Search

Pure Mg samples were processed by high-pressure torsion (HPT) for up to 10 turns at temperatures of 293 and 423 K. The microstructures of these samples were significantly refined and bimodal structures were obtained after 10 turns of HPT processing at both 293 and 423 K. Tensile experiments were conducted at room temperature to reveal the mechanical properties of pure Mg subjected to HPT processing at different temperatures. The yield strength increased with increasing numbers of turns after processing at 293 K whereas the yield strength showed almost no variation with increasing numbers of turns at 423 K. Pure Mg processed at 423 K exhibited a higher strain hardening ability and a larger uniform elongation than after processing at 293 K. Calculations show the grain size, bimodal structure and dislocation density are the main factors affecting both the yield strength of the material and the work hardening behavior.

This research describes the micro-mechanical behavior of an Al–Mg alloy system synthesized from two separate commercial Al-1050 and ZK60 alloys through the application of high-pressure torsion (HPT) for five turns at room temperature. The essential mechanical characteristics of the alloys are observed at the corresponding phases in a multi-layered structure at the disk center but the edge contains a metal matrix nanocomposite (MMNC) exhibiting an apparent plastic instability and decreasing strain rate sensitivity under strain rates of 10−4−10−3 s−1. The results demonstrate a significant opportunity for using HPT processing to prepare new alloy systems involving a wide range of MMNCs.

This work is focused on an investigation of the high-cycle fatigue behavior of a Ti–6Al–4V alloy subjected to a combination of equal-channel angular pressing and extrusion. Samples with a bimodal microstructure and with an ultrafine-grained (UFG) structure are tested using a cyclic rotational bending. The fatigue endurance of the UFG alloy is almost 40% higher as compared to the alloy with a bimodal microstructure. The main differences in the fracture surfaces of the UFG and bimodal Ti–6Al–4V alloy are considered. The effect of the UFG structure on the resistance of the alloy to fatigue crack initiation and propagation is also discussed.

It is known that magnesium (Mg)-hydroxyapatite (HA) composites can be produced by the room temperature consolidation of particles. The present study analyzes the corrosion behavior of an Mg-HA composite and makes a direct comparison with pure Mg. Samples of Mg-HA and of pure Mg were immersed in Hank`s solution for up to 60 hours and the microstructure and corrosion products were characterized by scanning and transmission electron microscopy and X-ray diffraction. Electrochemical tests were used to evaluate the corrosion behavior and a hydrogen evolution test was undertaken to determine the corrosion rate. The results show the corrosion rate of the Mg-HA composite is higher than for pure Mg but decreases significantly after ~10 hours of immersion in Hank`s solution. The increase in corrosion resistance of the composite is attributed to the formation of a protective layer of corrosion products with an external surface layer rich in Ca, P and O.

A grade 2 pure titanium with an initial grain size of ≈50 μm is processed by high-pressure torsion (HPT) at room temperature under an imposed pressure of 6.0 GPa. The microhardness variations are examined and the results show that the disks are reasonably homogeneous after 10 turns of torsional straining. The microstructural evolution is systematically characterized by optical microscopy, X-ray diffraction, and transmission electron microscopy to provide information on the effect of shear strain on grain size and microstructure. The results demonstrate that the initial coarse structure is gradually refined from the edge to the center of the disk under the shear stress during HPT processing and an ultrafine-grained pure Ti is achieved with an average grain size of ≈96 nm after 10 turns. A model is developed by considering the formation of subgrain boundaries, twins, and high-angle grain boundaries for the grain refinement of pure Ti processed by HPT.

An Al-0.1% Mg alloy was processed by high-pressure torsion (HPT) at room temperature. The Al-0.1% Mg alloy displays strain softening phenomenon through hardness evolution: the hardness values in the disc centre area are higher than at the disc edge area after 1/2, 1 and 3 turns, and the size of the hard region in the disc centre gradually reduces as the numbers of turns increases from 1/2 to 3 turns. The hardness values evolve towards homogeneity along the disc diameters after 5 and 10 turns. Electron backscatter diffraction (EBSD) and X-ray line profile analysis suggest that the lower hardness values at the disc edge area in the Al-0.1% Mg alloy are related to a recovery / recrystallization mechanism where the material is subjected to heavy straining.

