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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.

Equal-channel angular pressing (ECAP) was attempted at room temperature to refine grain sizes of six different commercial Al alloys, 1100, 2024, 3004, 5083, 6061 and 7075. Transmission electron microscopy revealed that submicrometer grain sizes are attained in these alloys. Tensile tests at room temperature showed that the strength increases with an increase in the number of pressings but the elongation to failure remains little changed following a large decrease after the first pressing. Static annealing experiments demonstrated that the extensive grain growth occurs above ∼200°C in 1100, 3004, 5083 and 6061 but the submicrometer-grained structures are stable in 2024 and 7075 even at 300°C. It was confirmed that the Hall–Petch relationship holds for the ECA-pressed alloys. The effect of sample size was further examined and the applied load was measured during ECAP for the possibility of scaling-up the process.

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

Ultrafine grain sizes may be achieved in materials subjected to severe plastic deformation through processing procedures such as equal-channel angular pressing (ECAP). Experiments were conducted to evaluate the factors influencing the superplastic characteristics of Al-Mg-Sc alloys. Specifically, detailed testing was undertaken to determine the influence of (i) the processing route, (ii) the strain introduced by ECAP and (iii) the Mg content.

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...

Ultrafine grain sizes, of the order of approximately 0.2 μm, may be introduced into Al–Mg–Sc alloys by subjecting the material to severe plastic deformation through the process of equal-channel angular pressing (ECAP). Experiments were conducted to evaluate the influence of the solution treatment temperature on the ductility of an Al–3% Mg–0.2% Sc alloy after ECAP. The results show the highest ductilities are achieved when the solution treatment temperature is within the narrow range of approximately 878 to about 883 K, immediately below the temperature associated with the onset of partial melting. These high temperatures serve to maximize the amount of scandium in solid solution and this leads, on subsequent heating, to an extensive precipitation of fine secondary Al3Sc particles which inhibit grain growth at the higher temperatures. Conversely, solution treatments at temperatures below approximately 878 K give less Sc in solid solution within the matrix and the precipitation of the Al3Sc particles is then insufficient to retain a uniform ultrafine microstructure.

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.

A multi-pass facility was fabricated for equal-channel angular (ECA) pressing which gave a total strain of approximately 5 on a single passage through the die. Experiments on high purity aluminum show that, when comparisons are made at the same total strains, both the hardness and the evolution of the microstructure are identical when using the multi-pass facility or when repetitively pressing samples through a standard die containing a single shearing plane.

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.

Equal-channel angular (ECA) pressing is a processing procedure which subjects a material to severe plastic deformation. Tests were conducted on a commercial cast aluminum alloy to evaluate the properties associated with samples subjected to three different ECA pressing procedures. The results show that all three procedures lead to an ultrafine microstructure and each procedure is capable of producing samples which exhibit high strain rate superplasticity. Optimum superplastic properties were achieved in samples subjected to ECA pressing to a strain of ∼12. Under these conditions, the measured elongations to failure at a temperature of 673 K were 1210 and 950% at strain rates of 10−1 and 1 s−1, respectively.

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.

A transmission electron microscopy (TEM) investigation has been performed on the morphologies of L1 2 -(Al,Ag) 3 Ti and Ti 3 AlC precipitates in L1 0 -ordered TiAl(Ag). During aging at temperatures around 1073 K after quenching from 1273 K, TiAl(Ag) hardens by the precipitation of (Al,Ag) 3 Ti and Ti 3 AlC. TEM observations revealed that plate-like (Al,Ag) 3 Ti precipitates lie on {001} planes of TiAl(Ag) matrix in the short aging period and the habit plane changed from {001} to {hhl} after a long period aging or high-temperature aging and finally to {112} of the matrix lattice. At the same time needle-like precipitates Ti 3 AlC, which lie only in one direction parallel to the [001] direction of the TiAl(Ag) matrix, appear after long period aging or high-temperature aging. The anisotropic misfits between TiAl(Ag) matrix and L1 2 -(Al,Ag) 3 Ti and Ti 3 AlC are considered to explain the morphologies of L1 2 -(Al,Ag) 3 Ti and Ti 3 AlC precipitates.

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.

