WO2006137911A2

WO2006137911A2 - Method and apparatus for an equal channel angular pressing (ecap) consolidation process for cryomilled nanocrystalline metal powders

Info

Publication number: WO2006137911A2
Application number: PCT/US2005/036919
Authority: WO
Inventors: Enrique Lavernia; Kyung H. Chung; Yizhang Zhou
Original assignee: The Regents Of The University Of California
Priority date: 2004-10-30
Filing date: 2005-10-11
Publication date: 2006-12-28
Also published as: WO2006137911A3

Abstract

A method of densifying a metallic powder comprises the steps of cryomilling the metallic powder to produce a nanocrystalline metallic powder; degassing the nanocrystalline metallic powder; preforming the degassed nanocrystalline metallic powder for a subsequent equal channel angular pressing (ECAP) process according to the metallic powder being processed; and equal channel angular pressing (ECAP) the preformed degassed nanocrystalline metallic powder at a predetermined temperature and pressure.

Description

METHOD AND APPARATUS FOR AN EQUAL CHANNEL ANGULAR

PRESSING (ECAP) CONSOLIDATION PROCESS FOR CRYOMILLED NANOCRYSTALLINE METAL POWDERS

The invention was made with the support of funds from the Office of Naval Research, Contracts N00014-03-1-0149 and N00014-02-1-1053. The U.S. Government has certain rights to the invention.

Background of the Invention

Field of the Invention

[0001] The invention relates to the field of equal channel angular pressing

(ECAP) as a consolidation process for cryomilled nanocrystalline metal powders.

Description of the Prior Art

[0002] Nanostructured materials exhibit microstructures in which the characteristic length scale is typically less than 100 nm. The properties of nanocrystalline materials differ from those of single crystals or coarse-grained polycrystals with the same chemical composition as a result of the reduced size of the crystallites and the extent of interfacial area. The synthesis, processing and characterization of such nanocrystalline materials are vital parts of an emerging and rapidly growing field referred to as "nanotechnology". The remarkable potential of nanocrystalline materials in the form of bulk materials, composites or coatings, is widely recognized. [0003] in addition to excellent physical properties in the fields of magnetics, catalysis, and optics, nanocrystalline materials exhibit a broad range of mechanical behavior. For example, superplastic deformation behavior has been reported in ceramic nanoscale powders at temperatures that are significantly lower than those required by conventional materials. Moreover, ultrahigh hardness values have been measured in nanoscale superlattices made of metallic and ceramic materials. Tensile and compressive strengths in nearly all materials systems have shown anomalously high values at the nanometer-length scale, comparable to those found for structural amorphous metals. The dependence of tensile strength on the microstructural scale size in Al-based alloys is illustrated in Fig. 1. A greatly enhanced yield strength as high as 1600 MPa becomes possible when the grain size of an Al alloy falls into the range of about 10 nm.

[0004] Chung et.al., "Continuous Shear Deformation Device" U.S. Patent

6,571 ,593 (2003), assigned to the Korea Institute of Science and Technology, ^■ describes shear deformed materials which are continuously produced by passing them through an ECAP mold which is a sharply bent channel type mold to give fine grain structure to the product material. The resultant materials have improved stiffness and plasticity properties. The sharply bent zone of the mold is generally known as equal channel angular pressing (ECAP). Chung can be understood as describing the process of continuously supplying materials to this type of mold. [0005] Kawazoe etal., "Method For Extrusion Of Aluminum Alloy And Aluminum Alloy Material Of High Strength And High Toughness Obtained Thereby" U.S. Patent 5,826,456 (1998), assigned to YKK Corp, describes an aluminum alloy material that is made up of crystal grains of particle diameters of not more than one micron are obtained in the process. The extrusion consists of exerting shear deformation in a lateral direction on an aluminum material through two extruding containers with the same cross-sections which are located at an angle with respect to one another. The material that results has a strength that surpasses materials that are obtained using conventional work hardening techniques. The resultant grain sizes within the microstructure of the material do not exceed an average of one micron in diameter.