Texture evolution is studied by EBSD in Mg–9Gd–4Y–0.4Zr (GW94), Mg–5Gd–4Y–0.4Zr (GW54), and Mg–5Gd–0.4Zr (GW50) alloys after processing with two extrusion-ratios of 19:1 and 8:1. While the alloys show common extrusion textures at the low extrusion-ratio of 8:1, an unusual extrusion texture is observed in the GW94 alloy at the high extrusion-ratio of 19:1 with the (0001) basal planes aligned perpendicular to the extrusion axis. This unusual texture decreases at lower extrusion-ratios and/or lower alloy contents. It is proposed that rare-earth (RE) elements modify the texture by retarding recrystallization in solute drag effects on grain boundaries and pinning effects of RE-rich precipitates.

Pure copper is subjected to severe plastic deformation at room temperature using quasi-constrained high-pressure torsion through 1/4 to 10 turns. The evolution of the microstructure is monitored using X-ray diffraction and electron backscatter diffraction. Specimens are analyzed both before and after micro-compression of ?50%. The results show that after 10 turns there is a homogeneous ultrafine-grained (UFG) microstructure with an average grain size of ?250?nm and with a simple shear texture. The micro-compression testing shows that UFG specimens exhibit a better surface quality after compression thereby demonstrating an excellent potential for using UFG copper in micro-forming.

A very high-purity (99.999%) aluminum was processed by equal-channel angular pressing (ECAP) at room temperature through 1 to 8 passes using a die with a channel angle of 90°. Analysis shows that processing by ECAP produces an ultrafine-grained (UFG) structure with a grain size of ?1.3??m and with microhardness and microstructural homogeneity. The mechanical properties and the fracture behavior were evaluated using micro-tensile testing after ECAP processing. A micro-forming process was used to fabricate a micro-turbine at ambient temperature and subsequent examination demonstrated that UFG pure aluminum gives higher strength and more uniform mechanical properties by comparison with conventional coarse-grained pure aluminum. The results confirm the very significant potential for using UFG pure aluminum for micro-forming at ambient temperature.

This research examines the creep behavior of an ultrafine-grained (UFG) Ti−6Al−4V alloy processed by equal-channel angular pressing followed by extrusion. It is shown that modifying the surface of the UFG alloy with nitrogen ions and then applying of a coating of (Ti + V)N inhibits the softening of the UFG alloy at temperatures up to 700 K due to a barrier effect in which the coating hinders the release of dislocations onto the surface. The differences in the mechanisms of crack initiation and failure of UFG samples are also examined both with and without a coating. The prospects of the proposed approach to the improving of titanium alloys are discussed, including the formation of an UFG structure in the bulk of the material and subsequent modification by ion-plasma methods for the manufacture of highly loaded parts operating at elevated operating temperatures.

This review provides an overview of the mechanical bonding of dissimilar bulk engineering metals through high-pressure torsion (HPT) processing at room temperature. A recently developed procedure of mechanical bonding involves the application of conventional HPT processing to alternately stacked two or more disks of dissimilar metals. A macro-scale microstructural evolution involves the concept of making tribomaterials and, for some dissimilar metal combinations, micro-scale microstructural changes demonstrate the synthesis of metal matrix nanocomposites (MMNCs) through the nucleation of nano-scale intermetallic compounds within the nanostructured metal matrix. Further straining by HPT during the mechanical bonding provides an opportunity to introduce limited amorphous phases and a bulk metastable state. The mechanically-bonded nanostructured hybrid alloys exhibit exceptionally high specific strength and an enhanced plasticity. These experimental findings suggest a potential for using mechanical bonding for simply and expeditiously fabricating a wide range of new alloy systems by HPT processing.

Experiments were conducted to investigate the low-cycle deformation behavior of an ultrafine-grained (UFG) Ti–6Al–4V alloy produced by severe plastic deformation (SPD) through a combination of equal-channel angular pressing (ECAP) and extrusion. The fatigue properties were examined by stress-controlled fatigue testing at stress amplitudes in the range of 700–1050 MPa. The specimens exhibited a dimpled final overload fracture mode. The results show the UFG alloy has a longer fatigue life and a finer dimple size than the coarse-grained material.

