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The microstructure of a metal may be very significantly changed by subjecting the material to severe plastic deformation (SPD) through procedures such as equal-channel angular pressing (ECAP) and high-pressure torsion. These procedures lead to a substantial refinement in the grain size so that the grains are reduced to, typically, the submicrometer or even the nanometer range. This paper describes the application of SPD to pure aluminum and aluminum-based alloys, with emphasis on the factors influencing the development of homogeneous microstructures of equiaxed grains separated by high-angle grain boundaries. Materials subjected to ECAP are capable of exhibiting exceptional mechanical properties including superplastic ductilities at very rapid strain rates. Examples of this behavior are presented and results are described showing the potential for using this approach in superplastic forming applications at high strain rates.

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

Equal-channel angular pressing (ECAP) is a processing method in which a metal is subjected to an intense plastic straining through simple shear without any corresponding change in the cross-sectional dimensions of the sample. This procedure may be used to introduce an ultrafine grain size into polycrystalline materials. The principles of the ECAP process are examined with reference to the distortions introduced into a sample as it passes through an ECAP die and especially the effect of rotating the sample between consecutive presses. Examples are presented showing the microstructure introduced by ECAP and the consequent superplastic ductilities that may be attained at very rapid strain rates.

The grain size of many metals may be reduced to the submicrometer or nanometer level using various procedures incorporating the application of severe plastic deformation. This paper examines the procedure and potential of equal-channel angular pressing (ECAP) in which a material is pressed through a die and a high strain, is imposed without any change in the cross-sectional dimensions of the workpiece. Various factors influence the ultrafine grains attained using ECAP and examples are presented showing the application of ECAP in fabricating materials exhibiting high strain rate superplasticity and rapid forming capabilities.

The imposition of severe plastic straining on an Al–3% Mg–0.2% Sc alloy through equal-channel angular pressing (ECAP) reduces the grain size to ca. 0.2 μm and provides an opportunity to achieve superplastic elongations at high strain rates. Experiments were conducted to evaluate the procedure for optimizing the ECAP process in order to attain maximum superplastic ductilities. The results show that: (i) high elongations are achieved only after each sample has been subjected to a sufficiently high strain to establish a homogeneous array of grains separated by high angle boundaries and (ii) elongations are a maximum when using processing route B C in which the sample is rotated by 90° in the same sense between each consecutive pressing through the ECAP die.

Equal channel angular pressing (ECAP) is a processing procedure in which a sample is pressed through a die containing a channel bent into an L shaped configuration. This procedure introduces a high strain into the sample without any change in the cross-sectional area and it may be used to attain an ultrafine grain size with dimensions lying typically within the submicrometer range. This paper describes a series of experiments where ECAP was applied to a commercial Al–Mg–Li–Zr alloy having an initial grain size of ∼400 µm. The results demonstrate a refinement in the grain size of this alloy to a size of ∼1 µm and it is shown that these small grains are stable up to temperatures >600 K because of the presence of β′-Al3Zr particles. The stability of these ultrafine grains at elevated temperatures provides an opportunity to achieve superplastic ductilities in this alloy at very high strain rates: for example, the measured elongations to failure under optimum pressing conditions exceed 1000% at a strai...

Severe plastic deformation provides a processing tool for introducing fine grain sizes into polycrystalline materials. This paper describes the principles of equal channel angular pressing (ECAP) in which a material is pressed through a die where two channels form an L shaped configuration. The process of ECAP imposes a severe strain on the sample but there is no concomitant change in the cross-sectional dimensions so that repetitive pressings may be undertaken to achieve very high total strains. Several factors influence the nature of the microstructures attained in ECAP including the processing route by which the sample is rotated between consecutive pressings, the angle subtended by the two channels within the die, and the speed and temperature associated with the pressing. These various factors are described for a series of experiments conducted on samples of pure aluminium.

An Al-3% Mg-0.2% Sc alloy was fabricated by casting and subjected to severe plastic deformation through equal-channel angular pressing to a strain of ~8. The grain size after pressing was ~0.2 ?m and increased to ?1.1 ?m when holding at 673 K for 10 min. Very high tensile elongations were recorded at 673 K with a maximum elongation of ~2280% when testing with an initial strain rate of 3.3 × 10?2 s?1. The strain rate sensitivity was measured as ~0.5 at strain rates in the vicinity of 10?2 s?1. Small disks were cut from the rods after pressing and these disks were successfully formed into domes at 673 K using a biaxial gas-pressure forming facility and forming times up to a maximum of 60 s. Measurements of the local thicknesses at selected points around the domes revealed reasonably uniform thinning which is consistent with the high strain rate sensitivity of this alloy.

