US20040060620A1

US20040060620A1 - High performance nanostructured materials and methods of making the same

Info

Publication number: US20040060620A1
Application number: US10/425,207
Authority: US
Inventors: Ev Ma; Yinmin Wang; Mingwei Chen
Original assignee: Johns Hopkins University
Current assignee: Johns Hopkins University
Priority date: 2000-10-05
Filing date: 2003-04-29
Publication date: 2004-04-01

Abstract

In accordance with the invention, nanostructured metallic materials having high tensile strength and increased ductility are prepared by providing a metallic material, deforming the metallic material to form a plurality of dislocation cell structures, annealing the material at a temperature from about 0.3 to about 0.7 of its absolute melting temperature, and cooling the annealed metallic material. The result is a nanostructured metal or alloy having increased tensile strength as compared with the corresponding coarse-grained material and substantially greater ductility as compared with nanostructured material made by conventional processes. Using this process applicants have made nanostructured alloys with tensile strengths in excess of 1.5 Gpa and ductility greater than 1 per cent strain-to-failure. They have also made nanostructured metals with tensile strength in excess of 400 MPa and ductility in excess of 50% strain-to-failure. The new materials are useful in a variety of applications such as rotors, electric generators, magnetic bearings, microelectromechanical devices and biomedical systems.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application is continuation-in-part of U.S. patent application Ser. No. 09/970,402 filed by T. Weihs et al. on Nov. 3, 2001 and entitled “High Performance Nanostructured materials and Method of Making the Same” which, in turn, claims the benefit of U.S. Provisional Application Serial No. 60/237,732 filed by C. H. Shang et al. on Oct. 5, 2000 and entitled “High Performance Nanostructured Materials and Methods of Making the Same”. Application Ser. Nos. 09/970,402 and 60/237,732 are incorporated herein by reference.[0001]

GOVERNMENT INTEREST

[0002] The United States Government has certain rights in this invention pursuant to Contract Number N00014-98-10600 supported by ONR and pursuant to Grant Number CMS-9877006 supported NSF.

This application also claims the benefit of U.S. Provisional Application Serial No. 60/445,700 filed by E. Ma et al. on Feb. 7, 2003 and entitled “High Tensile Ductility in a Nanostructured Metal”, which is incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to nanostructured metallic materials having high strength and ductility and to methods of making them.

BACKGROUND

Nanostructured materials are of considerable interest due to their unique mechanical properties and structural versatility. Materials with grain sizes less than one micrometer can have significantly improved mechanical properties compared to conventional coarse-grained materials. However, the starting materials, physical treatments, and fabrication conditions can significantly impact the performance of nanostructured materials.

Nanostructured materials with high yield strength and hardness have previously been fabricated. However, poor ductility was observed, especially in high-strength intermetallic compounds. Nanostructured intermetallics failed in the elastic regime under tensile stresses with virtually zero plastic strain-to-failure at room temperature, severely limiting their usage in industrial applications. This brittleness is attributed in part to flaws or porosity produced during fabrication.

Nanostructured materials are conventionally fabricated by synthesizing various powders of nanometer size and then consolidating them, as by hot pressing, into bulk articles. However, this processing does not prevent the formation of micro-flaws or porosity.

One-step methods of synthesis, such as electro-deposition, crystallization of amorphous solids, and severe plastic deformation, can produce materials without residual porosity, but these methods have several disadvantages. First, nanostructured intermetallics made by one step methods are extremely brittle. For example, nanostructured FeA1 intermetallic had high strength of 2.3 GPa, but the material exhibited such poor ductility that the strength was measurable only under compression. Second, it is difficult to form bulk nanostructured intermetallics because of the accumulation of deposition stresses. Forming bulk amorphous solids is technically complex and not practical for single-phase metallic materials. Single phase solids can be simpler to make, more stable, and may be desirable due to their magnetic, electrical, or optical properties. However, single-phase intermetallics have not shown a combination of high strength and good ductility. Nor has this problem been solved by decreasing the grain size. Decreasing the grain size is important for increasing strength, but grain size should be decreased while reducing or eliminating the flaws (cracks) and porosity in the materials.

Similar problems are encountered with nanostructured (nanocrystalline) metals. Nanocrystalline metals, particularly those with grain sizes of less than 100 nanometers, have strengths exceeding those of coarse-grained metals and alloyed metals. But conventional nanostructured metals suffer severe loss of ductility. For example, pure nanocrystalline Cu has a yield strength in excess of 400 MPa, which is six times higher than coarse-grained Cu. Its ductility, however, is greatly reduced as compared with coarse grained Cu, with a tensile elongation to failure of less than about 5 per cent and an even smaller regime of uniform deformation.

Accordingly there is a need for a method for making nanostructured metals and alloys having high tensile strength and good ductility.