The transformation between the face centered cubic austenitic and hexagonal close-packed martensitic phases during high-pressure torsion processing was observed in a Fe–Mn–C–Al twinning-induced plasticity steel. This phase transformation was not found in the same material processed by unidirectional compressive and tensile deformation. Initiated by the high-pressure loading, the martensite phase initially increased with torsional strain but diminished subsequently. Texture evolution of the austenitic phase was compared with the ideal texture distribution of face-centered cubic materials after shear deformation.

Cu/Nb multilayer composites with minimum individual layer thicknesses of ≈2.8 μm are achieved by accumulative roll bonding (ARB). The microstructural evolution and mechanical properties of these composites are investigated with different layer thicknesses after ARB processing. The results show that there is no visible interfacial reaction between the Cu and Nb layers, and the kernel average misorientation (KAM) distributions in electron backscatter diffraction (EBSD) maps remain in steady state during the third to seventh ARB cycles. The tensile testing results demonstrate that the yield strength increases with decreasing layer thickness in Cu/Nb multilayer composites. A simultaneous increase of strength and elongation is achieved by regulating the laminated structures. Microstructure and fracture analysis indicate that the simultaneous increase of strength and elongation is attributable to the high density of bimetal interfaces, which act as a barrier for dislocation mobility and crack propagation.

Two different processing routes are used to investigate the microstructure and strength of commercial purity (CP) titanium of grade 4 processed by equal‐channel angular pressing (ECAP). In the combined temperature (CT) route, the specimens are pressed at 723 K in the first pass and at 373 K in the second pass, but in the warm temperature (WT) route, the specimens are pressed through two passes at 723 K. Both routes lead to an inhomogeneous microstructure with an average grain size of ≈1.5 and ≈1.7 μm after the CT and WT routes, respectively. Both routes give improved strengthening and higher hardness, but the CT route with a lower temperature step gives the highest ultimate tensile strength of ≈790 MPa. The inclusion of a lower temperature processing step may be important for optimizing the strength of CP Ti for the use in medical implants.

High-pressure torsion (HPT) is a significant procedure for achieving substantial grain refinement but it may be used also to consolidate metallic particles to form bulk samples or composites where two (or more) different phases are mixed and consolidated. Herein, the consolidation of particles of the magnesium AZ91 alloy and a composite with an AZ91 matrix combined with 1% alumina powder is investigated. The results show that it is possible to fully consolidate this alloy after a large number of turns. As a consequence of the severe plastic deformation, the grain structure is significantly refined, with average grain sizes of 116 and 98 nm in the unreinforced alloy after 20 or 50 HPT turns and 76 nm in the composite after 50 HPT turns, respectively. This grain refinement is associated with a decrease in hardness and an increase in the strain rate sensitivity due to the onset of a grain boundary diffusion–assisted creep mechanism at room temperature. The results are consistent with the theoretical prediction of a breakdown in the Hall–Petch relationship at very small grain sizes.

A long‐term career at the University of Southern California, followed by an appointment at the University of Southampton, provided an opportunity to conduct extensive research into the flow behavior of polycrystalline metals. Initially, research is conducted on creep properties at elevated temperatures and it is shown that solid solution metallic alloys exhibit transitions in creep behavior with dislocation climb and viscous glide as the dominant rate‐controlling mechanisms. There are transitions between climb and glide with increasing stress and also a breakaway from the glide process at high stresses. These transitions are predicted theoretically and the results are in excellent agreement with the experimental data for a wide range of alloys. Attention is directed to the process of superplasticity and it is shown that the flow occurs by grain boundary sliding with accommodation by a limited amount of intragranular slip. Separate rate equations are developed for sliding in coarse‐grained materials and in superplastic materials where the grain sizes are generally

A ternary TiNiCu shape memory alloy is subjected to high‐pressure torsion (HPT) followed by post‐deformation annealing (PDA) to study the effect of Cu (5 at%) on amorphization after HPT processing and to investigate the microstructural evolution and shape memory effect (SME) after PDA. The results show that even after 20 revolutions the ternary alloy contains nanocrystalline areas and the microstructure is not fully amorphous. An easier martensite‐to‐austenite transformation and minor remaining austenite in the ternary alloy are responsible for suppressing amorphization. Post‐deformation annealing at 673 K provides nanocrystalline microstructures containing an R‐phase with a minor martensitic B19' phase in the ternary alloy. The SME of this alloy after PDA is not as satisfactory as that of the binary alloy processed through similar conditions because of the existence of a high volume fraction of the R‐phase. However, the total recovered strain of the ternary alloy after PDA for 30 min has a maximum value of 6.5%.