[0006] Semiatin etal. "Equal Channel Angular Extrusion of Difficult to

Work Alloys" U.S. Patent 5,904,062 (1999), assigned to U.S. Air Force, describes a homogeneous, microstructure materials. Referring to the Figures, the ingot or prior-worked billet 10 is extruded through a channel 11 comprising two channel portions 12 and 13 which are substantially identical in cross-sectional areas, but with center lines that are disposed at an angle with respect to one another. [0007] Zhu etal., "Ultrafine-Grain Titanium For Medical Implants " U.S.

Patent 6,399,215 (2002), assigned to the Regents of the University of California, describes a material is produced by working a coarse-grained titanium billet through multiple extrusions in a pre-heated equal channel angular extrusion die. The resultant material is an ultra-fine-grain titanium which has greatly improved mechanical properties. [0008] Dunlop etal., "Sputtering Target With Ultra-Fine Oriented Grains And Method Of Making Same" U.S. Patent 5,809,393 (1998), assigned to Johnson Matthey Electronics Inc., describes a method which involves the extruding of a workpiece through a die that has contiguous, transverse inlet and outlet channels of substantially identical cross-section. The extrusion provides a product material with a small grain size and a controlled grain structure.

Brief Summary of the Invention

[0009] The illustrated embodiment of the invention is a method of densifying a metallic powder comprising the steps of cryomilling the metallic powder to produce a nanocrystalline metallic powder, such as to a size of 100nm in diameter or less; degassing the nanocrystalline metallic powder; preforming the degassed nanocrystalline metallic powder for a subsequent equal channel angular pressing (ECAP) process according to the metallic powder being processed; and equal channel angular pressing (ECAP) the preformed degassed nanocrystalline metallic powder at a predetermined temperature and pressure. ^' [0010] In one embodiment the step of preforming the degassed nanocrystalline metallic powder comprises using the degassed nanocrystalline metallic powder in the ECAP process without further treatment. [0011] In another embodiment the step of preforming the degassed nanocrystalline metallic powder comprises cold isostatic pressing (CIP) the degassed nanocrystalline metallic powder to aid in densification. [0012] In still another embodiment the step of preforming the degassed nanocrystalline metallic powder comprises hot isostatic pressing (HIP) the degassed nanocrystalline metallic powder at shortened pressing durations and lower temperatures than conventional hot isostatic pressing (HIP) to aid in densification. For example, the shortened pressing durations are 30-60 minutes or less and the lower temperatures are 200-400⁰C or less; the actual temperature and pressure may vary, depending on the alloy composition of interest. [0013] The step of equal channel angular pressing (ECAP) the preformed degassed nanocrystalline metallic powder may be repeated until desired densification of the metallic powder is obtained. The magnitude of the temperature and pressure and the number of repetitions of the step of equal channel angular pressing (ECAP) are selected in combination according densification of the metallic powder desired.

[0014] The invention also contemplates within it scope the densified metallic powder manufactured by the above method as well as the apparatus for manufacturing the densified metallic powder according to the above method. [0015] While the apparatus and method has or will be described for the ^' sake of grammatical fluidity with functional explanations, it is to be expressly understood that the claims, unless expressly formulated under 35 USC 112, are not to be construed as necessarily limited in any way by the construction of "means" or "steps" limitations, but are to be accorded the full scope of the meaning and equivalents of the definition provided by the claims under the judicial doctrine of equivalents, and in the case where the claims are expressly formulated under 35 USC 112 are to be accorded full statutory equivalents under 35 USC 112. The invention can be better visualized by turning now to the following drawings wherein like elements are referenced by like numerals.

Brief Description of the Drawings

[0016] Fig. 1 is a graph showing the known variation of tensile strength as a function of grain size for nanocrystalline materials.

[0017] Fig. 2 is a microphotograph of the crystalline structure of a prior art cryomilled Al alloy.