SUMMARY OF THE INVENTION

In accordance with the invention, nanostructured metallic materials having high tensile strength and ductility are prepared by providing a metallic material, deforming the metallic material to form a plurality of dislocation cell structures, annealing the material at a temperature from about 0.3 to about 0.7 of its absolute melting temperature, and cooling the annealed metallic material. The result is a nanostructured metal or alloy having increased tensile strength as compared with the corresponding coarse-grained material and substantially greater ductility as compared with nanostructured material made by conventional processes. Using this process applicants have made nanostructured intermetallics with tensile strengths in excess of 1.5 Gpa and ductility greater than 1 per cent strain-to-failure. They have also made nanostructured metals with tensile strength in excess of 400 MPa and ductility in excess of 50% strain-to-failure. The new materials are useful in a variety of applications such as rotors, electric generators, magnetic bearings, microelectromechanical devices and biomedical systems.

BRIEF DESCRIPTION OF THE DRAWINGS

The advantages, nature and various additional features of the invention will appear more fully upon consideration of the illustrative embodiments described in connection with the accompanying drawings. In the drawings: [0012]
FIG. 1 shows differential scanning calorimetry traces of as-rolled and annealed Hiperco Alloy 50HS, measured at a heating rate of 40° C. per minute; [0013]
FIG. 2 shows X-ray diffraction profiles for Hiperco Alloy 50HS: (a) as-rolled, and (b) annealed at 438° C. for five hours; [0014]
FIG. 3 is an image of a nanocrystalline FeCo-based intermetallic material taken by transmission electron microscopy, showing grain size ranging from tens to hundreds of nanometers of nanostructured material. The inset shows the discontinuous ring diffraction pattern. The clusters of diffraction spots are evidence for the growth of subgrains with low-angle grain boundaries; [0015]
FIG. 4 is an image of a fracture surface for a nanocrystalline FeCo-based intermetallic with submicron dimples clearly showing the fracture is ductile; [0016]
FIG. 5 shows the results from room temperature tensile tests for nanocrystalline FeCo-based intermetallics; [0017]
FIG. 6 demonstrates room temperature strengths versus grain size of Hiperco Alloy 50HS samples; [0018]
FIG. 7 shows room temperature ductility versus grain size of Hiperco Alloy 50HS samples; [0019]
FIG. 8 shows Vickers hardness versus grain size for FeCo-based intermetallics; [0020]
FIG. 9 shows Vickers hardness as a function of annealing time for Hiperco Alloy 50HS; [0021]
FIG. 10 graphically illustrates engineering stress-strain curves for pure Cu in various forms; [0022]
FIG. 11 graphically represents tensile properties of pure Cu in various forms; [0023]
FIGS. 12A and 12B show transmission electron micrographs of the evolution of the Cu microstructure; [0024]
FIGS. 13A and 13B show transmission electron micrographs of Cu after different tensile strains; and [0025]
FIG. 14 is a schematic flow diagram showing the steps involved in fabricating nanostructured materials having increased ductility.[0026]

DETAILED DESCRIPTION

In accordance with the invention, nanostructured metallic materials are provided having a unique combination of ultrahigh tensile yield strength and large tensile ductility. The nanostructured materials may be formed from any suitable metallic material including, but not limited to pure metals (e.g., copper, nickel, iron), alloys, and intermetallic compounds (i.e. particular metallic chemical compounds based on a definite atomic formula). Nanostructured alloys and intermetallics are particularly advantageous for high strength, and nanostructured metals are particularly advantageous for high ductility. [0027]
The preferred nanostructured material has microstructures of less than one micrometer, typically grain size ranging from about 10 nanometers to about 900 nanometers. The tensile yield strength of an exemplary nanostructured alloy can exceed about 1.5 GPa, while the plastic strain-to-failure ratio is at least 0.5% and preferably at least 1%. An exemplary nanostructured metal has a tensile strength in excess of 400 MPa and a ductility in excess of 50% strain-to-failure. The precise mechanical properties desired can be achieved through controlled heat treatment. [0028]
The preferred nanostructured metallic materials are fully dense and essentially free of flaws and porosity. By “metallic materials” is meant metals, alloys and intermetallic compounds. “Fully dense” refers to materials that have a density within 0.1% of their theoretical density and “free of flaws and porosity” refers to materials that have less than 0.1 vol % pores and are essentially free of cracks at grain boundaries. “Controlled heat treatment” or annealing of deformed starting materials refers to heating the specimen in a controlled atmosphere with prescribed heat-up and ramp-down temperature rates and time periods, resulting in the formation of small, nanometer scale grains. [0029]
This description is divided into three parts: Part I describes the method of the invention. Part II describes its application in the fabrication of preferred materials for high strength applications, and Part III describes its use in fabricating preferred materials of high ductility. [0030]