The microstructural and mechanical properties of an ultrafine-grained (UFG) Al–Zn alloy processed by high-pressure torsion (HPT) are investigated using depth-sensing indentations, focused ion beam, scanning electron microscopy and scanning transmission electron microscopy. Emphasis is placed on the microstructure and the effects of grain boundaries at room temperature. The experiments show the formation of Zn-rich layers at the Al/Al grain boundaries that enhance the role of grain boundary sliding leading to unique plastic behavior in this UFG material. The occurrence of significant grain boundary sliding at room temperature is demonstrated by deforming micro-pillars. Our results illustrate a potential for using UFG materials as advanced functional materials in electronic micro-devices.

A new design of billet, based on a core–sheath configuration, was used for the processing of Ni, Fe, and a NiTi alloy by equal-channel angular pressing (ECAP) at room temperature. This configuration involves inserting metal cores within Fe sheaths prior to processing and it is designed especially for use with hard-to-deform materials. Billets were processed through one or two ECAP passes at room temperature and the microhardness values were recorded across the transverse directions within the cores to evaluate the flow process. As in conventional ECAP, the hardness increased significantly after the first pass and there were regions of lower hardness along the bottom surfaces of each core. The gradient of hardness decreased with increasing core diameter but the average microhardness values remained unchanged. Three-dimensional finite element simulations were used to evaluate the flow behavior after one pass of ECAP using different core metals. These simulations show the lower areas of the cores undergo less deformation than the upper areas and the homogeneity increases with increasing levels of friction at the core–sheath interface.

Processing through the application of severe plastic deformation (SPD) is attractive because it produces significant grain refinement and high strength. The standard procedure for performing SPD processing is through the use of equal-channel angular pressing (ECAP) but in practice it is difficult to perform ECAP on the magnesium AZ31 alloy at room temperature because the material cracks or exhibits segmentation. This difficulty was avoided in the present investigation by processing the alloy using high-pressure torsion (HPT). The results show that HPT provides an excellent procedure for producing significant grain refinement in the AZ31 alloy. At temperatures of 296 and 373 K, the processed grain sizes are in the submicrometer range and there is an evolution toward microstructural homogeneity after 5 turns. By contrast, at the higher temperature of 473 K, which is a typical temperature for ECAP, the grains grow during the processing operation.

Pure Cu was processed by ECAP at five different temperatures from room temperature (RT) to 523?K. The influence of pressing temperature on microstructure evolution and tensile behavior was investigated in detail. The results show that as the ECAP temperature is increased the grain size and ductility both increase whereas the dislocation density and yield strength decrease. In the case of ECAP processing in the range of RT to 473?K the mechanism governing microstructural refinement is continuous dynamic recrystallization (CDRX), whereas at 523?K the mechanism changes to discontinuous dynamic recrystallization (DDRX). At higher ECAP temperatures, the kinetics of CDRX are retarded leading to a lower fraction of equiaxed grains/high-angle grain boundaries and a higher fraction of dislocation cell structures. At 523?K, DDRX induces a high fraction of equiaxed grains with a very low dislocation density which appears responsible for the observed high tensile ductility. The sample processed at 523?K possessed a good combination of strength and ductility, suggesting that processing by ECAP at elevated temperatures may be a suitable alternative to RT ECAP processing followed by subsequent annealing.

A nanocrystalline bcc Ti67.4Nb24.6Zr5Sn3 alloy is shown to fracture in an intrinsically ductile manner with exceptionally large dimples (up to 10 mu m) which are two orders of magnitude greater than the grain size (approximate to 40nm). This large plasticity length scale is attributed to a combination of low shear modulus (approximate to 27GPa), high Poisson's ratio (approximate to 0.4) and ultrahigh strength (UTS approximate to 1.1GPa), close to the ideal shear stress, which facilitates ideal shear deformation to promote transgranular shear.

