Preparation of nanostructured materials having improved ductility

[0001] This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/714,794, filed Sep. 7, 2005, hereby incorporated by reference.

[0002] This invention was made with government support under Contract No. W-7405-ENG-36 awarded by the U.S. Department of Energy. The government has certain rights in the invention.

[0003] The present invention relates generally to nanostructured materials and more particularly to the preparation of nanostructured materials having improved ductility.

[0004] Nanostructured (NS) materials formed by severe plastic deformation techniques such as rolling, drawing, and extrusion are much stronger than their coarse-grained (CG) counterparts. However, applications for these types of nanostructured materials are limited because these techniques, which reduce the grain size and increase the strength, also reduce the ductility. As a result, these types of nanostructured materials possess very low (nearly zero) uniform tensile elongation, which results in necking immediately after yielding in tension. The onset of localized deformation in tension is governed by the equation

$\begin{array}{cc}{\left(\frac{\partial \sigma }{\partial \varepsilon }\right)}_{\varepsilon }\le \sigma & \left(1\right)\end{array}$

where σ is the true stress, and ε is the true strain. The loss of strain hardening during severe plastic deformation results from the reduction of the dislocation storage capacity in the tiny grains of the materials. The loss of dislocation storage capacity in these high strength materials makes them prone to plastic instability (early necking), which leads to lower uniform elongation.

[0005] Recent efforts have been made in improving the ductility of nanostructured materials by increasing the dislocation storage capacity of the grains to regain the strain hardening that is lost due to small grain sizes. The strategies employed in regaining the strain hardening generally involve tailoring the microstructures of the materials and changing the tensile conditions. Y. Wang et al. in “High Tensile Ductility in a Nanostructured Metal,” Nature, vol. 419, October 2002, pp. 912-916, for example, describe a method for improving strain hardening in copper by rolling copper at liquid nitrogen temperature to suppress dynamic recovery and allow the density of accumulated dislocations to reach a higher steady state level than what can be achieved at room temperature. Afterward, the material is annealed at a temperature of 180 degrees Celsius. The result is a material having a bimodal grain size distribution of micrometer size grains embedded in a matrix of nanocrystalline and ultrafine grains. The matrix grains impart high strength while the inhomogeneous microstructure induces strain hardening in the material. This method has also been described in U.S. Published Patent Application Number 2004/0060620 to Ma et al. entitled “High Performance Nanostructured Materials and Methods of Making the Same”.

[0006] Valiev et al. in “Paradox of Strength and Ductility in Metals Processed by Severe Plastic Deformation,” Journal of Materials Research, vol. 17, no. 1, January 2002, pp. 5-8, describe that treatment of copper by cold rolling to a thickness reduction of 60 percent significantly increased the strength but dramatically decreased the elongation to failure (which is a quantitative measure of ductility). Valiev et al. report that treatment of copper to two passes through an equal channel angular pressing (ECAP) die also increased the strength of the copper but decreased the ductility. However, continued deformation of the copper for a total of sixteen passes through the ECAP die increased both the strength and the ductility, and the increase in ductility was even greater than the increase in strength. Similar results were observed by subjecting titanium to high-pressure torsion. Transmission electron micrographs of the resulting ultrafine-grained materials show that the mean grain size for these materials is about 100 nm for copper and for titanium.

[0007] Youssef et al. in “Ultratough Nanocrystalline Copper With a Narrow Grain Size Distribution,” Applied Physics Letters, vol. 85, no. 6, 9 Aug. 2004, pp. 929-931, describe a method of preparing high strength copper with good ductility. The method involves milling copper powder at liquid nitrogen temperature, then flattening the milled powder, and then welding the powder to form thin flakes. Continued milling at room temperature and at cryogenic temperatures induced in situ consolidation of the flakes into fully dense nanocrystalline copper spheres having high yield strength (770 MPa) and good ductility.

[0008] Wang et al. in “Tough Nanostructured Metals at Cryogenic Temperatures,” Advanced Materials, vol. 16, no. 4, 17 Feb. 2004, pp. 328-331, describe a method for preparing nanostructured metals by equal channel angular pressing followed by cold rolling. The study found that strength and ductility can be improved at low temperatures and high strain rates. However, the method does not work at room temperature and quasi-static service conditions.

[0009] Horita et al. in “Achieving High Strength and High Ductility in Precipitation-Hardened Alloys,” Advanced Materials, vol. 17 (2005) pp. 1599-1602, describe processing an Al-10.8 wt % Ag by first heating the alloy to dissolve second phase particles, then refining the grain size by equal channel angular pressing (ECAP), and then annealing. This process rendered the alloy with both high strength and good ductility.

