Method and apparatus for making high strength metals with a face-centered-cubic structure

Abstract

A process for increasing the strength of pure copper and other fcc matrix alloys while maintaining ductility. The method is particularly applicable to face-centered-cubic materials that undergo dynamic recovery when strain-hardened at room temperature. The material is first subjected to equal channel angular pressing to create an ultra fine grain structure (“UFG”). The UFG sample is then subjected to cryogenic drawing and finally to subcryogenic deformation.

Description

MICROFICHE APPENDIX

Not Applicable

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the field of materials. More specifically, the invention comprises a method and apparatus for strengthening metals while limiting the dynamic recovery phenomenon, and maintaining ductility and electrical conductivity.

2. Description of the Related Art

It is possible to strengthen many metallic materials through work-hardening (sometimes called “strain hardening”). Work hardening is applied very often to materials having a face-centered cubic (“fcc”) structure. Such materials form atomic matrices, where the basic unit is a cube having an atom located at each of the cube's eight corners and in the middle of each of the cube's six faces. Austenite stainless steels, aluminum and copper alloys can assume this structure, with substutional atoms in the matrix at the corners and the faces of the cube.

The cubic structure only remains uniform within a particular region, often called a “grain.” If the boundary between the uniform regions is composed of dislocations, the uniformed cubic region is called “cell”. Thus, when viewed on a microscopic level, a piece of fcc metal is comprised of many interlocking grains or cells. Within each of these grains or cells is a relatively uniform fcc matrix.

Plastically deforming a metal introduces dislocations or second phase into its crystalline structure within grains. These generally tend to make the material harder and stronger. A typical example is drawing rod-shaped metal samples into smaller and smaller wires. The drawing process creates strain hardening. As the drawing strain increases, the ultimate tensile strength (“UTS”) increases. Drawing copper wire in this fashion produces hardened wire, which has a much higher UTS than annealed copper.

It is generally not desirable for a strain-hardening rate to diminish. This depends upon the retention of the dislocations added to the material. For most materials that have ability for forming the second phase by deformation, retention of the strain hardening is not an issue. However, some materials experience a phenomenon known as “dynamic recovery.” When this occurs, added dislocations are “undone” over time. The material lapses partially or fully back into its undeformed state, thereby destroying the desirable effects of strain hardening. Copper is a good example of a material which exhibits dynamic recovery.

Those skilled in the art will know that copper and its alloys have been used for many years in many electrical devices. In the design of such devices, one must consider the mechanical strength of the conducting material and its conductivity. These considerations are often in opposition. Pure copper is relatively weak and ductile. It can be strengthened by adding other alloying elements. However, the addition of such elements reduces its conductivity significantly.

Pure copper can also be strengthened by strain hardening. This process reduces the copper's conductivity moderately. However, the strength achievable is limited due to the dynamic recovery phenomenon. A method of strain hardening copper while reducing the effect of the dynamic recovery phenomenon would therefore be desirable. The retention of suitable ductility in the strain-hardened copper would also be desirable.

BRIEF SUMMARY OF THE PRESENT INVENTION

The present invention comprises a process for increasing the strength of pure copper and other fcc matrix alloys while maintaining good ductility and electrical conductivity if the materials are intended to be used as conductors. A material sample is first subjected to equal channel angular pressing to produce an ultra fine grain (“UFG”) structure. The UFG structure is then subjected to cryogenic drawing to reduce the cross-section and increase the strain density. Finally, the sample can be produced into a variety of shapes by different techniques, as long as the sample is precooled in liquid nitrogen. The inventive process can produce materials having relatively large sizes and long lengths.

The inventive method attains high strength through the stable accumulation of very high dislocation densities. The work hardening rate is changed by deforming the material under cryogenic conditions. The methodology can potentially be applied to many different materials which suffer dynamic recovery and consequent low strain hardening when deformed at room temperatures.

The inventive method can also produce high density twins. Neither the twins nor the dislocations will have a significant effect on the wire's conductivity.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a comparison of strain hardening for two copper materials deformed at room temperature.

FIG. 2 is a comparison of the stress-strain curves for the same Cu+Al₂O₃ material deformed at room temperature and under cryogenic conditions.

FIG. 3 is a comparison of the strain hardening rates for the same Cu+Al₂O₃ material deformed at room temperature and under cryogenic conditions.

FIG. 4 is an elevation section view showing the use of a diameter reducing die to strain harden a material under cryogenic conditions.

FIG. 5 is an elevation view, showing the use of reels to pass material through a diameter reducing die.