I. Method of Fabricating Nanostructured Metallic Materials With High Strength And Increased Ductility

Referring to the drawings, FIG. 14 is a schematic flow diagram showing the steps in making nanostructured materials having high strength and increased ductility. The first step shown in Block A is to provide a metallic material such as a body comprising a metal alloy, an intermetallic compound or a pure metal. [0031]
The second step (Block B of FIG. 14) is to deform the metallic material to form a plurality of dislocation cell structures. Advantageously the deformation comprises plastic deformation produced by a cold-rolling process, as described generally in U.S. Pat. No. 5,501,747. The deformation advantageously achieves a reduction ratio typically from between about 50% to about 95%. In a preferred embodiment, the deformation achieves the reduction ratio is at least 80%, and preferably more than 90%. The deformation is preferably at room temperature but can also be done at lower temperatures (e.g. liquid nitrogen temperature). [0032]
The third step (Block C) is to anneal the deformed material. The annealing temperature ranges from about 0.30 to about 0.70 of the material's absolute melting temperature for time periods ranging from less than about one hour to more than about 100 hours. The annealing can be conducted in a variety of atmospheres (e.g., hydrogen, argon, and nitrogen, or air). [0033]
Following annealing, the material is cooled (Block D). The cooling rate can vary from less than about 1° C./minute to more than about 500° C./s. This process produces nanostructured materials having ultrafine grains with grain sizes from tens to hundreds of nanometers without noticeable grain growth when used at temperatures below the annealing temperature. [0034]
The steps of FIG. 14 provide a method of producing nanostructured materials by forming grains of nanometer scale (less than a micron) in the heavily deformed bulk articles through controlled heat treatments. Dislocation cell structures, ordering domains, and other chemical or phase defects act as driving forces to form nanometer-sized grains. Recrystallization and grain growth are employed to develop nanostructured microstructures of diversified grain sizes The properties of nanostructured materials depend sensitively on the grain sizes. Varying grain sizes permits one to tailor the tensile strength and ductility to meet particular needs of the material. The heat treatments can be conducted for a controlled period of time at a wide range of temperatures to drive the recovery and recrystallization processes. [0035]

II. Fabrication of Preferred Materials For High Strength Applications

For applications requiring high strength with increased ductility, the metallic material is preferably an intermetallic compound or alloy. Advantageous intermetallic compounds are single-phase alloys which form highly ordered crystalline materials. The preferred intermetallic compounds include the FeCo-based intermetallic Hiperco Alloys 50 and 50HS, available from Carpenter Technology Inc. and described in U.S. Pat. No. 5,501,747, which is incorporated by reference herein. The chemical composition of the Hiperco Alloys in weight percent is:



	Alloy Element	Composition in weight percent

	C	0.003-0.02
	Mn	0.10 max.
	Si	0.10 max.
	P	0.01 max.
	S	0.003 max.
	Cr	0.1 max.
	Ni	0.2 max.
	Mo	0.1 max.
	Co	48-50
	V	1.8-2.2
	Nb	0.03-0.5
	N	0.004 max.
	O	0.006 max.

with iron as a balance. The preferred annealing temperature is generally between 0.30 and 0.70 of the absolute melting temperature (250° C.-950° C. for Hiperco Alloy 50HS) with an annealing time from 1000 hours to several seconds. More preferred is an annealing temperature in the range 0.37-0.53 of the absolute melting temperature with an annealing time from 50 hours to several minutes. The most preferred annealing temperature is from 0.39-0.44 of the absolute melting temperature with the annealing time ranging from 20 hours to about one hour. Recrystallizing plastically deformed ingots through controlled heat treatments results in nanostructured metals, alloys, and high strength intermetallics that are fully dense and free of flaws or porosity. [0037]
Grain size can be limited to less than about one micrometer by controlling the annealing temperature and time. The controlled annealing process results in the release of energy as the defects in the material are eliminated. [0038]
FIG. 1 is a Differential Scanning Calorimetric (“DSC”) scan of Hiperco Alloy 50HS showing the exothermic heat flow as a function of temperature in comparing the “as-rolled” condition of the Hiperco Alloy to its condition after annealing. As shown in FIG. 1, the major recovery and recrystallization process of the Hiperco Alloy 50HS material occurs from between about 350 to about 705° C. Since FeCo 50HS melts at 1470° C., these temperatures correspond to 0.36 to 0.56 of the material's absolute melting temperature of 1743 Kelvin. A DSC scan is one of many tools known in the art that may be used to determine the temperature range of the recovery and recrystallization process for any given starting material. The process of cold-rolling deformation and subsequent controlled recrystallization may be repeated one or more times to obtain still finer grains and higher mechanical strengths. [0039]
Advantageously the nanostructured materials contain niobium carbide (NbC[0040] _x) particles as retarders for grain growth. Compared with the more than 99 wt % major phase, however, these second phase particles occupy only a small portion in volume. Microalloying elements such as Nb contained in the nanostructured material preferably impede grain growth by nucleating particles at grain boundaries or by Nb atoms preferentially segregating to grain boundaries to act as a grain refiner. The use of Nb in the nanostructured materials is a preferred method of maintaining the structural stability of the materials.
The fabrication for high strength applications may be more clearly understood by consideration of the following specific examples: [0041]

Example 1

Nanostructured Materials With Tensile Strength Between 1.9 and 2.3 GPA and Plastic Strain-to-Failure Between 1.3% and 5.5%