[0010] The strategies used in the above methods attempt to regain the strain hardening of nanostructured materials and improve the ductility by increasing the dislocation storage capacity of the materials. However, improvements in the ductility are always accompanied by a decrease in the strength.

[0011] There remains a need for nanostructured materials having improved ductility, but not at the expense of strength.

[0012] In accordance with the purposes of the present invention, as embodied and broadly described herein, the present invention includes a method for preparing a nanostructured aluminum alloy comprising heating an aluminum alloy workpiece at a temperature sufficient to produce a single phase of coarse grained aluminum alloy; refining the grain size of the workpiece at a temperature of room temperature or below until the average grain size is less than 1000 nanometers and the strength of the workpiece increases; and aging the workpiece to induce the formation of second phase particles in the nanosized grains that increase the ductility without decreasing the strength of the workpiece.

[0013] The invention also includes a method for preparing a metal alloy comprising refining the grain size of a coarse grained single phase metal alloy until the grains of the coarse grained metal alloy are refined to an average grain size of less than 1000 nanometers, the deformed metal alloy comprising a first strength S1 and a first ductility D1; and thereafter aging the metal alloy at a temperature sufficient to induce the formation of second phase particles in the grains of the alloy, the annealed alloy comprising a second strength S2 and a second ductility D2, wherein S2≧S1, and wherein D2>D1.

[0014] The invention also includes a method for increasing the ductility of a workpiece comprising introducing small second phase particles into grains of a workpiece comprising an average grain size ≦1000 nm, whereby the ductility of the workpiece is increased without decreasing the strength of the workpiece.

[0022] The invention is concerned with preparing nanostructured materials having improved ductility. The invention was demonstrated using a 7000 series aluminum alloy known in the art as aluminum 7075. Samples of 7075 aluminum alloy were heat treated to form single-phase coarse-grained aluminum 7075 alloy, which was subsequently rolled at a cryogenic temperature and then subjected to aging at an elevated temperature. The result of the rolling was nanostructured 7075 alloy with much higher strength than the coarse-grained 7075 alloy but with low ductility, and the result of aging was an improvement in both the strength and ductility of the nanostructured alloy. Transmission electron micrograph (TEM) images and x-ray diffraction spectra demonstrate that the aging resulted in the formation of second phase particles in a nanostructured matrix. While not wishing to be bound by the present explanation, the formation of the second phase particles is believed to contribute to the observed enhancement in the ductility of the nanostructured alloy sample as measured by an improvement in the uniform tensile elongation.

[0023] Also while not wishing to be bound by the present explanation, it is believed that the measured improvement in ductility is largely a result of an increase in the strain hardening and dislocation storage capacity of the alloy sample. This increase in strength and ductility is due to the resistance to the movement of dislocations as well as the accumulation of dislocations in the matrix of the nanostructured alloy provided by the second phase particles that form upon aging of the cold rolled alloy sample.

[0024] A sample of coarse grained (CG) 7075 aluminum alloy was prepared by heating a sample of 7075 aluminum alloy at a temperature of about 500 degrees Celsius for about 5 hours. The effect of heating the aluminum alloy under these conditions was to dissolve all second phase particles to form a single solid phase composed of aluminum and alloy elements. The alloy sample was then quenched in liquid nitrogen. TEM images of the quenched sample show that the average grain size, d, of the sample is about 8 μm, and the orientation between neighboring grains is high-angle. While quenching was performed by cryogenic cooling using liquid nitrogen was used for this example and is preferred, it should be understood that quenching may also include other types of cooling (ice bath, dry ice/ethanol, dry ice/acetone, liquid nitrogen/solvent, and the like).

[0025] After quenching, the sample was immediately rolled to an 80 percent reduction in thickness. The rolling procedure required several passes, and was accomplished by immersing the sample in liquid nitrogen between consecutive rolling passes. The sample temperature before and after each rolling pass is estimated to be in the range of from about −150 degrees Celsius to about −100 degrees Celsius. It should be understood that while immersing the sample in liquid nitrogen between consecutive passes is preferred, the sample may be cooled using other cold liquids (ice bath, dry ice/ethanol, dry ice/acetone, liquid nitrogen/solvent, and the like) or gases. In between rolling passes, the sample temperature should be a temperature at or below room temperature.

[0026] FIG. 1a shows a dark-field TEM image of the sample after the cold rolling procedure. FIG. 1b shows the grain size distribution of the sample. The inset is the selected area diffraction pattern (SAD) from a circular area with a diameter of 1 μm. As FIG. 1a-b show, the cold-rolling procedure resulted in refinement of the coarse grains into nanosized grains with an average grain size of about 110 nm and low-angle grain boundaries (GBs).