[0018] Fig. 3 is a microphotograph of the crystalline structure of a cryomilled Al alloy after a prior art HIP consolidation. Dark areas are coarse grain structures and gray areas are submicron grain structures. [0019] Fig. 4 is a microphotograph of the TEM microstructure which shows grain structure of cryomilled Al-alloy after a prior art HIP which yields grain size of 100 - 500 nm in the sub-micron region.

[0020] Fig. 5 is a microphotograph of the crystalline structure of a prior art cryomilled Al alloy after prior art HIP consolidation and extrusion. Dark areas are coarse grain structures and gray areas are submicron grain structures. [0021] Fig. 6 is a diagrammatic side cross-sectional view of the prior art equal channel angular pressing (ECAP) process of the invention. [0022] Fig. 7a is a flowchart illustrating the prior art ECAP process.

[0023] Fig. 7b is a flowchart illustrating the ECAP process of the invention.

[0024] Fig. 8 is a simplified perspective diagram of a cryomill. [0025] The invention and its various embodiments can now be better understood by turning to the following detailed description of the preferred embodiments which are presented as illustrated examples of the invention defined in the claims. It is expressly understood that the invention as defined by the claims may be broader than the illustrated embodiments described below.

Detailed Description of the Preferred Embodiments

[0026] Consider first the fabrication of nanocrystalline powder materials by cryomilling. The properties of nanocrystalline materials can be changed dramatically by their porosity. Porosity-free, full dense, bulk nanocrystalline materials are essential for high material performance. One widely used method to synthesize nanocrystalline materials is mechanical attrition or milling, mechanical alloying,. which induces heavy cyclic deformation in powders and the formation of nanostructures by the structural decomposition of coarse grained structures as a result of severe plastic deformation. Mechanical alloying can be used to generate a variety of nanostructured powder material systems, such as single phase metals with bcc, fee and hep crystalline structures, intermetallic compounds, and numerous equilibrium and non-equilibrium alloys. The major advantages of this technique are simplicity, the relatively inexpensive equipment at least at the laboratory scale which is needed, applicability to essentially all classes of materials, and the possibility for scaling up to production of tonnage quantities for commercial applications. [0027] However, in the case of ductile materials, such as aluminum-based alloys, the relatively strong tendency of the material to adhere to the container walls and to sinter to larger particles of several millimeters in diameter during milling leads to difficulties during processing by conventional mechanical alloying techniques. The introduction of cryogenic liquid media into the mechanical alloying process represents one approach to meet the challenge. The cryogenic ball milling or " cryomilling process " can be effectively used to produce a number of bulk nanocrystalline alloys, such as Al-based alloy, Fe-based alloy, and Ni- based alloy. Fig. 2 shows a typical microstructure of cryomilled Al-based alloy of 20 - 50 nm grain size.

[0028] Consider now the consolidation of nanocrystalline powders via conventional techniques. To produce bulk nanocrystalline materials from cryomilled metallic powders, a consolidation process is essential, and several consolidating processes have been studied, including vacuum hot pressing (VHP) and hot isostatic pressing (HIP), HIP is the most commonly used technique for the fabrication of bulk nanocrystalline materials from nanocrystalline powders. Usually, isotropic microstructures and properties are observed after HIP, which are desirable for structural applications. With sufficient pressure, pressing time and temperature, the full density is attainable by HIP, but a certain amount of grain growth is inevitable because of the high operation temperature, normally well above 0.5T_m, during consolidation, where T_m is the melting point temperature of the alloy at atmospheric pressure. [0029] Fig. 3 is a microphotograph which shows the microstructure of nanocrystalline Al alloy after HIP, observed under scanning electron microscopy (SEM). The relatively bright area is a region of nano-sized grains and dark area shows recrystallized coarse grains with a micrometer size. As shown in Fig. 3, notable amounts of coarse grains exist due to abnormal grain growth during HIP. Obviously, the greater the amount of coarse grain, the lower the mechanical strength to be observed after HIP. Normal grain growth also occurs during HIP, in addition to abnormal grain growth.