Severe plastic deformation by high pressure torsion (HPT) is used to process and refine the grain structure of commercial purity magnesium and AZ31, AZ91, and ZK60 magnesium alloys. Transmission electron microscopy shows that the microstructure of pure magnesium is characterized by a bi-modal grain size distribution with grains in the range of a few microns and ultrafine grains after HPT, whereas the magnesium alloys display a homogeneous ultrafine grain structure after processing. X ray diffraction analysis reveals that the AZ91 alloy displays the largest lattice microstrain and this alloy also exhibits the highest hardness after processing. The processed AZ31 and the ZK60 alloys show similar microstructures and maximum values of hardness. Contrary to earlier reports of significant improvements in the corrosion resistance of magnesium alloys in biological environments, the present results show that processing by HPT has no significant effect on the corrosion behavior of magnesium alloys in a 3.5% NaCl solution. By contrast, pure magnesium exhibits an increased corrosion resistance after HPT.

In recent years a breakthrough has developed in the studies of nanostructured metals and alloys as advanced structural and functional materials associated both with the development of new routes for the fabrication of bulk nanostructured materials using severe plastic deformation (SPD) and with investigations of the fundamental mechanisms that lead to the new properties of these materials. This review paper discusses new concepts and principles in using SPD processing to fabricate bulk nanostructured metals with advanced properties. Special emphasis is placed on the relationship between microstructural features and properties, as well as the innovation potential of SPD-produced nanomaterials.

This report describes a study of the regularities in the kinetics of microstructure evolution and recrystallization processes in the Ti5.7Al3.8Mo1.2Zr1.3Sn alloy (Russian analogue VT8M-1) during processing by equal-channel angular pressing (ECAP). To produce a duplex (globular-lamellar) structure, the billets were subjected to preliminary heat treatment, including water-quenching from a temperature below the β-transus followed by annealing at 700ºС. For the ECAP temperature selection, the deformation behavior of the alloy with a duplex structure was investigated under upsetting in the temperature range of 650-800ºС. The evolution of the globular and lamellar fractions was examined during ECAP processing, and an emphasis was placed on the role of phase transformations, dynamic recrystallization and spheroidization of the α-phase which is realized with increasing accumulated strain when processing by ECAP. It is demonstrated that after 6 ECAP passes an equiaxed ultrafine-grained structure is formed with a mean α-phase grain/subgrain size of ~0.6 µm. The investigation includes an examination of the effect of microstructure on the mechanical properties of the alloy.

The Mg-5.5 % Zn-0.5 % Zr alloy exhibits large superplastic elongations after processing by Equal-Channel Angular Pressing and these are strongly dependent on the number of passes through the die. Grain refinement improves the elongations up to a peak of 3050 % after 2 passes but grain growth limits ductility after larger numbers of passes.

The innovation potential is high for bulk nanostructured materials (BNM) produced by methods of severe plastic deformation and accordingly this report focuses on very recent developments demonstrating the potential of using BNM for advanced and functional applications in engineering and medicine.