An Al-0.2%Sc alloy and an Al-3%Mg-0.2%Sc alloy were subjected to equal-channel angular pressing to a strain of - 8 to produce grain sizes of ∼ 0.7 and 0.2 μm, respectively. Because of the presence of some areas of subgrain boundaries in the Al-3%Mg-0.2%Sc alloy, additional samples of this alloy were subjected to ECA pressing to a strain of ∼12. In both alloys, the grain sizes were reasonably stable up to annealing temperatures above 700 K. Tensile specimens were machined from the as-pressed samples for testing at elevated temperatures. The Al-3%Mg-0.2%Sc alloy pressed to a strain of ∼ 12 exhibited exceptional ductility with elongations up to a maximum of ∼ 1560% at 673 K at the high strain rate of 3.3 x 10 -2 S -1 . By contrast, the Al-0.2%Sc alloy failed under all conditions at elongations

The interface structure of α-Mo fib ers in directionally solidified Ni-Al-Mo eutectic alloys has been investigated by transmission electron microscopy and computer-aided graphical techniques. The Mo fibers have square or rectangular cross sections with well defined flat facets and fundamentally have the Bain orientation relationship with respect to the γ matrix lattice. The crystal lattices of the Mo fibers, however, are often rotated with respect to the matrix lattice about their longitudinal axes. Correspondingly, the facet planes deviate from the exact rational (1 1 0)γ′//(1 0 0)α relationship. A computer aided graphical plotting of the atom arrangement at the interfaces suggests that good atomic matching is kept even after the rotation of fibers. High resolution electron microscopy revealed the incorporation of single atomic structural ledges to keep the coherency of the lattices of the matrix and rotated fibers. The relation between the lattice rotation and the facet plane rotation is well explained by the moire and O-lattice models.

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.

Analytical electron microscopy (AEM) has been applied to the interdiffusivity measurements and microstructural observations in Ni/Ni–8 mol% Ti diffusion couples. The high spatial resolution attained by AEM revealed that there is an effect of annealing atmosphere on the concentration profiles and microstructures within a range of several microns around the Kirkendall interface: the distortion of concentration profiles and the formation of Ti-rich particles and/or regions were observed near the interface. The interdiffusivities measured using the Sauer–Freise technique from the concentration profiles without the effect of annealing atmosphere were in good agreement with the reported values of the interdiffusivities and the impurity diffusivities of Ti in Ni, but were higher by an order of magnitude than the self-diffusivities of Ni in Ni.

Ultrafine grain sizes may be introduced into bulk samples by using the intense plastic straining technique equal-channel angular pressing. This article describes the principles of equal-channel angular pressing and demonstrates the application of this procedure to attain ultrafine grain sizes in an Al-3Mg solid-solution alloy and a commercial cast Al-Mg-Li-Zr alloy. Provided there is stability of these ultrafine grains at elevated temperatures, as in the Al-Mg-Li-Zr alloy, equal-channel angular pressing may be used as a processing tool to achieve high-strain-rate superplasticity in materials that are not potentially superplastic. These results have important implications for reducing the long production times that are associated with the fabrication of complex parts using superplastic forming.

Experiments were conducted on high purity aluminum to investigate the process of grain refinement during equal-channel angular (ECA) pressing. Samples were subjected to 1 to 4 pressings and then sectioned for microstructural examination in three mutually perpendicular directions. The results show that a single pressing leads to the development of subgrain bands. The subsequent microstructural development in additional pressings depends upon whether these repetitive pressings are conducted with no rotation of the sample (route A), with a rotation of 90° between each pressing (route B) or with a rotation of 180° between each pressing (route C). The subgrains evolve more rapidly into arrays of high angle boundaries using route B, less rapidly using route C and the evolution is slowest using route A. The results are explained by considering the shearing patterns developed in the samples during each processing route.

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°

Submicrometer-grained (SMG) structures were produced in Cu and Ni using an intense plastic straining technique, and the grain boundaries and their vicinities were observed by high-resolution electron microscopy. The grain boundaries exhibited zigzag configurations with irregular arrangements of facets and steps, and thus they were found to be in a high-energy nonequilibrium state. A similar conclusion was reached earlier for SMG Al–Mg solid solution alloys which have much lower melting points than Cu and Ni, suggesting that nonequilibrium grain boundaries are a typical feature of metals processed by intense plastic straining.