Hiperco Alloy 50HS (Co 48.68%, V 1.89%, Nb 0.31%, C 0.01%, Ni 0.11%, Mn 0.04%, Si 0.03%, Cr 0.05%, and balanced with Fe) was cold-rolled to 152.4 micrometers with a rolling reduction of 92.6%. The cold-rolled sheets were annealed in an ultrahigh purity hydrogen atmosphere at a temperature of 438° C. for five hours. The ramping rate was 2-3° C./minute. To establish ordered intermetallic structures that possess superior soft magnetic properties, the cooling rate after annealing was set at 1° C./min to 316° C. Based on the examination results of differential scanning calorimetric, cross-section high-resolution field emission electron microscopy, and transmission electron microscopy the nucleation period of the recrystallization process was largely completed after the above heat treatment, and the cold-rolled alloys were successfully transformed into nanostructured materials. [0042]
The grain sizes of the above processed nanostructured materials ranged from tens to hundreds of nanometers, with an average grain size of about 99 nanometers. The lower yield strengths ranged from 1.9 GPa to more than 2.3 GPa depending on the test orientation with respect to the rolling direction. The plastic strain-to-failure was 1.3% to more than 5.0% depending on the loading direction. The in-plane Vickers hardness was as high as 6.4 GPa. [0043]

Example 2

Nanostructured Materials With Tensile Strength Between 1.3 and 1.5 GPA and Ductility Between 11% and 18%

Hiperco Alloy 50HS alloy sheets were annealed at 650° C. for one hour. The other conditions were the same as those in EXAMPLE 1. The average grain size in these samples was 287 nanometers. The lower yield strengths ranged from 1.3 GPa to more than 1.5 GPa depending on the test orientation with respect to the rolling direction. The strain-to-failure was 11% to more than 18% depending on the loading direction. [0044]

Example 3

Nanostructured Intermetallic Materials With Fine Grain Size and High Ductility

Nanostructured intermetallics with an average grain size of 99 nm were fabricated by annealing Hiperco Alloy 50HS at 438° C. in a hydrogen atmosphere for five hours (FIG. 3). Fractographic studies show that the dominant fracture mode for the fabricated nanostructured intermetallics is ductile with submicron dimples (FIG. 4). [0045]

Example 4

Adjusting the Mechanical Properties of Nanostructured Materials by Varying Grain Size and Heat Treatment

The mechanical properties of the nanostructured materials of the invention are adjusted by varying the grain size and heat treatment of the materials. Decreasing the grain size (i.e., through use of a lower annealing temperature) increases the tensile strength and decreases the ductility (FIGS. 5 and 6). In contrast, increasing the grain size (i.e., through use of a higher annealing temperature), decreases tensile strength while increasing ductility (FIGS. 5 and 6). The lower yield tensile strengths follow a similar Hall-Petch relationship, whether samples are strained in the rolling or the transverse directions, with a slope of about 0.4 (FIG. 6). The ductility shows a peak around 500 nm, and decreases with reducing grain sizes (FIG. 7). The lowest ductility observed, about 1.3% plastic strain-to-failure, is significantly larger than that of as-rolled materials, and much larger than any other reported values for nanostructured intermetallics made by other methods. [0046]

Example 5

Vickers Hardness of the Nanostructured Materials

The hardness of the samples was measured on a LECO microhardness tester (M-400) with Vickers indents (FIG. 8). At a temperature within the major recovery and recrystallization process, the Vickers hardness was found to increase logarithmically with the annealing time (FIG. [0047] 9), suggesting that the degree of recrystallization and grain growth increases with time at a fixed annealing temperature.

Example 6

Additional Nanostructured Materials

The methods described in EXAMPLES 1-4 are applied to an a FeCo-based alloy consisting essentially of 48.78% cobalt, 1.92% vanadium, 0.05-0.31% niobium, 0.012% carbon, 0.1% nickel, balanced with iron cold-rolled to a reduction percentage of about 82.7% in thickness. [0048]