Equal-channel angular pressing is an effective tool for attaining ultrafine grain sizes in bulk materials. An important advantage of this technique over conventional metalworking processes, such as extrusion and rolling, is that very high strains may be attained without any concomitant change in the cross-sectional dimensions of the sample. The microstructures introduced by equalchannel angular pressing critically depend on a number of experimental factors, including the nature of the slip systems introduced during the pressing operation and the total strain imposed on the sample. These factors are illustrated by reference to experiments conducted on pure aluminum; results are also included to demonstrate the influence of alloying additions and especially the remarkably small grain sizes that may be achieved in materials having low rates of recovery.

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.

A Ni–Al–Cr alloy with an initial grain size of ∼60 μm was subjected to torsion straining to a strain of ∼7 at room temperature, thereby reducing the grain size to ∼34 nm. Similar torsion straining with samples of pure Cu and pure Ni gave grain sizes of ∼170 and ∼130 nm, respectively. Inspection of the Ni–Al–Cr alloy after torsion straining revealed highly strained regions containing dislocations associated with lattice distortions but with an absence of any Ni3Al ordered phase. The ultrafine grains in the Ni–Al–Cr alloy were extremely stable at high temperatures, and it was possible to retain a grain size of less than 100 nm after annealing at temperatures up to ∼900 K. By contrast, there was rapid grain growth in the samples of pure Cu and Ni at annealing temperatures in the vicinity of ∼500 K. The stability of the grains in the Ni–Al–Cr alloy is attributed to the formation of a Ni3Al-based ordered phase after annealing at ∼650–700 K. The presence of this phase also leads to an apparent negative slope in the standard Hall–Petch relationship.

Equal-channel angular (ECA) pressing is a procedure having the capability of introducing an ultrafine grain size into a material. Experiments were conducted to examine the effect of the low stacking fault energy in pure Cu on microstructural development during ECA pressing at room temperature. The results show that the low 0stacking fault energy and the consequent low rate of recovery lead to a very slow evolution of the microstructure during pressing. Ultimately, a stable grain size of −0.27 μm was established in pure Cu but the microstructure was not fully homogeneous even after pressing to a total strain of ∼10. It is shown by static annealing that the as-pressed grains are stable up to ∼400 K, but at higher temperatures there is grain growth. These results lead to the conclusion that a low stacking fault energy is especially favorable for the introduction of an exceptionally small grain size using the ECA pressing procedure.

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.

Superplastic deformation occurs at high temperatures and requires the presence of a very small grain size. Experiments demonstrate that equal-channel angular (ECA) pressing is capable of introducing ultrafine grain sizes in pure Al, Al–Mg and Al–Zr alloys, with grain sizes lying in the sub-micrometer range for the alloys. It is shown by static annealing that these ultrafine grain sizes are not stable in pure Al and the Al–Mg alloys at elevated temperatures but in the Al–Zr alloys the grains remain small up to temperatures of ≈600 K.

Equal-channel angular (ECA) pressing is capable of introducing an ultrafine grain size into bulk materials and the precise nature of the microstructure is dependent upon the number of pressings and the combinations of the planes for shear deformation. When the strain is sufficiently high, it is possible to achieve equiaxed fine grains with high angle grain boundaries. The number of pressings required to establish the ultimate equiaxed grains increases with an increase in the alloy content. Tensile experiments at elevated temperatures under different initial strain rates have demonstrated that ECA pressed Al-5.5%Mg-2.2%Li-0.12%Zr (Al-1420) and Al-6%Cu-0.5%Zr (Supral) show excellent superplasticity, thereby providing the potential for substantially increasing the viability of superplastic forming in the metal forming industry.

Equal-channel angular (ECA) pressing is a processing method for introducing an ultra-fine grain size into a material. In practice, it is a procedure that may be used to achieve high total strains by subjecting a sample to repetitive pressings. There is experimental evidence showing that the nature of the microstructural evolution in ECA pressing depends upon whether the sample is rotated between each passage through the die. This paper examines the shearing characteristics associated with ECA pressing for six different processing routes and reaches conclusions concerning the optimum processing procedure and the development of texture.

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.

Equal-channel angular (ECA) pressing has been used to introduce a very small grain size into a commercial cast Al-Mg-Li-Zr alloy. Tensile testing reveals the potential for achieving superplastic ductilities in this alloy at very -high strain rates (greater than 10-2s-1) but experiments show that this potential is realized only if a sufficiently high strain is introduced during the ECA pressing such that the microstructure is homogeneous and consists of an array of grains having large-angle grain boundaries.

