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An ultrafine-grained (UFG) aluminum nano-composite was fabricated using two severe plastic deformation steps: cryomilling of powders (and subsequent consolidation of blended powders by forging) followed by high-pressure torsion (HPT). The forged bulk composite featured a trimodal structure comprised of UFG, coarse grain (CG) regions, and ceramic particles. The additional HPT processing introduced finer grain sizes and altered the morphology and spatial distribution of the ductile CG regions. As a result, both strength and ductility increased substantially compared to those of the Al nano-composite prior to HPT. The increases were attributed to the more optimal shape and spacing of the CG regions which promoted uniform elongation and yielding during tensile loading. Microstructural changes were characterized at each processing step to establish the evolution of microstructure and to elucidate structure-property relationships. The toughening effect of the CG regions was documented via fracture analysis, providing a potential strategy for designing microstructures with enhanced strength and toughness.

Experiments show that the magnesium AZ31 (Mg-3 pct Al-1 pct Zn) alloy exhibits excellent superplastic properties at 623 K (350 °C) after processing by equal-channel angular pressing using a die with a channel angle of 135 deg and a range of decreasing processing temperatures from 473 K to 413 K (200 °C to 140 °C). A maximum elongation to failure of ~1200 pct was achieved in this alloy at a tensile strain rate of 1.0 × 10−4 s−1. Microstructural inspection showed evidence for cavity formation and grain growth during tensile testing with the grain growth leading to significant strain hardening. An examination of the experimental data shows that grain boundary sliding is dominant during superplastic flow. Furthermore, a comprehensive review of the present results and extensive published data for the AZ31 alloy shows the exponent of the inverse grain size is given by p ≈ 2 which is consistent with grain boundary sliding as the rate-controlling flow mechanism.

Disks of an extruded AZ31 magnesium alloy were processed by high-pressure torsion (HPT) at a pressure of 6.0 GPa through 1/4, 1, and 5 turns either at room temperature (296 K (23 °C)) or at an elevated temperature of 463 K (190 °C). The cross-sectional planes of the disks were examined after processing, and it is shown that there is a high level of heterogeneity throughout all of the samples. This heterogeneity is revealed through the nature of the flow patterns, through the distributions of grain sizes, and by comprehensive microhardness measurements. The results demonstrate that it is possible to double the strength of the alloy in some areas of the disk after processing through 5 turns at room temperature.

The processes of equal-channel angular pressing (ECAP) and high-pressure torsion (HPT) are now established for the fabrication of ultrafine-grained metals having superior properties by comparison with their coarse-grained counterparts. This article examines the recent developments designed to improve the processing technology of ECAP and HPT and to establish these techniques as viable procedures for use in industrial applications. Based on these developments, it is reasonable to anticipate these processing procedures will experience increasing use in the fabrication of commercial products.

Magnesium alloys such as ZK60 exhibit strain softening when processed by equal-channel angular pressing (ECAP). Finite element modeling (FEM) was used to examine the flow process during ECAP with an emphasis on the importance of the strain-rate sensitivity m. The simulations show there is unstable flow and shear localization for values of m of 0 and 0.01, but the flow is stable for values of 0.05 and 0.1. The flow softening reduces the cross-sectional area of the billet and leads to an enhanced accumulation of damage at the upper surface. The simulations show that the presence of a back pressure increases the ability of the billet to fill the exit channel but does not remove the development of plastic instabilities such as shear concentrations. It is shown that an imposed back pressure reduces the level of the maximum principal stresses in the area in which deformation takes place, and this reduces the tendency for cracking of the billet during the pressing operation.

The processing of age-hardenable aluminum alloys by equal-channel angular pressing (ECAP) was investigated using three different Al-Zn-Mg alloys. The results show that it is relatively easy to conduct the ECAP at an elevated temperature of 473 K, but this leads to a weakening of the alloy rather than a strengthening. The processing by ECAP may be performed successfully at room temperature provided it is conducted fairly quickly (within ~10 minutes) after quenching from the solution treatment. It is necessary also to optimize the solution treatment conditions for each alloy composition. Under optimum conditions, good strengthening is achieved even after a single pass in ECAP.