[0027] The XRD spectrum of the coarse grained sample indicates that the sample is texture free with a lattice parameter (a=4.0599 Å) larger than that for pure aluminum (a=4.0494 Å). The microstrain (<ε²>^1/2) of the sample, calculated from the XRD peak broadening (see: Zhao et al., Phys. Rev. B, vol. 56 (1997) pp. 14332-14329 (1997)), is about 0.069%. The dislocation density, ρ, is calculated from <ε²>^1/2 and d according to the formula

$\rho =\frac{2\sqrt{3}<{\varepsilon }^2{>}^{1/2}}{\mathrm{db}}$

where b is the absolute value of Burgers vector.

[0028] The conclusion from the data is that the cold rolling procedure induced a texture in the alloy sample with (220) orientation, and increased <ε²>^1/2 and ρ, but did not change the lattice parameter (See TABLE 1).

[0029] The cold rolled sample was subjected to an aging procedure, which involved heating the sample at temperature of about 50 degrees Celsius for about 5 hours, and then at a temperature of about 120 degrees Celsius for about 10 hours, then at a temperature of about 80 Celsius for about 9 hours.

[0030] FIG. 3a shows a bright field transmission electron microscope (TEM) image of the cold rolled sample after aging, and FIG. 4b shows a bright field image of the coarse grained sample after aging.

[0031] As FIG. 3a-b show, after aging, that both the coarse grained sample and the nanostructured (cold rolled) sample include G-P zones (cluster of alloy elements having a diameter of less than about 10 nm) and meta-stable η′ phases (second phase particles having a diameter of greater than about 10 nm) (see, for example: Jia et al., “Deformation Behavior and Plastic Instabilities of Ultrafine-Grained Titanium,” App. Phys. Lett., vol. 79, no. 5, July 2001, pp. 611-613). As a result of the aging process, a large amount of G-P zones and meta-stable η′ were formed in both the coarse grained sample and in the cold rolled sample, as shown in FIG. 3. A larger number of second phase particles formed in the nanostructured sample after aging compared to the coarse grained sample after aging, and the particle sizes of the cold rolled sample were smaller than those of the coarse grained sample. Accompanied with the formation of second phase particles, the lattice parameters for the aluminum matrix for the coarse grained sample and the nanostructured sample were reduced from 4.0599 to 4.0563 and 4.0569 Å, respectively.

[0032] Room-temperature tensile tests were performed using a Shimadzu Universal Tester. Each sample was cut and polished into a bone-shaped specimen with a gauge length of 10.0 mm and a cross-section of 2.0×1.0 mm for tensile tests at a strain rate of 1.7×10⁻⁴ s⁻¹. A long gauge length of 10.0 mm was used, because it was found that the ductility (especially the post-necking part) of nanostructured materials depends significantly on the specimen size (gauge length), where a longer gauge length results in a shorter but more credible ductility. For each sample, five specimens were used to get repeatable tensile curves. The tensile engineering and true stress-strain curves are shown in FIG. 6. For the coarse grained aluminum alloy samples, the aging increased the yield stress, σ_y, from 145 to 385 MPa, and the ultimate tensile stress, σ_UTS, from 374 to 596 MPa, respectively, but decreased the ductility (ε_E, elongation to failure) from 33.8 to 20.4%. For the nanostructured aluminum alloy samples, the age-produced second phase particles not only increased the strength of each sample but also enhanced the ductility. The σ_y, σ_UTS and σ_E of the un-aged NS 7075 Al sample are 550 MPa, 594 MPa and 5.9%, respectively. After low-temperature aging, these values are increased to 615 MPa, 680 MPa and 12.0%, respectively, as summarized in TABLE 1 below. The uniform tensile elongation of the aged NS+2^nd-P sample is about 7.0%, while the un-aged NS sample was only about 3.1%.

[0033] The average grain size (d), texture, lattice parameter (a), microstrain (<ε²>^1/2), type of grain-boundaries (GBs), and dislocation density (ρ) are summarized below in TABLE 1. Also included in TABLE 1 are tensile mechanical properties including yield strength (σ_Y), ultimate tensile strength (σ_UTS), elongation to failure (ε_E), uniform tensile elongation (ε_U) and work hardening exponent (n) of the quenched CG 7075 Al (written as CG), cold-rolled NS 7075 Al (NS), aged CG sample (CG+aging) and aged NS sample (CG+rolling+aging) before and after tension, which are described later.

