron—36 weight percent nickel alloy (Invar 36), the mixture could be solid state sintered for about 48 hours at 1350° C. in a vacuum or inert atmosphere to cause densification of the mixture into a metal matrix composite material.

The mechanical alloying process includes adding a mixture of master alloy powder or pre-proportioned metal powders together with nonmetal nanophase powder for the target composition to a high-energy ball mill for working. The nanophase particles endure mixing through severe plastic deformation in an inert atmosphere. The resulting mixture is consolidated by hot isostatic pressing or extrusion. Depending on the desired product, the billet can then be thermomechanically processed through a descending temperature range by press-forging to final size. A billet can also be rolled to plate or bar, or drawn to wire.

The liquid metal infiltration process includes forming a porous compact of the nanophase particles and then infiltrating the compact with a liquefied low CTE alloy, preferably the iron—36 weight percent nickel alloy (Invar 36). As an example, a porous compact of Y2O3 could be formed and then the iron—36 weight percent nickel alloy (Invar 36) could be melted at 1500° C. and poured into the porous compact to fill the pores. Similarly, vacuum displacement could be applied to pull the liquid alloy into the pores of the compact. The resulting metal matrix composite material would be approximately 30 volume percent nanophase particles dispersed in a metal matrix of the iron—36 weight percent nickel alloy (Invar 36).

A number of companies can provide the nanophase reinforcing powder as well as either the high purity iron and nickel powder or the high purity alloy powder. The nanophase reinforcement as a ceramic preform for liquid metal infiltration is also available commercially. In this instance, the metal for infiltration could be produced by melting high purity iron and nickel or, for quicker turn around, obtained commercially under conventional alloy designated by UNS No. K93603.

The above described method differs from previous attempts to make a low CTE alloy that has high strength, low CTE and microstructural stability in four important ways. First, commercial attempts to form an in situ composite by introducing solutes that later react and precipitate through heat treatment have met with limited success because the iron-nickel matrix always has a finite solubility for ad-elements and because of the extreme sensitivity of the thermal expansion properties on the presence of unreacted elements that remain in solution. Examples include (a) the addition of Ti and C into the low-expansion iron-nickel alloy family to produce titanium carbide, (b) the addition of W, Cr, and C as for example with Elinvar to precipitate tungsten and chromium carbides, or (c) the addition of Ti and Nb as in NILO alloy 365 to precipitate gama prime. Secondly, alloying iron and nickel with cobalt, as with Kovar, Super Invar 32-5 and Pernifer 29-18, produce a low CTE over limited temperature ranges but are microstructurally unstable at cryogenic temperatures. Third, the proposed process preferably uses high purity iron and nickel starting materials that will ensure optimum thermal expansion properties by minimizing the presence of tramp impurities. Fourth, the composition of the dispersed phase will be chosen to minimize thermal expansion mismatch between the low CTE alloy matrix, and the iron—36 weight percent nickel alloy (Invar 36) matrix in particular, and the individual nanophase particle thus minimizing thermal stresses in the matrix over a wide range of temperature change.

It will be apparent to those skilled in the art having regard to this disclosure that other modifications of this invention beyond those embodiments specifically described here may be made without departing from the spirit of the invention. Accordingly, such modifications are considered within the scope of the invention as limited solely by the appended claims.

Claims

1. A metal matrix composite material comprising a low coefficient of thermal expansion (CTE) alloy strengthened by nanophase dispersed particles.

2. The metal matrix composite material according to claim 1, wherein said allow alloy is selected from the group consisting of iron-nickel and iron-nickel-cobalt alloys strengthened by nanophase dispersed particles.

3. The metal matrix composite material of claim 2 wherein the low CTE iron-nickel alloy comprises 36 weight percent nickel, remainder iron.

4. The metal matrix composite material of claim 2 wherein the low CTE iron-nickel-cobalt alloys are selected from the group consisting of a 32 weight percent nickel, 5 weight percent cobalt, remainder iron alloy and a 26 weight percent nickel, 17 weight percent cobalt, remainder iron alloy.

5. The metal matrix composite of claim 2 wherein the nanophase dispersed particles comprise about 10 to 45 volume percent of the metal matrix composite, wherein the remainder of the metal matrix composite is the low CTE alloy.

6. The metal matrix composite of claim 2 wherein the nanophase dispersed particles comprise about 30 volume percent of the metal matrix composite, wherein the remainder of the metal matrix composite is the low CTE alloy.

7. The metal matrix composite of claim 2 wherein the nanophase dispersed particles have a grain size distribution of about 10 nm to 200 nm with the majority of the nanophase particles having a grain size less than about 100 nm.

8. The metal matrix composite of claim 2 wherein the nanophase dispersed particles are selected from the group consisting of refractory oxides, carbides and nitrides.

9. The metal matrix composite of claim 8 wherein the refractory oxide is selected from the group consisting of Al2O3, TiO2, Y2O3, HfO2, ZrO2, SiO2, Ta2O5, and ZrSiO4.

10. The metal matrix composite of claim 8 wherein the nitride is selected from the group consisting of TaN, TiN, ZrN, AlN, Si3N4, VN, CrN, NbN, and HfN.

