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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

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.

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 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.

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.

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.

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.

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°

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.

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.

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.

The morphology of deformation-induced dislocations in polycrystalline Ni3 (Al,Cr) containing M23C6 precipitates has been investigated using transmission electron microscopy. Fine polyhedral precipitates of M23C6 were formed in the Ni3 (Al,Cr) matrix by aging at temperatures around 973 K after solution annealing at 1423 K. The aging behavior was investigated by microhardness measurements and the temperature dependence of the yield stress of Ni3 (Al,Cr) containing M23C6 precipitates was investigated by compression tests over the temperature range 298–1173 K. Typical Orowan loops were observed surrounding the M23C6 particles after deformation at temperatures below 973 K. At higher deformation temperatures, the Orowan loops disappeared and the morphology of dislocations at the particle-matrix interfaces suggested the existence of attractive interaction between dislocations and particles. The change in the modes of interaction between the dislocation and particles with increasing deformation temperature can be considered as a result of strain relaxation at the interface between the matrix and particles.

The effect of carbon addition on the microstrucrures and mechanical properties of TiAl intermetallic alloys has been characterized. It is shown that carbon is not only an efficient solid solution strengthener in TiAl, it is also an efficient precipitation strengthener by fine dispersion of carbide. Transmission electron microscope observations have revealed that fine needle-like precipitates, which lie only in one direction parallel to the (001) axis, appear in carbon doped TiAl during aging at temperatures around 1073 K after quenching from 1523 K . Selected area electron diffraction analyses have shown that the needle-shaped precipitate is of a perovskite type, Ti 3 AlC (P-phase). The orientation relationship between the P-phase and the TiAl matrix is found to be (001) P //(001)TiAl and (010) P //(010) TiAl . By aging at higher temperatures or for longer periods at 1073 K, plate-like precipitates of Ti 2 AlC (H-phase) with a hexagonal crystal structure are formed on the planes of the TiAl matrix. The orientation relationship between the H-phase and the TiAl matrix is (0001) H //(111) TiAl and (1120) H //(101) TiAl . TiAl doped with carbon hardens appreciably by the fine precipitation of carbide. The deviation from the stoichiometry has a significant effect on the aging behavior of TiAl. High temperature strength of TiAl increases appreciably by the precipitation of fine carbide. Dislocations bypass the carbide needles at further higher temperatures. The calculation of Orowan stress shows a good agreement with the experimentally measured result.

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

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

The crystallographic structures of carbide and nitride formed in TiAl containing carbon and nitrogen have been investigated using transmission electron microscopy. The needle-shaped carbide and nitride are formed parallel to only the [001] direction of the L10-ordered TiAl matrix. Detailed analyses in terms of selected area electron diffraction and dark-field electron microscopy revealed that the needle-shaped carbide and nitride have a crystal structure of perovskite type and have long-range carbon-vacancy and nitrogen-vacancy ordering below a critical temperature. The domain of the long-range order rotates periodically about the axis of the needle by 90[ddot] resulting in the repeated formation of antiphase boundaries along the needle. At higher temperatures, long-range ordering disappears indicating uniform distribution of carbon and nitrogen in the needle-shaped perovskite carbide and nitride.

A correction procedure for the delocalization effect when determining the site occupancy of an impurity element by atom location by channelling-enhanced microanalysis is proposed. The procedure utilizes a disordered structure with a composition similar to the ordered structure. Correction factors are derived by illuminating a sample with a disordered structure in the same orientation as for the ordered structure. The correction procedure is applied to a determination of the Ti occupancy in a Ni3Al intermetallic compound.

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

Etude par microscopie electronique a transmission, par mesure de durete, par diffraction d'electrons de la precipitation de phases stables ou metastables entre 873 et 1373 K dans des alliages Ni-7% a 30% poids Mo-2% a 11% poids Al. Durant le vieillissement on observe la formation des phases γ', NiMo(δ), Ni 2 Mo, Ni 3 Mo et Ni 4 MO dependante de la composition de l'alliage et de la temperature de vieillissement

In this paper, we examine the applicability of the differential X-ray absorption (DXA) method of Morris, Ball and Statham which is capable of determining foil thickness and local composition concurrently in an analytical electron microscope. The method is first modified to take a general form where the thickness determination is achieved with a pair of characteristic lines from different elements. When this form is used, the pair is not necessarily limited from the same elements. It is shown then that there are four important factors affecting the precision and accuracy of the foil thickness. A comparison is carried out between the thicknesses determined with the DXA method and those measured with the convergent-beam electron diffraction method. The composition analysed with the DXA method is also compared with the nominal composition of the sample.

The detailed mechanism of the transformation of various mixtures of ferrite and austenite was studied byin situ experiments in a high voltage electron microscope. Various phenomena were observed which seemed to be controlled by the rate of carbon diffusion when the γ/α interface was curved. The effect of manganese was studied by using commercial steels. In agreement with theoretical predictions based upon the local equilibrium model, the dissolution of cementite in ferrite can be more or less prevented by manganese. The effect on the dissolution in austenite is smaller. The reaction from lamellar pearlite to austenite is more complex, the cementite lamellae providing carbon to the austenite but also acting as barriers.