	TABLE 1
				Nanostructured
			Nanostructured	sample formed by
	Coarse	Coarse	sample formed	rolling and aging
	grained	grained	from by rolling	coarse grained
	sample after	sample after	coarse grained	sample
	quenching	aging	sample	(CG + rolling +
	(CG)	(CG + aging)	(CG + rolling)	aging)
Relative number of second	Few	Many	Few	Many
phase particles
Structure of	Grain	High-angle	High-angle	Low-angle	Low-angle
the aluminum	boundaries
matrix	d (nm)	8150	8160	111	108
before	Texture	Free	Free	(220)	(220)
tension	a (À)	4.0599	4.0563	4.0599	4.0569
	<ε2>1/2 (%)	0.069	0.069	0.362	0.309
	ρ(1013 m−2)	0.10	0.10	39.53	34.50
Structure of	d (nm)	8150	8160	110	101
the aluminum	<ε2>1/2 (%)	0.354	0.303	0.394	0.445
matrix after	ρ(1013 m−2)	0.533	0.448	43.23	53.74
tension
Tensile	σY(MPa)	145	385	550	615
mechanical	σUTS(MPa)	374	596	594	680
properties	εE(%)	33.8	20.4	5.9	12.0
	εU(%)	27.1	16.3	3.1	7.0
	n	0.35	0.24	0.11	0.15

[0034] Based on the true stress-strain curves, the normalized work hardening rate, Θ, can be calculated using the equation

$\Theta =\frac{1}{\sigma }{\left(\frac{\partial \sigma }{\partial \varepsilon }\right)}_{\varepsilon }.$

FIG. 5 shows Θ versus the plastic strain, ε_P, and true stress σ. When ε_P>1%, at a certain ε_P value, the quenched coarse-grained aluminum sample had the largest Θ value, and the aged coarse-grained sample with second phase particles had the next largest Θ. The cold-rolled sample has the smallest value of Θ and the aged cold rolled sample with second phase particles had the second smallest value for Θ. The Θ variation sequence is: Θ_CG>Θ_CG+2nd-P>Θ_NS+2nd-P>Θ_NS. In other words, the second phase particles in the nanostructured (cold rolled) 7075 Al alloy enhanced the strain hardening rate.

[0035] From the above analysis, the enhanced ductility of the aged nanostructured aluminum alloy (cold rolling+aging) sample is due to strain hardening. To find the origin of the enhance strain hardening in the aged nanostructured Al sample, XRD and TEM experiments were carried out on the tensile-tested samples. X-ray analysis indicated that in the Al sample processed by cold rolling+aging, the dislocation density increased from 34.5×10¹³ m⁻² before the testing to 53.74×10¹³ m⁻² after the tensile testing, suggesting that the strain hardening is from accumulation of dislocations.

[0036] The X-ray analysis was also confirmed using TEM images. FIG. 6 shows the high-resolution TEM images and its Fourier and inverse Fourier transformation for the aluminum alloy sample produced after rolling and aging. FIG. 6 shows many dislocations around and/or within the second phase particles. These dislocations were formed during the tensile deformation and blocked by the second phase particles.

[0037] Without wishing to be bound by any particular explanation, it is believed that the second phase particles (which in the case for the aluminum alloy are the G-P zones and meta-stable η′ phase) increase the resistance to dislocation movement by forcing dislocations to cut through, or circumvent, the fine precipitates, which increases the yield strength of the nanostructured sample. During tension, the second phase particles increase the dislocation density by blocking/trapping dislocations around and/or within particles. As a result, the second phase particles promote further dislocation storage during tensile deformation, thereby increasing the strain hardening and uniform tensile elongation and ductility of the nanostructured 7075 aluminum alloy sample.

[0038] It should be understood that while the invention has been generally described using an exemplary aluminum alloy, which is a preferred alloy, the invention is applicable to other alloys of aluminum, and more generally alloys (steel, for example) of any solid metal. Thus the invention is capable of providing nanostructured, high strength, ductile alloys of, for example, titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, iron, ruthenium, osmium, cobalt, rhodium, iridium, nickel, palladium platinum, copper, silver, gold, zinc, cadmium, and the like.

[0039] While cooling the workpiece using a liquid cryogen such as liquid nitrogen is preferable, it should be understood that other types of cooling may be used that include, but are not limited to cooling the workpiece using an ice bath, which also provides a cold workpiece but not as cold as one using a liquid cryogen. A cold workpiece may also be provided a dry ice/acetone or dry ice ethanol or liquid nitrogen/solvent cold baths. Similarly, cooling devices may be used to provide cold liquid such as cold methanol.

[0040] In summary, metal alloy was subjected to heating to provide the sample with a single-phase coarse-grain structure, and afterward the metal alloy was rolled at cryogenic temperature and thereafter subjected to aging to provide nanostructured grains with second phase particles in the grains. The second phase particles enhance both the strength and ductility of the nanostructured metal alloy.

[0041] The foregoing description of the invention has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the above teaching.

[0042] The embodiment(s) were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.