III. Fabrication of Nanostructured Materials of High Ductility

To achieve grain sizes as small as possible, very large amount of cold work can be applied to a metal or alloy. Also, the cold work can be done at subambient temperatures (e.g., by using liquid nitrogen cooling of the workpiece), especially for materials that dynamically recover fast at room temperature. The desirable mechanical properties can be obtained by manipulating the grain sizes and their distributions in the nanocrystalline and ultrafine-grained regimes: an example is the use of secondary recrystallization of the already-recrystallized material described before. By doing this one can achieve a bimodal grain size distribution and the deformation in such a nonuniform microstructure can lead to ductility approaching that of a conventional coarse-grained ductile metal, without sacrificing much of the strength. In general, the process of FIG. 14 can therefore lead to materials with strength above 1.5 GPa and yet with respectable ductility, or it can lead to materials with ductility as high as >50% or more elongation to failure in tension while maintaining a strength more than 3-5 times that of the coarse-grained and annealed counterpart. The materials also exhibit strain hardening that stabilizes the useful uniform tensile deformation to large plastic strains. [0049]
Application of the method of FIG. 14 to Cu results in a bimodal grain size distribution, with micrometer-sized grains embedded inside a matrix of nanocrystalline and ultrafine (<300nm) grains. The matrix grains impart high strength, as expected from an extrapolation of the Hall-Fetch relationship. The inhomogeneous microstructure induces strain hardening mechanisms that stabilize the tensile deformation, leading to a high tensile ductility—up to 65% elongation to failure, and 30% uniform elongation. These results permit the development of tough nanostructured metals for forming operations and high-performance structural applications including microelectromechanical and biomedical systems, as well as high strength, high conductivity metals. [0050]
FIG. 10 graphically illustrates engineering stress-strain curves for pure Cu in various forms. Curve A is annealed, coarse grained Cu. Curve B is Cu subject to room temperature rolling to 95% cold work (CW). Curve C is Cu subjected to liquid nitrogen temperature rolling to 93% CW. Curve D involves 93% CW and annealing at 180° C., 3 min. And Curve E is 93% CW and annealing at 200° C., 3 min. Note the coexisting high strength and large uniform plastic strain as well as the large overall percentage elongation to failure for the Cu of curve E. [0051]
The pure copper is very ductile in its annealed and coarse-grained form. It has an elongation to failure as large as 70% (curve A, FIG. 10), but a low yield strength (σ[0052] _y). Strengthening through heavy cold work results in a material with a tensile curve that peaks immediately after yielding (curve B, FIG. 10). Such a trend of strengthening accompanied by a loss of ductility is generally true for Cu and other metals processed in various ways.
FIG. 11 graphically represents the tensile properties of pure copper in various forms. The data are for Cu of conventional, ultrafine and nanocrystalline grain sizes and after cold rolling to various degrees of CW from applicants' own tests (filled black circles). Data points E, A, and B are from the corresponding curves in FIG. 10. Uniform elongation up to the peak in the engineering stress strain curve is compared here, not only because it is a desirable property but also because the overall percentage elongation to failure is often dominated by localized deformation (necking) whose magnitude depends on the gauge length used in the different experiments. The tough Cu developed here (E) is clearly separated from the general trend. [0053]
To provide nanostructured copper with high tensile ductility, our processing starts by rolling the Cu at liquid nitrogen temperature to a high value of percentage cold work (CW). The resulting material has a typical heavily deformed microstructure with high densities of dislocations in nanoscale networks, with some resolvable grains all below 200 nm in size. The use of the low temperature suppresses dynamic recovery, allowing the density of the accumulated dislocations to reach a higher steady-state level than that achievable at room temperature. [0054]
Transmission electron micrographs of the microstructures after recovery annealing and recrystallization are shown in FIG. 12[0055] a, 12 b. These electron micrographs show the evolution of the Cu microstructure. FIGS. 12a and 12 b show the samples used to obtain the curves D and E in FIG. 10, respectively. After annealing at 180° C. for 3 min. (FIG. 12a), recovery has occurred, and the dislocation density is much reduced. The vast majority of the grains are in the nanocrystalline/ultrafine range, with some recrystallized regions. Heat treating at 200° C. for 3 min. led to full recrystallization followed by secondary recrystallization (FIG. 12b). The recrystallized grains have well-defined, high-angle boundaries (FIG. 12b). The majority of the grains are in the nanocrystalline to ultrafine range. Meanwhile, abnormal grain growth (secondary recrystallization) has started to produce a volume fraction (˜25%) of coarser (1-3 μm) grains, some of which contain annealing twins.
The engineering stress-strain curves corresponding to the microstructures in FIG. 12 are compared in FIG. 10. After 93% CW at liquid-nitrogen temperature (curve C), the σ[0056] _yis much higher than that of the room-temperature-rolled Cu (95% CW; curve B). The elongation to failure is also significantly larger. After the 180° C. annealing, the σ_ydecreased slightly for the recovered and partially recrystallized grains, and the elongation to failure increased further (curve D). Such concurrent strengthening and toughening (in terms of post-necking elongation), as observed for both curves C and D when compared with curve B, can be attributed to the nanocrystalline/ultrafine grains that reduce the size of nucleating flaws and increase the resistance to crack propagation, leading to a higher fracture stress. Micrographs of the fracture surfaces (not shown) indicate that ductile fracture through the nucleation and coalescence of extremely fine microvoids was promoted. The radial unstable crack growth was delayed, and the stabilizing triaxial stress state was maintained to larger strains.
A marked improvement in uniform elongation was found concurrent with pronounced strain hardening, without sacrificing much of the strength, in material with the bimodal microstructure shown in FIG. 12[0057] b. The resultant stress-strain curve is shown in FIG. 10 (curve E). The large densities of defects and cold-work energy stored during processing at liquid nitrogen temperature allowed copious nucleation during recrystallization at a low temperature, so that the vast majority of the matrix grains are kept in the nanocrystalline/ultrafine grain regime to help maintain the high strength of the ‘composite’ material. Meanwhile, the grains with sufficiently large sizes obtained through secondary recrystallization, at a volume fraction of 25%, produced pronounced strain hardening to sustain the useful uniform deformation to large strains. Note that one should not restore strain hardening by allowing uniform growth of all grains or a large fraction of excessively grown grains, both of which would cause an additional unwanted drop in σ_y.