The Al-W alloy powders containing up to 4 at. pct W particles in a nanocrystalline Al matrix were synthesized by ball milling and then hot compacted (HC) at 573 K or cold compacted using high-pressure torsion (HPT). Hardness measurements were made to determine the effects on mechanical properties. Based on existing models, the hardening effect of W particles in HC samples was attributed to Orowan-particle strengthening. Composite-type strengthening due to large W particles appeared to be negligible. Both the compressive strain and shear strain in high pressure torsion added to the strengthening effects without producing a change in the grain size.

Conventional superplasticity is generally achieved in metals having grain sizes in the range of ∼2 to 5 μm, but processing by equal-channel angular pressing (ECAP) provides the opportunity of introducing exceptional grain refinement and producing materials with ultrafine grain sizes in the submicrometer range. These materials have the potential for exhibiting excellent superplastic properties when tested in tension at elevated temperatures and examples are presented for representative aluminum and magnesium alloys. When these ultrafine-grained materials deform in superplasticity, internal cavities develop as in conventional superplastic alloys. An example is presented for an aluminum-based alloy, and it is shown that the cavity growth processes are also similar to those in conventional alloys.

Microstructure and texture evolution during the severe-plastic deformation (SPD) of unalloyed aluminum were investigated to establish the effect of processing route and purity level on grain refinement and subgrain formation. Two lots of aluminum with different purity levels (99.998 pct Al and 99 pct Al) were subjected to large plastic strains at room temperaturevia four different deformation processes: equal-channel angular extrusion (ECAE), sheet rolling, conventional conical-die extrusion, and uniaxial compression. Following deformation, microstructures and textures were determined using orientation-imaging microscopy. In commercial-purity aluminum, the various deformation routes yielded an ultrafine microstructure with a ∼1.5-µm grain size, deduced to have been formedvia a dynamic-recovery mechanism. For high-purity aluminum, on the other hand, the minimum grain size produced after the various routes was ∼20 µm; the high fraction of high-angle grain boundaries (HAGBs) and the absence of subgrains/deformation bands in the final microstructure suggested the occurrence of discontinuous static recrystallization following the large plastic deformation at room temperature. The microstructure differences were underscored by the mechanical properties following four ECAE passes. The yield strength of commercial-purity aluminum quadrupled, whereas the high-purity aluminum showed only a minor increase relative to the annealed condition.

The extrusion/equal channel angular pressing (EX-ECAP) processing procedure, in which magnesium-based alloys are subjected to extrusion followed by ECAP, was applied to a Mg-7.5 pct Al-0.2 pct Zr alloy prepared by casting. Microstructural inspection showed the EX-ECAP process was effective in reducing the grain size from ∼21 µm after extrusion to an as-pressed grain size of ∼0.8 µm. It is shown through static annealing that these ultrafine grains are reasonably stable up to 473 K, but grain growth occurs at higher temperatures. Tensile specimens were cut from the billets prepared by EX-ECAP and testing showed these specimens exhibited superplasticity at relatively low temperatures with maximum elongations up to >700 pct. By processing through EX-ECAP to a higher imposed strain and thereby increasing the area fraction of high-angle boundaries, it is demonstrated that there is a potential for achieving high-strain-rate superplasticity.

Experiments were conducted to evaluate the utility of a new processing procedure developed for Mg-based alloys in which samples are subjected to a two-step processing route of extrusion followed by equal-channel angular pressing (designated as EX-ECAP). The experiments were conducted using a Mg-0.6 wt pct Zr alloy and, for comparison purposes, samples of pure Mg. It is shown that the potential for successfully using ECAP increases in both materials when adopting the EX-ECAP procedure. For the Mg-Zr alloy, the use of EX-ECAP produces a grain size of ∼1.4 µm when the pressing is undertaken at 573 K. By contrast, using EX-ECAP with pure Mg at 573 K produces a grain size of ∼26 µm. Tensile testing of the Mg-Zr alloy at 523 and 573 K after processing by EX-ECAP revealed the occurrence of significantly enhanced ductilities with maximum elongations of ∼300 to 400 pct.