[0030] Fig. 4 is a microphotograph which shows the grains in the range of

100 - 300 nm of the "HIPped" Al alloy. An extrusion process usually follows after HIP to break down the coarse structure and to improve ductility of bulk nanocrystalline material. With the application of extrusion after HIP, the coarse grain structure is broken down to a smaller one, but the regions containing micrometer grains still remain in the microstructure as shown in the microphotograph of Fig. 5.

[0031] The prior art process of equal channel angular pressing (ECAP) can now be considered in context of the foregoing prior art fabrication methodologies. Severe plastic deformation (SPD) has been studied as a process to produce ultra-fine grain materials from ordinary coarse grain bulk materials at the micron-meter scale. It is well known that heavy deformations, e.g. cold rolling or drawing, can result in the significant refinement of microstructure at low temperatures. Among the various methods of SPD, equal channel angular pressing (ECAP) has been gathering interest since it was introduced in the 1980s. As shown in the diagrammatic side cross-sectional view of Fig. 6, the billet 10 experiences heavy shear plastic deformation during its passing through the die 14 due to the pressure of plunger 12, but the cross sectional area remains the same before and after the ECAP. Due to that, repeated deformation is possible. In the 1990s, the method was further developed and applied as an SPD method for the processing of nanocrystalline materials and successfully produced submicron grain sized materials with grain sizes as small as a few hundred nanometers in various alloy systems. [0032] Consider now the ECAP process of the invention. As aforementioned above, to take advantage of the superior mechanical properties of cryomilled nanocrystalline powders for structural applications, we must consolidate the cryomilled powders into a bulk material. Conventional consolidation techniques currently available have demonstrated their limitations or drawbacks. Therefore, development of a novel consolidation process, which can produce fully dense bulk materials while maintaining the nano-size grains during the process, is a critical and necessary step for the commercialization of nanocrystalline structural materials.

[0033] The technique described in the invention introduces the ECAP process as a consolidation method for cryomilled metal powders, and accordingly eliminates the conventional HIP process, which causes significant grain growth. The ECAP process is conducted under relatively lower temperatures, which will minimize the grain growth, than those of conventional HIP, which is conducted at well above 0.5 T_m. Furthermore, severe plastic deformation, which is applied during ECAP, can further refine the fine grains already existing. So this new process can be expected to minimize the grain growth during consolidation without sacrificing its dense structure and the superior properties of nanocrystalline materials.

[0034] To produce nanocrystalline powders, a mixture of elemental metal powders or pre-alloyed powders is cryomilled. The cryomilling step 16 in Fig. 7a and 7b is conducted in a sealed chamber 18 with a continuous supply of liquid nitrogen 20 during the process. Fig. 8 is a schematic diagram of the chamber 18. The chamber 18 is filled with stainless steel balls 22 and the shaft 24 and its beaters 26 transfer high kinetic energy to the balls 22. A proper process control agent, e.g. stearic acid, can be added to the chamber 18 to promote a balance between the welding and fracturing of powders during the cryomilling process. [0035] In the canning step 28 and degassing step 30 in Fig. 7a and 7b the cryomilled powders are packed into a sealed metal can (not shown) with a pipe attached for degassing to prevent contamination by the atmosphere. The thickness of the can wall is usually less than 1 mm, and similarity of composition of the can with the powders is desirable. In order to eliminate any trapped gases, as well as process controlling agents, the powder should be degassed under a vacuum within 1 x 10^"6 Torr range by a turbomolecular pump for a few hours at a high temperature, for example, approximately 400⁰C for Ni-alloy. [0036] Next comes the preforming steps for the ECAP process of the invention as shown in Fig. 7b. In the prior art a conventional HIP step 32 is performed here instead. There are three alternatives for the performing step for ECAP consolidation of cryomilled powders. The canned and degassed powders can be used directly as a preform for ECAP. To aid densification, however, a cold isostatic pressing (CIP) step 34 can be applied prior to ECAP as shown in Fig. 7b. Moderate densification of powders is achievable by CIP without grain growth due to is lower operating temperature. In addition, a modified HIP step 36 can be also used to make a preform for ECAP, but the modified HIP preform fabrication requires lower temperature and pressure and shorter pressing time compared to those required for conventional HIP consolidation, because a full densification is not necessary at this stage. The choice of preform processes depends on the alloy systems of the powders and the required properties of the final product. [0037] For consolidation of the preforms to full density, the ECAP step 38 in Fig. 7b is performed, whereas the prior art finishes with an extrusion step 40 shown in Fig. 7a. The ECAP process parameters to be considered include temperature and number of ECAP passes. A higher temperature provides better formability, i.e. easy densification at a lower pressure, but the tendency of grain growth increases, resulting in larger grains after the process. The lower temperature can reduce the grain growth during ECAP₁ but may require more ECAP passes and a higher pressure for the same densification requirement. [0038] Many alterations and modifications may be made by those having ordinary skill in the art without departing from the spirit and scope of the invention. For example, reacting the cryomilled powders prior to consolidation to develop desirable target phases; modifying the cross section of the ECAP die to generate various geometries that are suitable for different applications, and others. Further, it must be expressly understood that the invention applies with full generality and equivalence to conventional hot pressing in place of cold or hot isostatic pressing steps CIP, HIP.