The strain hardening is needed because the onset of localized deformation (necking instability, peak in FIG. 10) in tension is governed by the Considére criterion [0058] $\begin{matrix} {(\frac{\partial σ}{\partial ɛ})}_{\dot{ɛ}} \leq σ & (1) \end{matrix}$
where σ and ε are true stress and true strain, respectively. The nanocrystalline and ultrafine matrix grains tend to lose the work hardening (left-hand side of equation (1)) quickly on deformation owing to their very low dislocation storage efficiency inside the tiny grains, especially when in presence of dynamic recovery. Such a high-strength material is therefore prone to plastic instability (early necking), severely limiting the desirable uniform elongation unless larger grains of appropriate sizes and volume fractions are present. [0059]
Our strategy is to efficiently use the moderate population of the larger grains to achieve a strain hardening rate much higher than that which would be expected from curve A (FIG. 10, for uniform deformation under uniaxial tension). We do this by taking advantage of the following three factors. First, these confined grains deformed in the inhomogeneous microstructure are under multi-axial stress states. There are complex strain paths and triaxial strain components, with very large strain gradients. It is known that a complex stress state, complicated straining patterns and dislocation configurations, and high densities of geometrically necessary dislocations are all beneficial for promoting grain refinement (or dislocation storage and strain hardening). For example, equal channel angular pressing (ECAP) uses similar conditions to make nanostructured metals in an efficient way. For a metal such as Cu, the non-uniform deformation over a length scale of the order of a few micrometers (FIG. 12[0060] b) is the realm of strain-gradient plasticity theory, which predicts significant strain hardening owing to an excessively large number of geometrically necessary dislocations that are forced to be present to accommodate the large strain gradient.
FIGS. 12[0061] a and 13 b show transmission electron micrographs of Cu after different tensile strains. The Cu sample is that shown in FIG. 12a after 6% plastic strain. The upper left inset shows the selected-area electron diffraction pattern, and the lower right inset shows the high-resolution image of the boundary region between a larger, micrometer-sized grain (L) and one of the surrounding much smaller ultrafine grains (S). A twin boundary (TB) is seen near the tip of the S protrusion into L, where twining is initiated. FIG. 13b shows the Cu after the maximum uniform strain of ˜30%.
Second, <112> {11{overscore (1)}} twinning, as shown in FIG. 13[0062] a and the selected-area electron diffraction pattern in the upper left inset, was observed unexpectedly after straining for 6% inside most of these larger grains. Deformation twins have not been observed for Cu before except at high strain rates or low, sub-ambient temperatures, and activation of such twins is known to require high stresses, especially when the grain sizes are small. Additional observations by transmission electron microscopy (for example, the high-resolution image in FIG. 13a lower right inset) show twin boundaries located preferentially near the protrusions of the surrounding nanocrystalline/ultrafine grains into the softer large grains, suggesting twinning initiation presumably due to stress concentration. The activation of the twinning mechanism suggests that these constrained larger grains plastically deform at high stresses, consistent with the high strength observed. In terms of enhancing strain hardening, twinning is known to be highly effective in conventional Cu, owing to dislocation pileups at the twin boundaries (FIG. 13a). In nanocrystalline Cu, the interfaces generated between twinned segments can act as strong barriers to dislocation motion.
Third, the larger (softer) grains accommodate strains preferentially. When the overall uniform elongation reached −30% (peak of curve E in FIG. 10), these larger grains had accumulated large numbers of twin boundaries, dislocations and subgrain boundaries such that the microstructure was refined to a level similar to the nanocrystalline/ultrafine grained matrix, FIG. 13[0063] b). Afterwards, the post-necking elongation is similar to that discussed for the sample annealed at 180° C. (curve D in FIG. 10). Overall, the nanostructured Cu of curve E (FIG. 10), when compared with the coarse grained starting material, represents an elevation of σ_yby a factor of 5-6 while maintaining comparable elongation to failure. The simultaneous high strength and ductility, especially the very large uniform deformation at the elevated strength, results in a notable gain in toughness (the area under the stress-strain curve). This is what sets this material apart from all previous treated copper materials, as demonstrated in FIG. 11.
To establish the reproducibility, three additional samples with or without the ECAP step were processed through similar CW and heat treatment. In all cases, coexisting high strength and ductility were observed. Further annealing beyond that shown in FIG. 12[0064] b caused additional grain growth and larger uniform elongation, but with a large decrease in σ_yand no gain in overall ductility owing to the decrease of the post-peak elongation (compare curve A with curve D, FIG. 10). Attempts to start with the room-temperature-rolled Cu only managed a σ_yof˜100 MPa when elongation to failure reached −50%. This emphasizes the importance of the step at liquid nitrogen temperature, which stores large cold work energy that leads to a lower recrystallization temperature (compared with room-temperature rolling, the calorimetric recrystallization/grain growth peak temperature decreased by 60° C.) and favors copious nucleation over growth. This makes it possible to achieve the nanocrystalline/ultrafine grained matrix structure through recrystallization, thus affording room for tailoring the microstructure through controlled secondary recrystallization.
Our approach does not use uniform nanocrystalline grains, which have to rely on grain boundary deformation mechanisms (such as grain boundary sliding) to contribute significantly to ductility and stabilize the plastic deformation through large increases in strain rate sensitivity. Experimental data so far (for example, FIG. 11) indicate that at ambient temperature the increase in ductility provided by grain boundary sliding in small grains is either insufficient to compensate for the loss of dislocation controlled ductility, or concurrent with a much reduced strength. [0065]
Our idea of improving strain hardening may be used to derive good ductility from other nanocrystalline materials, where abnormal grain growth is often observed. For example, after heating to a moderate temperature nanocrystalline nickel was reported to exhibit a bimodal microstructure and pronounced strain hardening under certain deformation conditions. In addition to achieving a combination of strength and ductility, our thermomechanical approach to the processing of bulk samples is also simpler than those processes required to produce uniform nanocrystalline grains; the latter processes are not only difficult or expensive to implement, but also are difficult to keep free of artifacts such as porosity and impurities. In fact, fracture due to sample flaws after consolidation or deposition, together with plastic flow instabilities such as necking and shear banding, are responsible for the very limited strain to failure observed so far in nanocrystalline materials. [0066]
The following specific examples further illuminate the nature and features of the invention. [0067]