High-purity aluminum (99.99 pct) was processed by equal-channel angular pressing (ECAP) at room temperature through a die with a 90 deg angle between the die channels. Samples were examined by transmission electron microscopy (TEM) and orientation imaging microscopy (OIM) methods after one, four, and 12 passes through the die. Repetitively pressed samples were rotated by 90 deg in the same sense between successive pressing operations (route Be). After one pressing, TEM showed a subgrain structure which was elongated in the shearing direction. Corresponding OIM data illustrated an inhomogeneous microstructure in which bandlike features were also aligned with the shearing direction. The lattice orientation varied from location to location in the material. The boundary disorientation distribution determined from the OIM data exhibited a peak at 2 to 5 deg, in agreement with a predominance of subgrains in the microstructure. After four pressings, the microstructure data obtained by TEM and OIM were mutually consistent. The disorientation data revealed a decrease in the population of 2 to 5 deg boundaries accompanied by an overall upward shift in the distribution. Two orientations were generally apparent in the texture, although specific orientations varied with location. Often, a (111) orientation tended to align with the shear direction. Following 12 ECA passes, the grain size was reduced further to about 1.0 µm. The populations of high-angle boundaries (2 ≥ 15 deg) increased in the disorientation distribution. A texture characteristic of shear deformation of fee metals became apparent, although the orientations and particular components varied with location. Microstructural refinement during severe straining includes the development of large fractions of high-angle boundaries. Office of Naval Research Grant no. DAAD19-00-1-0488 (ONR) Approved for public release; distribution is unlimited.

High-temperature creep experiments often reveal a transition at very low stresses to a region where the stress exponent is reduced to a value lying typically in the range of ∼1 to 2. This region is generally associated with the occurrence of a new creep mechanism, such as grain-boundary sliding, diffusion creep, and/or Harper-Dorn creep. Several recent reports have suggested that diffusion creep and Harper-Dorn creep may not be viable creep mechanisms. This article examines these two processes and demonstrates that there is good evidence supporting the occurrence of both creep mechanisms under at least some experimental conditions.

An Al-3 pct Mg-0.2 pct Sc alloy was fabricated by casting and subjected to equal-channel angular pressing to reduce the grain size to ∼0.2 µm. Very high tensile elongations were achieved in this alloy at temperatures over the range from 573 to 723 K, with elongations up to >2000 pct at temperatures of 673 and 723 K and strain rates at and above 10−2 s−1. By contrast, samples of the same alloy subjected to cold rolling (CR) yielded elongations to failure of

Using equal-channel angular (ECA) pressing at room temperature, the grain sizes of six different commercial aluminum-based alloys (1100, 2024, 3004, 5083, 6061, and 7075) were reduced to within the submicrometer range. These grains were reasonably stable up to annealing temperatures of ∼200 °C and the submicrometer grains were retained in the 2024 and 7075 alloys to annealing temperatures of 300 °C. Tensile testing after ECA pressing through a single pass, equivalent to the introduction of a strain of ∼1, showed there is a significant increase in the values of the 0.2 pct proof stress and the ultimate tensile stress (UTS) for each alloy with a corresponding reduction in the elongations to failure. It is demonstrated that the magnitudes of these stresses scale with the square root of the Mg content in each alloy. Similar values for the proof stresses and the UTS were attained at the same equivalent strains in samples subjected to cold rolling, but the elongations to failure were higher after ECA pressing to equivalent strains >1 because of the introduction of a very small grain size. Detailed results for the 1100 and 3004 alloys show good agreement with the standard Hall-Petch relationship.

The influence of pressing speed in equal-channel angular (ECA) pressing was investigated using samples of pure Al and an Al-1 pct Mg alloy and a range of pressing speeds from ∼10−2 to ∼10 mm s−1. The results show that the speed of pressing has no significant influence on the equilibrium grain size, at least over the range used in these experiments. Thus, the equilibrium grain sizes were ∼1.2 µm for pure Al and ∼0.5 µm for the Al-1 pct Mg alloy for all pressing conditions. However, it is shown that the nature of the microstructure is dependent on the pressing speed, because recovery occurs more easily at the slower speeds, so that the microstructure is then more equilibrated. There is also indirect evidence for the advent of frictional effects when the cross-sectional dimensions of the samples are at or below ∼5 mm.