[0039] Therefore, it must be understood that the illustrated embodiment has been set forth only for the purposes of example and that it should not be taken as limiting the invention as defined by the following claims. For example, notwithstanding the fact that the elements of a claim are set forth below in a certain combination, it must be expressly understood that the invention includes other combinations of fewer, more or different elements, which are disclosed in above even when not initially claimed in such combinations. [0040] The words used in this specification to describe the invention and its various embodiments are to be understood not only in the sense of their commonly defined meanings, but to include by special definition in this specification structure, material or acts beyond the scope of the commonly defined meanings. Thus if an element can.be understood in the context of this specification as including more than one meaning, then its use in a claim must be understood as being generic to all possible meanings supported by the specification and by the word itself.

[0041] The definitions of the words or elements of the following claims are, therefore, defined in this specification to include not only the combination of elements which are literally set forth, but all equivalent structure, material or acts for performing substantially the same function in substantially the same way to obtain substantially the same result. In this sense it is therefore contemplated that an equivalent substitution of two or more elements may be made for any one of the elements in the claims below or that a single element may be substituted for two or more elements in a claim. Although elements may be described above as acting in certain combinations and even initially claimed as such, it is to be expressly understood that one or more elements from a claimed combination can in some cases be excised from the combination and that the claimed combination may be directed to a subcombination or variation of a subcombination.

[0042] Insubstantial changes from the claimed subject matter as viewed by a person with ordinary skill in the art, now known or later devised, are expressly contemplated as being equivalently within the scope of the claims. Therefore, obvious substitutions now or later known to one with ordinary skill in the art are defined to be within the scope of the defined elements.

[0043] The claims are thus to be understood to include what is specifically illustrated and described above, what is conceptionally equivalent, what can be obviously substituted and also what essentially incorporates the essential idea bf the invention.

Claims

We claim:

1. A method of densifying a metallic powder comprising: cryomilling the metallic powder to produce a nanocrystalline metallic powder; degassing the nanocrystalline metallic powder; preforming the degassed nanocrystalline metallic powder for a subsequent equal channel angular pressing (ECAP) process according to the metallic powder being processed; and equal channel angular pressing (ECAP) the preformed degassed nanocrystalline metallic powder at a predetermined temperature and pressure.

2. The method of claim 1 where preforming the degassed nanocrystalline metallic powder comprises using the degassed nanocrystalline metallic powder in the ECAP process without further treatment.

3. The method of claim 1 where preforming the degassed nanocrystalline metallic powder comprises cold isostatic pressing (CIP) the degassed nanocrystalline metallic powder to aid in densification.