Bulk ultra-fine-grained (UFG) materials produced by severe plastic deformation (SPD) usually have high strength but relatively low ductility at ambient temperatures. This low ductility is attributed to insufficient strain hardening due to an inability to accumulate dislocations. For a singlephased UFG material where dislocation slip is the primary deformation mechanism, a long-standing fundamental question concerns the feasibility of developing microstructures that offer high ductility without sacrificing strength. The answer appears to be positive because there are some isolated examples where excellent mechanical behavior has been observed. Nevertheless, the structural features contributing to high strength and good ductility remain undefined, and this lack of understanding has hindered the search for effective procedures to simultaneously improve the strength and ductility of UFG materials. Here, we report a new process in which high ductility is achieved without sacrificing strength by plastically deforming UFG Cu in liquid nitrogen. The enhanced ductility is attributed primarily to the presence of a high density of preexisting deformation twins (PDTs) and also possibly to a large fraction of high-angle grain boundaries (HAGBs) formed during cryogenic processing. We conclude that this procedure provides a new strategy for increasing the ductility of UFG materials without any concurrent loss in strength. Strength and ductility are often mutually exclusive, i.e., materials may be strong or ductile but are rarely both. This also applies to bulk UFG materials. The low ductility of UFG materials has invariably limited their practical application and, accordingly, much attention has been paid to the development of strategies for improving this poor ductility. For singlephase UFG and nanostructured materials, several of the reports documenting high ductility and strength describe experiments on Cu where the stacking-fault energy is relatively low. In some investigations the high ductility was attributed to the development of a bimodal grain size distribution or pre-existing growth twins (PGTs), but in other investigations the reasons for the high ductility were not clearly defined. In practice, however, a bimodal grain size distribution must sacrifice some of the strength gained from nanostructuring. Another challenge is the need to fabricate UFG materials in large bulk form suitable for structural applications. This requirement has been hindered because the evidence suggests that PGTs occur only in electrodeposited thin films of nanostructured Cu, and in nanocrystalline Cu by inert-gas condensation (IGC) followed by compaction. However, the ductility of IGC-prepared nanocrystalline Cu is very low. The objectives of this study were twofold: First, to develop a procedure for increasing the ductility of large bulk UFG Cu without incurring any significant loss in strength. Second, to evaluate the mechanism contributing to high ductility in UFG Cu. A pure Cu (99.99%) bar was initially processed by equalchannel angular pressing (ECAP) to produce a UFG structure (hereafter designated the UFGECAP sample), then cryodrawn (D) to a reduction in area of ca. 95%, followed by cryorolling (R) with a reduction in thickness of ca. 96% (hereafter designated the UFGECAP+D+R sample). Figure 1a shows that the UFGECAP+D+R sample has superior mechanical properties compared to the UFGECAP sample. The UFGECAP Cu sample has a 0.2% yield strength of ca. 410 MPa ( ), which is significantly higher than the value of ca. 40 MPa in coarse-grained (CG) Cu. In addition, necking occurs rapidly after the stress reaches a maximum value, yielding a uniform elongation of only ca. 1.3% and an elongation to failure of only ca. 5.9% in the UFGECAP sample. By contrast, the yield strength is increased to ca. 500 MPa in the UFGECAP+D+R sample, and, more importantly, this sample undergoes strain hardening, giving a uniform elongation of C O M M U N IC A IO N S

Processing by severe plastic deformation (SPD) leads to very significant grain refinement in metallic alloys. Furthermore, if these ultrafine grains are reasonably stable at elevated temperatures, there is a potential for achieving high tensile ductilities, and superplastic elongations, in alloys that are generally not superplastic. In addition, the production of ultrafine grains leads to the occurrence of superplastic flow at strain rates that are significantly faster than in conventional alloys so that processing by SPD introduces the possibility of using these alloys for the rapid fabrication of complex parts through superplastic forming operations. This paper examines the development of superplasticity in various aluminum alloys processed by equal-channel angular pressing (ECAP).

The effect of the initial annealing temperature on the evolution of microstructure and microhardness in high purity OFHC Cu was investigated after processing by HPT. Disks of Cu were annealed for one hour at two different annealing temperatures, 400 and 800°C, and then processed by HPT at room temperature under a pressure of 6.0 GPa for 1/4, 1/2, 1, 5 and 10 turns. Samples were stored for 6 months after HPT processing to examine the self-annealing effects. Electron backscattered diffraction (EBSD) measurements were recorded for each disk at three positions: center, mid-radius and near edge. Microhardness measurements were also recorded along the diameters of each disk. Both alloys showed rapid hardening and then strain softening in the very early stages of straining due to self-annealing with a clear delay in the onset of softening in the alloy initially annealed at 800°C. This delay was due to the relatively larger initial grain size compared to the alloy initially annealed at 400°C. The final microstructures consisted of homogeneous fine grain sizes having average sizes of ~0.28 and ~0.34 μm for the alloys initially annealed at 400 and 800°C, respectively. A new model is proposed to describe the behavior of the hardness evolution by HPT in high purity OFHC Cu.

Severe plastic deformation may be used as a processing tool to achieve a refinement in grain size in metallic alloys to the submicrometer or nanometer range. This paper describes recent developments using the procedure of equal-channel angular pressing (ECAP) in which samples are pressed through a die containing a channel bent into an L-shaped configuration. The shearing associated with passage through the die introduces bands of subgrains which evolve, with additional pressings, into arrays of grains separated by boundaries having high angles of misorientation. The process of ECAP is a useful tool for both increasing the strength and toughness of an alloy at ambient temperatures and achieving a potential for superplastic forming of the alloy at rapid strain rates at elevated temperatures.