FIG. 6 is an elevation view, showing an alternate embodiment of the device shown in FIG. 5.

FIG. 7 is a graphical depiction of the grain boundaries within a strain-hardened sample.

FIG. 8 is a graphical depiction of the grain boundaries within a strain-hardened sample made according to the present invention.

FIG. 9 is a perspective view, showing a die used for equal channel angular pressing.

FIG. 10 is a perspective view of a sample.

FIG. 11 is an elevation view, showing the roll forming step of the process.

REFERENCE NUMERALS IN THE DRAWINGS

10	cell	12	cell boundary
14	wire axis	16	cryostat
18	wire	20	cryogenically deformed wire
22	drawing die	28	liquid nitrogen
30	deformation zone	32	ambient atmosphere
34	draw reel	36	payoff reel
40	idler pulley	42	ECAP die
44	die half	46	die half
48	entrance channel	50	exit channel
52	ram	54	sample
56	pin	58	hole
60	bend	62	longitudinal axis
64	subcryo rolled sample (first stage)
66	subcryo rolled sample (second stage)
68	rolling drum

DETAILED DESCRIPTION OF THE INVENTION

The present invention contemplates strain hardening materials wherein part of the inventive process is carried out under cryogenic conditions. Bulk ultrafine-grained (“UFG”) materials produced by severe plastic deformation usually have high strength, but relatively low ductility at ambient temperatures. This low ductility of UFG materials is attributed to the defects generated by severe plastic deformation and insufficient strain hardening due to an inability to accumulate dislocations. The present invention addresses this problem by first creating a UFG material, and then plastically deforming it under cryogenic conditions.

The use of plastic deformation under cryogenic conditions to actually increase ductility of a UFG material at room temperature is counterintuitive. Strength and ductility must often be traded off against each other, as processes tending to increase one also tend to decrease the other. This principle applies to UFG materials as well. The low ductility of UFG materials has invariable limited their applications. The present inventive process increases the ductility of UFG materials without any significant loss in strength.

Those skilled in the art will know that strain hardening can be produced by virtually any device which plastically deforms a sample of material. Strain hardening devices include rolling mills (sometimes called “rolling dies”), punches, breaks, roll formers, and drawing dies. One familiar example of strain hardening is drawing a metal wire through a diameter reducing die (a “wire drawing die”). Such a dies plastically deforms the wire to a smaller diameter. A series of such dies can produce progressively smaller and smaller diameters.

The present inventive method uses conventional strain hardening devices. However, for a portion of the process, both the strain hardening device and the material being strain hardened are immersed in a bath of cryogenic liquefied gas. This approach is used to reduce the dynamic recovery phenomenon in face-centered-cubic materials.

Some examples using cryogenic strain-hardening alone are illustrative. The cryogenic deformation process is well suited to the creation of strain hardened conductive wires. It can be used to create a high strength pure copper conductor, a copper alloy conductor made with copper+Al₂O₃, and other materials.

The copper material is preferably strain hardened at a temperature of about 77 K (the approximate temperature of liquid nitrogen boiling at atmospheric pressure). Once the strain hardening is performed, the drawn material is allowed to return to room temperature. Further shaping can also be done as long as the samples are pre-cooled with the liquid nitrogen.

FIG. 1 is a plot of strain hardening characteristics for two copper samples worked at room temperature. The copper samples are C10100 and UNS-C15715. C10100 is 99.99% pure copper (by weight). UNS-C15715 is a Cu+Al₂O₃ alloy. The unalloyed strength of the copper is increased in UNS-C15715 by the aluminum oxide dispersion in the existing copper matrix.

Both the pure copper and the aluminum oxide dispersion-strengthened copper were strain hardened at room temperature (295 K). For the pure copper (C10100), no further strain hardening occurred after a pure strain of about 1.5. The UNS-C15715 sample can be further strain hardened above the strength of pure copper. Pure copper can be strain hardened at room temperature to a strength level of about 450 MPa. UNS-C157115 can be strain hardened at room temperature to about 580 MPa.

In order to achieve higher strength one would naturally seek to increase the deformation strain. However, when this is done at room temperature, the rate of change in the strain hardening (dσ/dε) approaches zero. This occurs because fcc metals (such as copper) exhibit dynamic recovery beyond a certain amount of strain. For fcc metals strengthened with alloying elements the rate of change in the strain hardening remains greater than zero, but it is very small.