Example 7

Nanostructured Material of High Ductility

Pure Cu (99 99%) bar from a commercial source (ESPI) was first processed by severe cold rolling, with liquid-nitrogen-temperature (LNT) cooling of the workpiece between consecutive rolling passes (−150 −C. and −100° C. before and after each pass, respectively). The degree of LNT deformation is defined using per cent cold work, [0068]
%CW=(s ₀ −s)/s ₀×100%
where s[0069] ₀and s are the cross-sectional areas before and after rolling. Some samples were processed through eight passes of equal channel angular pressing (ECAP) at room temperature before rolling, while others were subjected directly to LNT rolling The ECAP step made no obvious difference to the eventual microstructure and properties, as the very large cold work energy stored at LNT controls the subsequent recrystallization behaviour. Microhardness, as well as the recrystallization temperature and enthalpy storage measured in a calorimetric scan, levels off after about 90% CW
For mechanical property measurements, all the samples were cut and polished to a cross-section of 1 mm×1.8 mm, and a gauge length of 5 mm (previously reported tensile tests of nanostructured metals typically used a gauge length in the range of 1-5 mm. Uniaxial tensile tests were conducted at room temperature at an initial quasi-static strain rate of 5×10[0070] ⁻⁴s⁻¹.
While preferred embodiments of the invention have been described and illustrated, it should be apparent that many modifications to the embodiments and implementations of the invention can be made without departing from the spirit or scope of the invention. While the illustrated embodiments have been described utilizing a cold-rolling and controlled annealing process to produce nanostructured materials of high tensile yield strength and high ductility, it should be readily apparent that other processes may be utilized (or steps added to the processes) to produce the unique nanostructured materials in accordance with the invention. Any form of plastic deformation, particularly a shape-changing process (e.g., forging, swagging, extrusion etc.), that results in the generation of numerous dislocation structures within existing grains may be utilized. To facilitate formation of fully dense ingots, the starting materials may be melted into a liquid state by vacuum induction melting or other suitable techniques, including vacuum-based resistive furnaces, electron beam melting, reduced atmosphere melting, etc. [0071]
It can now be seen that the invention includes a method of making a nanostructured material comprising the steps of providing the metallic material, deforming the material to form a plurality of dislocation cell structures, annealing the deformed material at a temperature from about 0.3 to about 0.7 of its absolute melting temperature and cooling the material to produce the nanostructured material. The annealing temperature is advantageously in the range 0.37-0.53 of the absolute melting temperature and preferably in the range 0.39 to about 0.44. The annealing time is advantageously from about 1000 hours to several seconds and preferably from about 50 hours to several minutes. In one form, the method involves annealing from about 0.39 to 0.44 of the absolute melting temperature from about 20 hrs to about 1 hr. with time and temperature to achieve a ductility of at least about 1% plastic strain-to-failure and a tensile elastic yield strain of at least about 0.5%. [0072]
The deforming step can advantageously comprise cold rolling with a thickness reduction ratio in the range from about 50% to about 95%. Advantageously the ratio is at least about 80% and preferably at least about 90%. [0073]
In another aspect, the invention comprises a nanostructured metallic material having a tensile yield strength of at least about 1.5 GPa and a ductility of at least about 1 percent strain-to-failure. The material is composed of microstructures with an average grain size of at least 10 nanometers and preferably in the range from about 10 nanometers to about 900 nanometers. The material can typically have a tensile elastic yield strain of at least about 0.5% and a ductility from about 1 to about 18% plastic strain-to-failure. It can have ductility typically from between about 1.3 to about 5.5 percent plastic strain-to-failure, and it can have a Vicker's hardness typically of about 5.5 to about 10 Gpa. [0074]
In yet another aspect, the invention comprises a nanostructured metallic material having a tensile yield strength of at least about 400 MPa and a ductility of at least about 5% strain-to-failure. The ductility can be 30 percent or higher. In preferred metals the strength can be in excess of three times that of the conventional coarse-grained metal and the ductility can be in excess of 50 percent strain-to-failure. [0075]
The deforming can comprise cold working a metal at reduced temperatures, e.g. liquid nitrogen temperature. The cold worked metal can then be heat treated to recrystalization and secondary recrystallization. [0076]
It is understood that the above-described embodiments are illustrative of only a few of the many possible specific embodiments, which can represent applications of the invention. Numerous and varied other arrangements can be made by those skilled in the art without departing from the spirit and scope of the invention. [0077]

Claims

What is claimed:

1. A method of making a nanostructured metallic material comprising the steps of:

providing a metallic material;

deforming the metallic material wherein a plurality of dislocation cell structures are formed;

annealing the metallic material at a temperature from about 0.3 to about 0.7 of its absolute melting temperature; and

cooling said metallic material to produce nanostructured material.