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.

Ultrafine grain sizes were introduced into an Al-3wt%Mg solid solution alloy and a commercial Al-Mg-Li-Zr alloy through intense plastic straining by equalchannel angular (ECA) pressing at room temperature and at 673 K respectively. Tensile testing of pressed samples at room temperature revealed markedly different stress-strain curves for these two alloys, with the Al-3wt%Mg alloy exhibiting a high yield stress with little subsequent strain hardening and the Al-Mg-Li-Zr alloy exhibiting a lower yield stress and extensive strain hardening after yielding. These and other experimental results are interpreted in terms of the nature of the microstructure introduced by the ECA pressing procedure. It is concluded that significant variations may occur in the mechanical properties of nominally similar ultrafine-grained materials depending upon the pressing conditions and the extent of any relaxation which may occur during the straining process.

Equal-channel angular pressing provides a convenient procedure for introducing an ultrafine grain size into a material. Using samples of pure Al, tests were conducted to determine the influence of the channel angle Φ, defined as the angle of intersection of the two channels within the die, on the subsequent microstructure attained by pressing. Experiments were performed using dies having channel angles from 90 to 157.5°. The results show that an ultrafine microstructure of essentially equiaxed grains, separated by high angle grain boundaries, is achieved only when a very intense plastic strain is imposed on the sample in each passage through the die as when using a die having a channel angle of Φ close to 90°

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.

Experiments were conducted on an Al5.5% Mg2.2% Li0.12% Zr alloy to investigate the feasibility of introducing an ultra-fine grain size using equal-channel angular (ECA) pressing and of retaining an ultra-fine grain size at elevated temperatures. It is shown that ECA pressing is capable of reducing the grain size from an initial value of ∼ 400 μm to a value of ∼ 1.2 μm. However, the microstructure after ECA pressing is heterogeneous, with many areas of equiaxed grains having high angle grain boundaries and some regions of subgrains with boundaries having low angles of misorientation. Unlike earlier experiments on AlMg binary alloys, it is demonstrated that the grain size of the AlMgLiZr alloy is reasonably stable up to temperatures as high as ∼ 700 K because of the presence in the matrix of a fine dispersion of β′-Al 3 Zr precipitates. Microhardness data confirm the Hall-Petch relationship for grain sizes above ∼ 2 μm produced by annealing at temperatures above ∼ 673 K, but the Hall-Petch relationship breaks down at smaller grain sizes because of variations in the volume fraction of the δ′-Al 3 Li precipitates.

Experiments were conducted to investigate the development of an ultra-fine grain size during equal-channel angular (ECA) pressing of high purity aluminum with an initial grain size of ∼1.0 mm. The results show that, under ECA pressing conditions giving a strain of ∼1.05 on each passage through the die, the microstructure is reasonably homogeneous after a single pressing and consists of parallel bands of elongated subgrains, having an average length of ∼4 μm, and these subgrains are further divided by boundaries with very low angles of misorientation. Repetitive pressings were conducted on the same samples, up to a total of 10 passages through the die, with the samples pressed either without rotation (route A) or after rotating through 180° between each pressing (route C). It is demonstrated that the misorientations of the subgrain boundaries increase with repetitive pressings until ultimately both routes lead to a similar equiaxed ultra-fine grain size of ∼1 μm after 10 pressings, but the microstructural evolution is enhanced using route C where there is a more rapid transition into an array of high angle grain boundaries. The results suggest that, at least for high purity aluminum, an ultra-fine microstructure close to optimum may be obtained after only 4 pressings provided the sample is rotated through 180° between each pressing.

This paper presents high-resolution electron microscopy studies of grain boundary structures in a submicrometer-grained Al-3%Mg solid solution alloy produced by an intense plastic straining technique. The studies include the effect of static annealing on the grain boundary structure. Many grain boundaries are in a high-energy nonequilibrium state in the as-strained sample. The nonequilibrium character is retained on some grain boundaries in samples annealed at temperatures below the onset of significant grain growth. The effect of electron irradiation on the grain boundary structure also is examined.

A submicrometer-grained (SMG) microstructure, with an average grain size of ∼0.2 μm, was introduced into an Al-3% Mg solid solution alloy by equal-channel angular pressing. The SMG structure was heterogeneous with regions of equiaxed and elongated grains and with grain boundaries in a non-equilibrium configuration. Samples were annealed for various times at a temperature of 473 K and then examined using transmission electron microscopy. The results show that the grain boundaries evolve with time into an equilibrium condition and some limited grain growth occurs primarily in the grains where recovery has taken place. The results establish that the SMG structure is reasonably stable in the Al-3% Mg alloy at a temperature of 473 K.