Transmission electron microscopy examinations of Ag-modified Al3Ti with an L12-ordered structure have revealed the various modes of precipitation of Al2Ti upon aging after quenching from higher temperatures. A fine uniform precipitation of Al2Ti occurs when the supersaturation is sufficiently high. On the other hand, a preferential precipitation at the antiphase boundaries can be observed in alloy with a low supersaturation. When L12Al3Ti is supersaturated with DO22Al3Ti, DO23Al11Ti5 with a multidomain structure is formed during aging. The L12 phase field in the TiAlAg system is severely skewed with respect to the temperature axis and is restricted into a much smaller field at lower temperatures. Appreciable hardening and overage softening during aging can be explained in terms of microstructural variations.

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.

Diffusion experiments have been conducted using the Ni 3 Al/NiAl couples at annealing temperatures of 873 and 923 K. It is shown that the Ni 5 Al 3 phase forms between the Ni 3 Al and NiAl phases during the diffusion annealing. Optical microscopy, transmission electron microscopy and electron probe microanalysis (EPMA) are employed for the identification of the Ni 5 Al 3 phase. Concentration profiles of Al across the diffusion-couple interfaces are measured using EPMA to determine the region of the Ni 5 Al 3 single-phase in the Ni-Al binary phase diagram. Interdiffusion coefficients in the Ni 5 Al 3 phase are also determined from the concentration profiles.

Transmission electron microscopy and electron diffraction studies were carried out in order to clarify the crystallographic properties of strengthening precipitates in B2-ordered (Ni,Co)Al supersaturated with Ni and Co. The precipitates observed in specimens aged at different aging temperatures are Co phase which has different crystal structure corresponding to different aging temperature. The Co phase has a face centered cubic (f.c.c.) structure at aging temperature higher than 973 K and the shape is rod whose long axis is parallel to the 〈111〉 direction of the matrix. The orientation relationship between the f.c.c.-Co precipitates and the B2-(Ni,Co)Al matrix is found to be the Kurdjumow-Sachs (K-S) orientation relation. By aging at temperatures below 873 K, the precipitates of Co phase take a hexagonal close packed (h.c.p.) structure and are rod in shape with the long axis parallel to the 〈111〉 direction of the matrix. The orientation relationship between the h.c.p.-Co precipitates and the B2-(Ni,Co)Al matrix is found to be the Burgers orientation relation. (Ni,Co)Al hardens appreciably by the fine precipitation of both the f.c.c.-Co phase and h.c.p.-Co phase.

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.

Microstructural variations and correlated hardness changes during phase decomposition of B2-ordered NiAl(β) containing 2 mol% of Hf have been investigated by means of transmission electron microscopy (TEM), hardness and creep tests. At the beginning of aging, fine platelets of L21-type Ni2AlHf (Heusler phase) precipitate on the {111} planes of the matrix NiAl phase keeping a perfect lattice coherency. With longer periods of aging, Ni2AlHf precipitates lose their lattice coherency and change their morphology to spherical ones surrounded by misfit dislocations. Compressive creep tests clearly indicated that the creep rate decreases appreciably by the fine precipitation of Ni2AlHf phase. TEM observations of the interaction between dislocations and spherical precipitates revealed that the dislocations tend to be strongly attracted to the particle interfaces during the creep deformation.

Compressive creep tests were performed under constant stress over a range of temperatures from 923 to 1073 K on precipitation strengthened L1 2 -ordered Co 3 Ti. The creep rates decrease appreciably by the fine precipitation of coherent disordered f.c.c. Co-rich phase when the applied stress is low. An analysis of the creep data is developed taking a threshold stress into account of the creep equations. The significance of the threshold stress is discussed based upon the transmission electron microscope observations of the interaction between dislocations and precipitates. The superdislocations produced during deformation tend to be strongly attracted and dissociate as they meet the coherent disordered precipitates because the anti-phase boundary energy in the disordered phase is zero. The extra stress necessary to pull the dislocation out of the precipitates may play an important role to establish the threshold stress which is expected to be beneficial to the improvement of the creep strength of L1 2 -ordered intermetallics for high temperature structural applications.

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.