2. The method of claim 1, wherein said temperature is from about 0.37-0.53 of its absolute melting temperature.

3. The method of claim 1, wherein said temperature is from about 0.39 to about 0.44 of its absolute melting temperature.

4. The method of claim 1, wherein said temperature is at least about 350 degrees Celsius.

5. A method of adjusting the tensile strength of a nanostructured material comprising:

providing a metallic material;

annealing the metallic material at a temperature from about 0.30 to 0.70 of its absolute melting temperature for a time from about 1000 hours to several seconds; and

cooling the metallic material.

6. A method of adjusting the ductility of a nanostructured crystalline material comprising the steps of:

providing a metallic material;

deforming said metallic material so that a plurality of dislocation cell structures are formed;

annealing said metallic material at a temperature from about 0.37 to 0.53 of its absolute melting temperature for a period of time from 50 hours to several minutes; and

cooling said metallic material after said annealing step.

7. A method of adjusting the ductility of a nanostructured crystalline material comprising the steps of:

providing a metallic material;

annealing said metallic material at a temperature from about 0.39 to about 0.44 of its absolute melting temperature for a period of time from about 20 hours to about 1 hour, wherein the temperature and time are selected to achieve a ductility of at least about 1% plastic strain-to-failure and a tensile elastic yield strain of at least about 0.5%; and

cooling said metallic material after said annealing step.

8. The method of claim 5 wherein said deforming step further comprises cold rolling said metallic material with a thickness reduction ratio in the range from about 50% to about 95%.

9. The method of claim 8 wherein said thickness reduction ratio is at least about 90%.

10. The method of claim 8 wherein said thickness reduction ratio is at least about 80%.

11. A nanostructured metallic material having a tensile yield strength of at least about 1.5 GPa and a ductility of at least about 1 percent strain-to-failure.

12. The nanostructured material of claim 11, further comprising microstructures with an average grain size ranging from about 10 nanometers to about 900 nanometers.

13. The nanostructured material of claim 11, further comprising microstructures with an average grain size of at least 10 nanometers.

14. The nanostructured material of claim 11 having a tensile elastic yield strain of at least about 0.5% and a ductility from about 1 to about 18 percent plastic strain-to-failure.

15. The nanostructured material of claim 11, wherein said ductility is from between 1.3 to about 5.5 percent plastic strain-to-failure.

16. The nanostructured material of claim 11, wherein said the nanostructured material has a Vicker's hardness of about 5.5 to about 10 GPa.

17. Nanostructured magnetic materials, wherein the materials are cold-rolled and annealed at a temperature ranging from about 350 to about 705 degrees Celsius, have a room temperature yield strength in excess of about 1.2 GPa and tensile ductility in excess of about 1% plastic strain-to-failure.

18. The nanostructured magnetic materials of claim 17, wherein the materials consist essentially of about 0.003% to about 0.02% C, no more than about 0.10% Mn, no more than about 0.10% Si, no more than about 0.01% P, no more than about 0.003% S, no more than about 0.1% Cr, no more than about 0.2% Ni, no more than about 0.1% Mo, from about 48 to about 50% Co, from about 1.8 to about 2.2% V, from about 0.03 to about 0.5% Nb, no more than about 0.004% N, and no more than 0.006% O, and iron as the balance.

19. The nanostructured magnetic materials of claim 17, wherein said materials consist essentially of 48.78% cobalt, 1.92% vanadium, 0.06% niobium, 0.012% carbon, 0.1% nickel, balanced with iron.

20. A nanostructured metallic material having a tensile yield strength of at least about 400 MPa and a ductility of at least about 5 percent strain-to-failure.

21. The nanostructured material of claim 20 wherein the ductility is at least 30 percent strain-to-failure.

22. The nanostructured material of claim 20 wherein the metal comprises copper.

23. The nanostructured material of claim 20 wherein the metal consists essentially of copper.

24. The nanostructured material of claim 20 wherein the nanostructured metal has a strength in excess of 3 times the strength of the conventional coarse-grained metal and a ductility in excess of 50 percent strain-to-failure.

25. The method of claim 1 wherein the nanostructured metallic material is metal and the deformation comprises cold working the metal.

26. The method of claim 25 wherein the metal is cold worked at liquid nitrogen temperature.

27. The method of claim 25 wherein the cold worked metal is heat treated to recrystallization and secondary recrystallization.