FIG. 2 shows the stress-strain curve for UNS-C15760 (another aluminum oxide dispersion-strengthened copper). The lower curve reflects strain hardening conducted at room temperature. The upper curve represents strain hardening conducted at 77 K. The reader will observe how the room temperature curve flattens, while the upper curve continues to increase in strength for increasing strain.

FIG. 3 shows a comparison of the strain hardening rate of UNS-C15760 for strain hardening performed at room temperature versus strain hardening performed at 77 K. The upper curve represents the 77K sample and the lower curve represents the room temperature sample. The reader will observe how the strain hardening rate is significantly increased for the sample hardened at 77 K.

From FIGS. 1 through 3, one may conclude that the application of strain hardening in a cryogenic environment has significant advantages. If the deformation is performed at cryogenic temperatures, much higher flow stress levels and a significantly higher strain hardening rate can be achieved.

FIGS. 2 and 3 refer to an aluminum oxide strengthened copper. In the region above about 800 MPa (for the 77 K sample), dislocation accumulation occurs due to the presence of the nano-scale alumina particles. This accumulation promotes the formation of dislocation cells. The size of the cell can reach even nano-grain ranges in the copper. In-face cells on the scale of 100 nm have been observed in the pure copper deformed at 77 K.

At a stress of 800 MPa, the material hardens at a rate of approximately 2000 MPa (strain being unit less). This strain hardening rate is approximately G/16, where G is the shear modulus of copper (about 33 GPa). For room temperature strain hardening, the rate is only about G/200. Thus, cryogenic deformation produces a drastic change in the material properties, with a significant increase in the strain hardening rate. The result is that a much higher UTS can be obtained using less strain.

Impurities introduced into copper often reduce the conductivity of the resulting alloy in comparison to pure copper. Impurities can change the resistivity by a factor of two per 1% of added impurity. Using the present cryogenic strain hardening process for pure copper or copper dispersion-strengthened by aluminum oxide only introduces dislocations. It does not produce a dissolution of any second phase, such as the aluminum oxide. Therefore, the decrease in conductivity resulting from the cryogenic strain hardening process is mainly due to dislocation accumulation.

This fact allows the change in resistivity to be used to calculate an estimation for dislocation accumulation. The resistivity of pure copper cryogenically drawn to a strain of 2.3 is 1.79×10⁻⁸ Ω·m (at room temperature). This figure represents only about a 4% increase over the resistivity of pure annealed copper. Therefore, although the cryogenic drawing process increases the strength considerably, the increase in resistivity is minimal.

In order to perform the strain hardening under cryogenic conditions, the sample must be immersed in a cryogenic liquid. FIG. 4 is a sectional elevation view through a cryostat 16 containing liquid nitrogen 28. The term “cryostat” will be understood to encompass any vessel capable of containing a cryogenic liquefied gas. Vessels using a Dewar-type evacuated wall construction work well in this application, but other constructions are possible.

The liquid nitrogen is vented to the atmosphere, resulting in the maintenance of its boiling temperature of about 77 K. An fcc metal is placed in the cryostat along with the strain-producing device. In the particular example shown, a copper or copper alloy wire 18 of a first diameter is placed within the vessel. It is forced through drawing or extrusion die 22 (a diameter reducing die) which draws or extrudes it down to a second reduced diameter as the wire passes through deformation zone 30. Cryogenically deformed wire 20 remains in the liquid nitrogen for some period before exiting the cryostat and returning to ambient atmosphere 32. Thus, the strain hardening occurs at cryogenic temperatures.

Those skilled in the art will realize that the device illustrated in FIG. 4 is only necessary when a large diameter rod is required. In this case, the sample length is usually short. Wire drawing operations generally involve long lengths of wire, requiring the use of drums or spools to maintain desirable tension. FIG. 5 shows an elevation view of a more sophisticated device. A liquid nitrogen cryostat 38 is used to house payoff reel 36. It pays out copper wire of a first diameter. The wire is drawn through a wire drawing die 22 as before (The die is also immersed in the liquid nitrogen contained within the cryostat). The wire is then taken up on the rotating draw reel 34, which also applies tension.

The top of the cryostat is depicted as being open, though in reality it would need to be mostly sealed in order to prevent an inordinately high boil-off rate. The drawn wire would likely exit the cryostat through a small opening through a well insulated wall.