4. The method of claim 1 where preforming the degassed nanocrystalline metallic powder comprises hot isostatic pressing (HIP) the degassed nanocrystalline metallic powder at shortened pressing durations and lower temperatures than conventional hot isostatic pressing (HIP) to aid in densification.

5. The method of claim 1 where preforming the degassed nanocrystalline metallic powder comprises hot pressing (HP) the degassed nanocrystalline metallic powder.

6. The method of claim 4 where the shortened pressing durations are 30-60 minutes or less and the lower temperatures are 200-400⁰C or less, for Al and its alloys with higher temperatures and pressures for metals and their alloys.

7. The method of claim 1 where equal channel angular pressing (ECAP) the preformed degassed nanocrystalline metallic powder is repeated until desired densification of the metallic powder is obtained.

8. The method of claim 7 where the magnitude of the temperature and pressure and the number of repetitions of the step of equal channel angular pressing (ECAP) are selected in combination according densification of the metallic powder desired.

9. A densified metallic powder fabricated from the steps comprising: cryomilling the metallic powder to produce a nanocrystalline metallic powder; degassing the nanocrystalline metallic powder; preforming the degassed nanocrystalline metallic powder for a subsequent equal channel angular pressing (ECAP) process according to the metallic powder being processed; and equal channel angular pressing (ECAP) the preformed degassed nanocrystalline metallic powder at a predetermined temperature and pressure.

10. The densified metallic powder of claim 9 where preforming the degassed nanocrystalline metallic powder comprises using the degassed nanocrystalline metallic- powder in the ECAP process without further treatment.

11. The densified metallic powder of claim 9 where preforming the degassed nanocrystalline metallic powder comprises cold isostatic pressing (CIP) the degassed nanocrystalline metallic powder to aid in densification.

12. The densified metallic powder of claim 9 where preforming the degassed nanocrystalline metallic powder comprises hot isostatic pressing (HIP) the degassed nanocrystalline metallic powder at shortened pressing durations and lower temperatures than conventional hot isostatic pressing (HIP) to aid in densification.

13. The densified metallic powder of claim 9 where preforming the degassed nanocrystalline metallic powder comprises hot pressing (HP) the degassed nanocrystalline metallic powder.

14. The densified metallic powder of claim 12 where the shortened pressing durations are 30-60 minutes or less and the lower temperatures are 200-400⁰C or less, for Al and its alloys; higher temperatures and pressures for other metals and their alloys.

15. The densified metallic powder of claim 9 where equal channel angular pressing (ECAP) the preformed degassed nanocrystalline metallic powder is repeated until desired densification of the metallic powder is obtained.

16. The densified metallic powder of claim 15 where the magnitude of the temperature and pressure and the number of repetitions of the step of equal channel angular pressing (ECAP) are selected in combination according densification of the metallic powder desired.

17. A method of densifying a metallic powder comprising: cryomilling the metallic powder to produce a nanocrystalline metallic powder; degassing the nanocrystalline metallic powder; and equal channel angular pressing (ECAP) the degassed nanocrystalline metallic, powder at a predetermined temperature and pressure.

18. The method of claim 17 where cryomilling the metallic powder to produce a nanocrystalline metallic powder comprises cryomilling the metallic powder to a grain size of not more than 100nm in diameter.

19. A method of densifying a metallic powder comprising: cryomilling the metallic powder to produce a nanocrystalline metallic powder; and equal channel angular pressing (ECAP) the degassed nanocrystalline metallic powder at a predetermined temperature and pressure.

20. The method of claim 19 where cryomilling the metallic powder to produce a nanocrystalline metallic powder comprises cryomilling the metallic powder to a grain size of not more than 100 nm.

21. An apparatus for densifying a metallic powder comprising: a cryomill to produce a nanocrystalline metallic powder; a chamber for degassing the nanocrystalline metallic powder produced by the cryomill; and a die for equal channel angular pressing (ECAP) the degassed nanocrystalline metallic powder at a predetermined temperature and pressure.

22. The apparatus of claim 21 where the cryomill produces nanocrystalline metallic powder with a grain size of 100nm or less in diameter.