11. The metal matrix composite of claim 8 wherein the carbide is devoid of free carbon and is selected from the group consisting of TiC, ZrC, HfC, VC, NbC, TaC, Cr3C2, MO2C, WC, and SiC.

12. A method of making a metal matrix composite material, the method comprising the steps of:

forming nanophase particles;

combining the nanophase particles with a low coefficient of thermal expansion (CTE) alloy to form a metal matrix composite material having the nanophase particles dispersed therein.

13. The method of claim 12 wherein the nanophase particles are selected from the group consisting of refractory oxides, carbides and nitrides and the low CTE alloy is selected from the group consisting of iron-nickel and iron-nickel-cobalt alloys.

14. The method of claim 12 wherein the step of combining comprises mixing powders of the nanophase particles with powders of the low CTE alloy to form a compact and then heating to a temperature sufficient to cause sintering of the low CTE alloy.

15. The method of claim 12 wherein the step of combining comprises mechanical alloying.

16. The method of claim 12 wherein the step of combining includes forming a porous compact of the nanophase particles and then infiltrating the compact with a liquefied low CTE alloy.

17. The method of claim 12 wherein the nanophase particles comprise about 10 to 45 volume percent of the metal matrix composite, wherein the remainder of the metal matrix composite is the low CTE alloy.

18. The method of claim 12 wherein the nanophase particles comprise about 30 volume percent of the metal matrix composite, wherein the remainder of the metal matrix composite is the low CTE alloy.

19. The method of claim 13 wherein the refractory oxide is selected from the group consisting of Al2O3, TiO2, Y2O3, HfO2, ZrO2, SiO2, Ta2O5, and ZrSiO4.

20. The method of claim 13 wherein the nitride is selected from the group consisting of TaN, TiN, ZrN, AlN, Si3N4, VN, CrN, NbN, and HfN.

21. The method of claim 13 wherein the carbide is devoid of free carbon and is selected from the group consisting of TiC, ZrC, HfC, VC, NbC, TaC, Cr3C2, Mo2C, WC, and SiC.

22. The method of claim 12 wherein the nanophase particles have a grain size distribution of about 10 nm to 200 nm with the majority of nanophase particles having a grain size less than about 100 nm.

23. A method of making a metal matrix composite material of low coefficient of thermal expansion comprising the steps of:

a) providing an iron-nickel alloy powder metal material of a low coefficient of thermal expansion;

b) providing refractory nanophase particles having a standard molar enthalpy of formation less than −300 kJ mol−1;

c) mixing said iron nickel allow and said refractory nanophase particles to yield a substantially uniform mixture wherein said refractory nanophase particles amount to 10-45% of said mixture by volume;

d) forming said mixture into a compact; and

e) then heating said mixture to a temperature sufficient to cause sintering of said iron-nickel alloy.

24. The method of making a metal matrix composite material according to claim 23, wherein said iron-nickel alloy is 36 weight percent nickel and 64 weight percent iron.

25. The method of making a metal matrix composite material according to claim 23, wherein said step of heating includes sintering said mixture in one of a vacuum and a inert atmosphere and causing densification of the mixture into said metal matrix composite material.

26. The method of making a metal matrix composite material according to claim 23, wherein said steps further comprising consolidated said mixture by one of hot isostatic pressing or extrusion.

27. The method of making a metal matrix composite material according to claim 23, wherein said refractory nanophase particles constitute about 30% of said mixture by volume.

28. A metal matrix composite material having of low coefficient of thermal expansion comprising:

an iron nickel alloy having a low coefficient of thermal expansion; and

refractory nanophase particles having a standard molar enthalpy of formation less than −300 kJ mol−1 dispersed substantially uniformly within said iron nickel alloy, wherein said refractory nanophase particles constitute between 10-45% by volume of said metal matrix composite material.

29. The metal matrix composite material according to claim 28, wherein said iron-nickel alloy is 36 weight percent nickel and 64 weight percent iron.

30. The metal matrix composite material according to claim 29, wherein said refractory nanophase particles constitute about 30% of said mixture by volume.

31. The metal matrix composite material according to claim 28, wherein said refractory nanophase particles constitute about 30% of said mixture by volume.

32. The metal matrix composite according to claim 28 wherein said metal matrix composite has

33. The metal matrix composite according to claim 28, wherein said nanophase particles have a grain size distribution in a range between 10-200 nanometers (nm).

34. The metal matrix composite according to claim 30, wherein said nanophase particles have a grain size distribution in a range between 10-200 nanometers (nm).

36. The metal matrix composite according to claim 28, wherein said metal matrix composite has a secant coefficient of thermal expansion less than or equal to 1.5 ppm/K over the temperature range between 4K to 330K.

37. The metal matrix composite according to claim 28, wherein said metal matrix composite has a secant coefficient of thermal expansion less than or equal to 1.5 ppm/K over the temperature range between 4K to 330K and said nanophase particles have a grain size distribution in a range between 10-100 nanometers (nm).

38. The metal matrix composite according to claim 34, wherein said metal matrix composite has a secant coefficient of thermal expansion less than or equal to 1.5 ppm/K over the temperature range between 4K to 330K.