Creep tests were conducted at elevated temperatures on an AZ91 alloy reinforced with 20 vol pct Al2O3 fibers. When the creep data are interpreted by incorporating a threshold stress into the analysis, it is shown that the true stress exponent, n, is ∼3 at the lower stress levels and increases to >3 at the higher stresses. The true activation energy for creep is close to the value anticipated for interdiffusion of aluminum in magnesium. This behavior is interpreted in terms of a viscous glide process with n =3 and a breakaway of the dislocations from their solute atom atmospheres at the higher stress levels. The threshold stresses in this composite appear to arise from an attractive interaction between mobile dislocations in the matrix alloy and Mg17Al12 precipitates. The experimental results reveal several important similarities between the creep behavior of this magnesium-based composite and the well-documented creep properties of aluminum-based composites.

The primary characteristics of the creep of metal matrix composites (MMCs) are reviewed, including the shapes of the creep curves, the origin of the threshold stresses, and the nature of the rate-controlling processes. A detailed analysis of two representative MMCs provides no support for the concept of a constant substructure model for creep with a stress exponent (n) of 8. Analysis of the data demonstrates that creep is controlled by deformation in the matrix alloys and, as in solid solution alloys, there is a division into control by viscous glide (class A) and climb (class M), respectively.

Experiments were undertaken to compare the equal-channel angular (ECA) pressing of Al-1 pct Mg and Al-3 pct Mg solid-solution alloys with pure Al. The results reveal both similarities and differences between these three materials. Bands of subgrains are formed in all three materials in a single passage through the die, and these subgrains subsequently evolve, on further pressings through the die, into an array of grains with high-angle boundaries. However, the addition of magnesium to an aluminum matrix decreases the rate of recovery and this leads, with an increasing Mg content, both to an increase in the number of pressings required to establish a homogeneous microstructure and to a decrease in the ultimate equiaxed equilibrium grain size. It is concluded that alloys exhibiting low rates of recovery should be especially attractive candidate materials for establishing ultrafine structures through grain refinement using the ECA pressing technique.

Creep tests were conducted on an Al-6092 alloy reinforced with 25 vol pct SiC particulates and on an unreinforced Al-6092 matrix alloy. Both materials exhibit creep behavior indicating the presence of a threshold stress and both have a true stress exponent of 3, but with meaured activation energies for creep of ∼135 and ∼230 kJ mol−1 in the unreinforced and reinforced materials, respectively. By incorporating a temperature-dependent load transfer into the analysis, it is shown that the activation energy for the composite is reduced to ∼130 kJ mol−1. Both materials therefore exhibit creep behavior consistent with viscous glide and the dragging of Mg solute atmospheres, and in addition the results for the composite are consistent with the proposal that the creep of metal matrix composites divides into two classes depending upon the rate-controlling process in the matrix alloys.

Ultrafine grain sizes were introduced into samples of an Al-3 pct Mg solid solution alloy and a cast Al-Mg-Li-Zr alloy using the process of equal-channel angular (ECA) pressing. The Al-3 pct Mg alloy exhibited a grain size of ∼0.23 µm after pressing at room temperature to a strain of ∼4, but there was significant grain growth when the pressed material was heated to temperatures above ∼450 K. The Al-Mg-Li-Zr alloy exhibited a grain size of ∼1.2 µm, and the microstructure was heterogeneous after pressing to a strain of ∼4 at 673 K and homogeneous after pressing to a strain of ∼8 at 673 K with an additional strain of ∼4 at 473 K. The heterogeneous material exhibited superplastic-like flow, but the homogeneous material exhibited high-strain-rate superplasticity with an elongation of >1000 pct at 623 K at a strain rate of 10−2 s−1. It is concluded that a homogeneous microstructure is required, and therefore a high pressing strain, in order to attain high-strain-rate superplasticity (HSR SP) in ultrafine-grained materials.