FIG. 6 shows an alternate embodiment in which the payoff reel and draw block are both located outside the cryostat. Two submerged “idler” pulleys 40 are preferably used to maintain tension on the wire as it passes through the cryostat. However, the larger payoff reel and draw reel remain outside the cryostat. This fact means that the cryostat can be smaller and that the payoff reel and draw reel do not need to be designed to withstand the extremely low temperatures found in the cryostat. As for the embodiment of FIG. 5, the top of the cryostat shown in FIG. 6 would actually be closed with only small openings provided for the passage of the wire.

FIGS. 7 and 8 illustrate some of the microstructures of the deformed fcc materials and serve to explain some of the advantages obtained by performing the present inventive method. FIG. 7 shows a graphical depiction of the cell/grain boundaries in a strain-hardened fcc metal. The image is a cross sectional view, with the section being transverse to the direction in which the metal was drawn. The left hand view shows a sample which was drawn at room temperature (“RT Sample”), whereas the right hand view shows a sample which was drawn under cryogenic conditions (“LNT Sample”) using the present process.

The RT Sample shows elongated cells. The LNT Sample, in contrast, shows cells having a generally circular cross section. A cell size for the LNT Sample is about 150 nm, whereas a cell size for the RT Sample is about 190 nm. The cell boundaries are composed of dislocations. The larger cell sizes are corresponding to lower densities of dislocations, which are a result of dynamic recovery during the strain hardening. The LNT sample shows more variation in microstructure, which again is a result of the reduction in the dynamic recovery phenomenon. For instance, one observes twins in LNT samples. Such microstructure differences contribute to the property differences between the two types of samples.

FIG. 8 shows another graphical depiction of the grain structure in an fcc metal sample, with the section this time being taken in the same plane as the direction in which the metal was drawn. The reader will observe how the dislocation cells are highly oriented, generally aligning with the direction of drawing.

In the present inventive process, an additional step is added before the cryogenic operations. The process preferably starts with the use of equal channel angular pressing (“ECAP,” which is also known as “equal channel angular extrusion”) to produce an ultra fine grain (“UFG”) sample in order to reduce the deformation required for cryogenic deformation but keep the sample geometry in a relatively large size. ECAP is a process which is capable of forming very high dislocation densities without significantly changing the cross-section of the sample. FIG. 9 shows a representative ECAP die 42. The die is split into two halves for manufacturing and assembly convenience. Die half 46 is mated to die half 44 by conventional registration devices such as pins 56 and holes 58. The two halves are clamped together using substantial force, such as through the use of a hydraulic system or simply bolting the two halves together.

The ECAP die includes entrance channel 48 and exit channel 50 joined by bend 60. Those skilled in the art will know that ECAP dies use various angles defining the relationship between the entrance channel and the exit channel. For the specific embodiment shown in FIG. 9, a 90 degree angle is formed between the entrance and exit channels. Sample 54 is placed in the entrance channel. The sample may be any reasonably ductile fcc material, with high purity copper (99.99%) being a good example. With the die assembled, ram 52 forces sample 54 around bend 60 and into exit channel 50.

FIG. 10 shows sample 53 in more detail. The sample is a cylindrical solid having a diameter of about 20 mm. Longitudinal axis 62 passes down its center. The sample as it emerges from the ECAP die will not typically have perfectly square ends, so the depiction in FIG. 10 should be viewed as representative without being completely accurate.

Once the sample emerges from the ECAP die, it is rotated 90 degrees about the longitudinal axis as shown in FIG. 10. The sample is then fed back through the ECAP die. This step is repeated up to 12 passes through the die, with the sample being rotated 90 degrees about the longitudinal axis between each pass. As those skilled in the art will know, this is referred to as a “processing route B_C” ECAP procedure.

The sample emerging from the ECAP process still has a diameter of about 20 mm. The processes will be repeated until the microstructure of the sample reaches UFG but before the defects form. If the defect forms before UFG, the deformation should stop before the defect formation. This material is then fed into the next stage of the process. FIGS. 4-6 previously disclosed simplified representations of devices which can be used for drawing operations under cryogenic conditions.

Of course, a large diameter ECAP UFG sample may have to be linearly fed through a die, as shown in FIG. 4, since it may be impractical to wind it around a spool. In any event, the cylindrical UFG sample emerging from the ECAP process is placed in the cryostat along with the strain-producing device. In the particular example shown, the UFG copper sample is forced through a drawing die 22 which draws it down to a second reduced diameter as the wire passes through deformation zone 30. The process can be a drawing process, an extrusion process, or a combination of both (The term “drawing die” should be understood to include any such variation). A single drawing die may be used, or multiple progressive drawing dies may be used.