The microstructural stability of an Al 3%Mg solid solution alloy with a submicrometergrained (SMG) structure (∼ 0.2 μm) was evaluated using both static annealing and transmission electron microscopy over a range of temperatures from 443 to 803 K and differential scanning calorimetry (DSC) up to 773 K. The results show that the SMG structure contains many non-equilibrium grain boundaries but recrystallization occurs at the higher temperatures giving large grains with boundaries having high-angle equilibrium configurations. There are significant differences between the DSC curves of the SMG alloy and a standard cold-rolled Al 3%Mg alloy, due primarily to the advent of significant heat release at low temperatures in the SMG alloy because of recovery at the non-equilibrium grain boundaries. A temperature of ∼ 500 K, close to half the absolute melting temperature, represents an effective upper limit for utilization of the SMG structure in this material.

Considerable interest has centered recently on the processing of materials with ultra-fine grain sizes, typically

High tensile ductility may be achieved in metallic alloys over a limited range of strain rates at elevated temperatures when the grain size is very small (typically less than ∼ 10 μm). These superplastic materials are often suitable for industrial forming operations. This paper considers the possibility of attaining a superplastic forming capability at lower temperatures and/or faster strain rates by a reduction of the grain size into the submicrometer range. Procedures are available for the fabrication of ultrafine-grained materials by imposing a very high plastic strain. These procedures are examined with reference to results obtained using Al-Mg solid solution alloys.

An Al-1.5% Mg solid solution alloy was produced with a submicron grain size of ≈ 0.2–0.5 μm by introducing intense plastic deformation through torsion straining. Tests were conducted to determine the yield stress of the alloy using a miniaturized bending procedure. Parallel tests were conducted using an Al-3% Mg alloy with a large grain size of ≈ 500 μm. The results demonstrate the feasibility both of producing a submicrometer-grained structure in an Al-1.5% Mg alloy by torsion straining and of obtaining a reasonable measure of the yield stress using a miniature testing facility.

A submicrometer-grained (SMG) Al−3% Mg solid solution alloy, with an initial grain size of ∼0.2 μm, was produced by intense plastic straining. Experiments show that tensile specimens of the SMG alloy exhibit high elongations to failure at low testing strain rates at the relatively low temperature of 403 K. The stress exponent is high (∼7–8) and calculations show deformation is within the region of power-law breakdown. The initial microstructure of the alloy consists of diffuse boundaries between highly deformed grains. At strain rates of ∼10−4 s−1 and lower, plastic deformation leads to dynamic recrystallization and the formation of highly nonequilibrium grain boundaries that gradually evolve into a more equilibrated configuration.

The Zn–22% Al eutectoid alloy is capable of exhibiting very high superplastic elongations, in excess of 2000% in tension, when the grain size is in the range of ∼ 1–10μm. This paper describes the fabrication of a submicrometer grain size in the Zn–22% Al alloy by subjecting the samples to intense plastic straining in torsion under high pressure (∼5 GPa) at room temperature. Observations after straining revealed a heterogeneous microstructure with grain sizes in the range of ∼0.1–0.5μm. As a result of the low melting temperature of the alloy, the high internal stresses introduced by torsion straining are relaxed and the grain boundaries are close to an equilibrium configuration.

Using the technique of equal-channel angular (ECA) pressing, it is possible to reduce the grain size of polycrystalline materials to the submicrometer level. Thus, this processing technique has the potential for producing materials which may exhibit superplasticity. This paper describes various factors affecting the development and evolution of the microstructure produced by ECA pressing. Optimization of such factors is then presented for the advent of superplasticity.

In the past decade, there has been an important emphasis in materials science on the production and use of ultrafine-grained materials. These materials offer wide-ranging advantages such as an improvement in strength at low temperatures and enhanced superplastic behavior at high temperatures. New techniques to process these materials have been developed. Good properties have been achieved by powder metallurgy and by techniques of bulk processing. Equal-channel angular (ECA) pressing is a bulkprocessing technique which has led to some remarkable achievements in the production of sub-microcrystalline materials with excellent superplastic properties at low temperatures and high strain rates. When considering the currently available physical and mechanical properties of these materials, it appears that some aspects cannot be explained solely by the very small grain size. In fact, the specific mode of deformation occurring in ECA pressing appears to have an influence on the final properties. This paper reviews the details of this metal working technique and then provides an explanation for the observed microstructures, their thermal stability and their remarkable superplastic properties.

Superplastic ductilities may be achieved in tension using polycrystalline materials with small grain sizes (typically