The shearing associated with equal-channel angular (ECA) pressing was examined using optical microscopy. Samples of pure Al with a large grain size were subjected to ECA pressing to different strains and then examined on three orthogonal planes. Samples were pressed without any rotation or with rotations of either 90 or 180 deg between each consecutive pressing. The experimental observations are compared with models which predict the shearing characteristics associated with ECA pressing under different conditions. It is demonstrated that there is good agreement, in terms of both the grain elongation and the shearing within individual grains, between the experimental results and the predictions of the models.

Experiments were conducted to determine the age-hardening characteristics and the mechanical properties of an Al-5.5 pct Mg-2.2 pct Li-0.12 pct Zr alloy processed by equal-channel angular (ECA) pressing to give a very fine grain size of ∼1.2 µm. The results show that peak aging occurs more rapidly when the grain size is very fine, and this effect is interpreted in terms of the higher volume of precipitate-free zones in the fine-grained material. Mechanical testing demonstrates that the ECA-pressed material exhibits high strength and good ductility at room temperature compared to conventional Al alloys containing Li. Elongations of up to ∼550 pct may be achieved at an elevated temperature of 603 K in the ECA-pressed condition, thereby confirming that, in this condition, the alloy may be a suitable candidate material for use in superplastic forming operations.

A new miniature double-shear testing procedure is introduced for the testing of semibrittle materials such as intermetallics. Detailed experiments on samples of pure Al and an Al-5 pct Mg solid solution alloy confirm that the miniature configuration provides consistent and reliable results when using either a circular or a square cross section. In addition, it is demonstrated that the data obtained using miniature specimens are consistent with results from conventional large double-shear specimens and from specimens tested under tensile conditions. The miniature testing technique is used to provide preliminary mechanical data on a Ni-45 pct Al-5 pct Fe intermetallic.

Experiments were conducted on a superplastic copper alloy to investigate the growth of single holes machined in the gage length prior to testing. Specimens were deformed in tension in the three regions of flow associated with superplastic materials. Within each flow region, three distinct stages of hole growth were identified. Initially, in stage 1, the hole simultaneously increases in length along the tensile axis but decreases in the dimension measured perpendicular to the tensile axis ('transverse contraction'). Subsequently, in stage 2, the hole grows both along and perpendicular to the tensile axis ('transverse growth'). Finally, in stage 3, a crack nucleates on either side of the hole and propagates to cause failure ('crack propagation'). It is shown that the transitions between the different stages of growth is dependent upon the initiation and development of macroscopic necking adjacent to the hole.

High-temperature creep tests were conducted on polycrystalline copper of commercial purity in order to investigate the inter-relationship between the extent and the rate of grain boundary sliding (GBS) and the development of internal cavitation. An image processing technique was used to provide quantitative information on the size and shape of the cavities formed over a range of stresses and at temperatures from 673 to 873 K. The results demonstrate that cavity development is inhomogeneous within any specimen, such that there is an increase in the level of cavitation in regions of higher local strain. It is shown by quantitative measurements that the total cavitated area increases at the faster sliding rates, but the number density of cavities decreases at the highest sliding rates because of cavity interlinkage. When interlinkage is taken into account, the results confirm that the overall level of cavitation is related to the occurrence of GBS.

It is now well established that cavities are often formed during superplastic deformation. However, experimental investigations suggest important differences in the nature of the cavitation in typical superplastic metals and ceramics. These differences are demonstrated with reference to a superplastic Cu-based alloy and yttria-stabilized tetragonal zirconia (Y-TZP). By using a quantitative metallographic procedure and scanning video images, measurements are presented showing the size, shape, and configuration of internal cavities in these two materials after deformation at high temperatures.

Experiments were conducted to compare the influence of temperature on the flow and strain-hardening characteristics of an Al-6061 metal matrix composite, reinforced with ∼20 vol pct of Al2O3-based microspheres, with the unreinforced monolithic alloy. At room temperature, the yield stresses and the strain-hardening rates are higher in the composite material in the asquenched condition and after aging at 448 K for periods of time up to 300 hours. The 0.2 pct proof stress and the strain-hardening rate decrease with increasing temperature in both materials, but the rate of decrease is faster in the composite so that the unreinforced monolithic alloy exhibits higher yield stresses and strain-hardening rates at temperatures in the vicinity of 600 K. Under conditions of constant stress at high temperatures, the composite exhibits both a higher creep strength than the monolithic alloy and higher values for the stress exponents for creep.