As an example, the diameter of the sample can be drawn down from 20 mm to about 4.5 mm. The cryogenically deformed sample remains in the liquid nitrogen for some period before exiting the cryostat and returning to ambient atmosphere. Thus, the strain hardening occurs at cryogenic temperatures.

A second plastic deformation operation is also preferably carried out under subcryogenic conditions in order to achieve various shapes necessary for applications. For instance, the wire/rod can be rolled into strips. The sub-cryogenic condition is achieved by cooling the sample to a temperature of 77 K. The reduced-diameter sample is roll-formed to produce a bar (or sheet) having a significantly reduced thickness. FIG. 11(A) shows this step in the process. A pair of rolling drums 68 (collectively referred to as a “rolling die”) are placed in a cryostat 16. Cryogenically deformed wire 20 is passed between the rotating drums to produce sub-cryo rolled sample (first stage) 64.

The roll forming may be repeated in multiple stages by using additional rolling drum sets or simply moving a single set of rolling drums closer together and passing the sample through multiple times. In multiple stage configurations, the samples need to be cooled between stages. FIG. 11(B) shows a separate rolling operation which further reduces the thickness of the cryo rolled sample. In this example, the rolling process (which is performed in multiple stages) continues until the 4.5 mm diameter wire is transformed into a sheet having a thickness of about 0.2 mm.

The sample created using the inventive process is thereby a UFG material which has undergone an ECAP step, a cryogenic drawing step, and a sub-cryogenic rolling step.

Such a material may then be referred to as “UFG_ECAP+D+R.”

The use of strain hardening at cryogenic temperatures is believed to: (1) produce a stable accumulation of very high dislocation densities; (2) therefore alter the microstructure (as will be explained in more detail subsequently); and (3) increase the work hardening rate.

The process is particularly suitable for strengthening pure copper and copper matrix materials. These materials can be used as electrical conductors in many areas, such as high field pulsed magnets, high efficiency motors, and medical sensors. The process achieves the increase in strength without significantly reducing conductivity.

The application of cryogenic strain hardening to the UFG ECAP material produces synergistic advantages. The higher strength of UFG_ECAP+D+R copper is in part a direct consequence of the smaller cell size when compared with copper passing through the ECAP process alone. However, it is reasonable to assume that the high density of twin boundaries in UFG_ECAP+D+R copper act as barriers inhibiting dislocation slip and thereby further strengthens the material. In addition, UFG_ECAP+D+R copper contains a large fraction of high angle grain boundaries in comparison with material which has only undergone the ECAP process.

The high twin density and large fraction of high angle grain boundaries also produce a high strain-hardening rate and consequently a higher uniform elongation and elongation to failure. Pre-existing deformation twins likely play a role in improving the strain hardening rate.

An example of the transformation of mechanical properties through the steps employed in the inventive process may benefit the reader's understanding. Coarse-grained copper has a 0.2% (strain) yield strength of about 40 MPa. An ECAP-processed UFG copper has a 0.2% yield strength of about 410 MPa. By contrast, the 0.2% yield strength of UFG_ECAP+D+R copper is increased to about 500 MPa. Even more significantly, the UFG_ECAP+D+R sample undergoes strain-hardening giving a uniform elongation of about 3.5% and a subsequent elongation to failure of about 11.8%.

The reader will note in the preceding that the ECAP process alone substantially increases the strength of the copper. It does so, however, at the expense of ductility (the classic trade-off). The ECAP sample actually strain softens after a small plastic strain (only about 2%). Thus, the ECAP sample cannot be practically “worked” into higher strength than ECAP+D.

The UFG_ECAP+D+R sample, in contrast, displays a positive strain hardening up to about 7%. Thus, one can conclude that the substantially higher ductility of the UFG_ECAP+D+R sample is caused primarily by its higher strain-hardening rate.

The mechanical properties of bulk solids are controlled by the microstructure. Investigations by transmission electron microscopy (“TEM”) indicate that the ECAP-only sample contains a large fraction of low-angle grain/cell boundaries. After being subjected to the subsequent cryogenic deformation processes, the same sample had numerous high angle grain boundaries and deformation twins. The subcryo-deformation (rolling in the example given in this document) provides the required geometry of the samples. Thus, the three-stage process (ECAP+cryo drawing/extrusion+subcryo deformation) produces significant advantages.

One can perform the ECAP preliminary deformation if large sizes of components are needed so that the preliminary dislocation densities can be established. At the end of the ECAP stage, it is expected that dislocation cell sizes can reach about 200 nm and the dislocation wall thickness can reach about 10 nm. The material can then be further deformed at cryogenic temperatures to further increase the dislocation densities and create twins.