Experiments were conducted to determine the age-hardening behavior of an Al-6061 metal matrix composite reinforced with ∼20 vol pct of A12O3 microspheres and prepared using liquid metallurgy processing. The presence of alumina microspheres increases the dislocation density in the matrix of the composite in the as-quenched state and, by comparison with the monolithic alloy, this leads to a significant increase in the yield stress in the as-quenched and unaged condition. From measurements of the 0.2 pct proof stress, there is no evidence for any significant acceleration in the aging process in the composite material: both materials attain similar peak-aged conditions after essentially the same aging times. The microstructures of the composite and the monolithic alloy are similar in the peak-aged condition, with a high density of fine needlelike β″ precipitates and, in the over-aged condition, with a reasonably homogeneous distribution of the rod-shaped β′ phase. It is proposed that the aging behavior is better quantified by determining the yield stress rather than by taking hardness measurements. Formerly Visiting Scholar, Kyushu University

Equal-channel angular (ECA) pressing is rapidly becoming an established procedure for inducing microstructural refinement in polycrystalline materials through the process of intense plastic straining. To date, there have been numerous reports of the use of the ECA pressing procedure to produce grain refinement down to the submicrometer, or occasionally the nanometer, level in a range of metallic materials[1–10] and in some intermetallic compounds.[11,12] In ECA pressing, a sample is pressed through a die having two channels, equal in cross section, which intersect at an angle of F, with simple shear occurring as the sample moves through the angle subtended at the intersection of the two channels.[13,14] Since the cross section of the sample remains unchanged on passage through the die, the procedure is readily amenable to repetitive pressings of the same sample in order to introduce high total strains. Pressings are generally conducted using dies having F . 90 deg, and under these conditions the equivalent strain introduced on each passage through the die is approximately equal to 1.[15] In practice, experiments have demonstrated that the microstructural characteristics introduced by ECA pressing, including the structural homogeneity, the average shapes of the individual grains, and the misorientation angles of the boundaries between adjacent grains, are all dependent upon experimental conditions such as the number of passages through the die,[5,9,10,16] the shearing directions in each separate passage as manifested by any rotation of the sample between separate pressings,[5,9,16] and the temperature at which the straining is conducted.[10] When repetitive pressings are undertaken, it is a standard procedure to identify three different routes, designated A, B, and C, in which the pressings are conducted without any rotation of the sample, with rotation about the longitudinal axis by 90 deg between each pressing and with rotation by 180 deg between each pressing, respectively. Two recent reports have compared directly the efficiency of these different processing routes for establishing a homogeneous structure consisting of an array of equiaxed grains. In experiments by Ferrasse et al.[9] on Cu and Al alloys, it was concluded that processing by route C produces a more ho

In a recent investigation of the creep behavior of an Al-4 wt pct Mg composite reinforced with 10 vol pct of SiC particles, henceforth designated Al-4 pct Mg-10 vol pct SiC(p), Pandey et al. obtained a plot of the steady-state creep rate, {dot {epsilon}}, vs the applied stress, {sigma}, over the temperature range from 573 to 673 K. The present analysis leads to two conclusions. First, the experimental data reported by Pandey et al. for an Al-4 pct Mg-10 vol pct SiC(p) composite divides into two distinct creep regimes at all testing temperatures, with a transition with increasing stress from region I to region II at a strain rate of {approximately}3 {times} 10{sup {minus}8} s{sup {minus}1}. Second, a detailed examination of the data at the higher stresses in region II does not support a constant structure model for creep, as proposed by Pandey et al., but rather it leads to a true stress exponent ({approximately}3.0) and a true activation energy for creep ({approximately}125 kJ mol{sup {minus}1}) which are consistent with control by the viscous glide of dislocations dragging Mg solute atom atmospheres within the Al-Mg matrix.