Although the preceding description contains significant detail, it should not be construed as limiting the scope of the invention but rather as providing illustrations of the preferred embodiments of the invention. As an example, although copper and copper alloys have been discussed in the descriptions, the inventive method could be applied to other fcc metals which exhibit dynamic recovery. The inventive process could likewise be carried out in many different ways. As an additional example, sheet material could be fed through a small rolling mill (which would reduce the sheet thickness and increase its width) contained in a cryostat. The rolling mill would be the strain producing device and would be equivalent to the drawing dies shown in the illustrations. Thus, the scope of the invention should be fixed by the following claims rather than the examples given. The inventors acknowledge the helpful discussion provided by Dr. J. D. Embury.

Claims

1. A method for strain hardening a length of metal having a longitudinal axis while reducing dynamic recovery, maintaining ductility and electrical conductivity, comprising: a. passing said length of metal through an equal channel angular pressing die for a number of passes sufficient to transition said metal to an ultra fine grained structure;b. providing a drawing or extrusion die, located within a vessel and immersed in a cryogenic liquefied gas;c. passing said length of metal through said drawing/extrusion die while a portion of said length of metal proximate said drawing die and said drawing die remain immersed in said cryogenic liquefied gas;d. providing a rolling die; ande. passing said length of metal through said rolling die while a portion of said length of metal precooled with cryogenic liquefied gas.

2. A method of strain hardening as recited in claim 1, wherein said cryogenic liquefied gas is liquid nitrogen.

3. A method of strain hardening as recited in claim 1 wherein said length of metal is rotated 90 degrees about said longitudinal axis between each successive pass through said equal channel angular pressing die.

4. A method of strain hardening as recited in claim 1, wherein said length of metal is fed into said drawing die from a payoff reel, through said drawing die, and onto a draw reel.

5. A method of strain hardening as recited in claim 4, wherein said payoff reel is contained within said vessel and said draw reel is located outside said vessel.

6. A method of strain hardening as recited in claim 4, wherein both said payoff reel and said draw reel are located outside said vessel.

7. A method of strain hardening as recited in claim 6, wherein said wire is fed from said payoff reel to a first idler pulley, then through said drawing die, then to a second idler pulley, and then to said draw reel.

8. A method of strain hardening as recited in claim 7, wherein said first and second idler pulleys are contained within said vessel.

9. A method for strain hardening a length of metal having a longitudinal axis while reducing dynamic recovery and maintaining ductility, comprising: a. providing a volume of cryogenic liquefied gas;b. providing a drawing or extrusion die;c. providing a rolling die;d. immersing said drawing die and said deformation die in said volume of cryogenic liquefied gas;e. passing said length of metal through an equal channel angular pressing die for a number of passes sufficient to transition said metal to an ultra fine grained structure;f. thereafter passing said length of metal through said rolling die while immersed in said volume of cryogenic liquefied gas; andg. thereafter passing said length of metal through said rolling die while said metal is precooled by cryogenic liquefied gas

10. A method of strain hardening as recited in claim 9, wherein said cryogenic liquefied gas is liquid nitrogen.

11. A method of strain hardening as recited in claim 9 wherein said length of metal is rotated 90 degrees about said longitudinal axis between each successive pass through said equal channel angular pressing die.

12. A method of strain hardening as recited in claim 9, wherein said length of metal is fed into said drawing die from a payoff reel, through said drawing die, and onto a draw reel.

13. A method of strain hardening as recited in claim 12, wherein said payoff reel is contained within said vessel and said draw reel is located outside said vessel.

14. A method of strain hardening as recited in claim 13, wherein both said payoff reel and said draw reel are located outside said vessel.

15. A method of strain hardening as recited in claim 14, wherein said wire is fed from said payoff reel to a first idler pulley, then through said drawing die, then to a second idler pulley, and then to said draw reel.

16. A method of strain hardening as recited in claim 15, wherein said first and second idler pulleys are contained within said vessel.

17. A method of strain hardening as recited in claim 3, wherein said cryogenic liquefied gas is liquid nitrogen.

18. A method of strain hardening as recited in claim 4, wherein said cryogenic liquefied gas is liquid nitrogen.

19. A method of strain hardening as recited in claim 5, wherein said cryogenic liquefied gas is liquid nitrogen.

20. A method of strain hardening as recited in claim 6, wherein said cryogenic liquefied gas is liquid nitrogen.