Disks of an extruded magnesium WE43 alloy are processed by high-pressure torsion (HPT) and the evolutions of microstructure and mechanical properties are investigated in detail. Excellent grain refinement is achieved by HPT processing with a reduction in grain size from an initial value of ?12??m to a final value of ?200–300?nm after 10 turns. The microhardness increases significantly with HPT processing but low hardness values are recorded at the centers of the disks. The tensile strength initially increases and then decreases while the elongation decreases. Observations of the fracture surfaces reveal a corresponding transition of the fracture mode from ductile to brittle.

The processing of bulk metals through the application of severe plastic deformation (SPD) provides the potential for achieving exceptional grain refinement leading to ultrafine-grained (UFG) materials. These materials generally exhibit high strength but very limited ductility at room temperature. The recent development in the nanoindentation technique provides significant feasibility in evaluating the micro-mechanical behavior of UFG materials. Accordingly, this review examines the available experimental results showing the enhancement in strength and ductility through nanoindentation analysis in various materials after different SPD techniques. A comprehensive tabulation is also presented listing the available data for the strain rate sensitivity, m, in a variety of UFG metals processed by SPD.

The cover demonstrates the innovation potential of bulk nanostructured materials obtained by severe plastic deformation (SPD) in various industrial sectors. The microstructure of Al wire processed by continuous SPD processing in ECAP-Conform together with a schematic illustration of an ECAP-Conform set-up are shown in the background. Further information about the perspectives of commercialization of bulk nanostructured materials produced by SPD processing can be found in the review paper by R. Z. Valiev et al. on page 527. The innovation potential is high for bulk nanostructured materials (BNM) produced by methods of severe plastic deformation and accordingly this report focuses on very recent developments demonstrating the potential of using BNM for advanced and functional applications in engineering and medicine.

Experiments were conducted on monolithic Al[sub 2]O[sub 3] and Al[sub 2]O[sub 3] composites reinforced with SiC-whiskers in which the load was removed during high-temperature creep. The monolithic material exhibited no anelastic recovery on load removal, but the composites showed a significant anelastic recovery which was reproducible and essentially independent of the total creep strain at which the load was removed. The results were consistent with the hypothesis that an interconnecting whisker network may develop during processing of the composites.

SUMMARY There are two distinct groups of superplastic materials, depending on whether the final fracture surface has a large cross-sectional area at optimum ductility or the specimen gradually pulls down to a very fine point. Experiments on a commercial superplastic copper alloy and the Zn-22% Al eutectoid indicate that the formation of internal voids is important in materials of both groups. In addition, the results show that the size and shape of the internal voids is dependent both on the material and the testing conditions.

Six types of polycrystalline MgO, one nominally fully dense and the others having porosities of ∼1 to 2%, were tested in compression up to 1400°C. At 1200°C and above, all materials deformed plastically; two of the materials, both porous, also exhibited plastic flow at temperatures down to 800°C, and a third at 1000°C. A qualitative analysis of the microstructures of these materials indicated that the differences in behavior arose primarily because of variations in the size and distribution of pores and in the concentration of impurities at the grain boundaries. It is suggested that the following factors aid plasticity below ∼1200°C: (1) Strong grain boundaries, in the absence of excessive impurities, permit buildup of stress concentrations with consequent nucleation of slip on the {100} system and the extension of slip across the boundaries and (2) clusters of very fine pores within the grains allow some mass accommodation.

For NaCl-KCl solid solutions, the shape of creep curves was consistent with that anticipated for class II alloy behavior. For class II alloys the stress component is approximately 3, there is little or no instantaneous strain on application of load, the creep curve exhibits little or no primary creep, the steady-state creep rate appears to be independent of the stacking-fault energy of the alloy, and subgrains either are not formed during creep, or, if formed, are not very extensive. (WDM)

Die Grosenordnung des Korngrenzengleitens wird interferometrisch an 6 polykristallinen MgO-Proben (Korngrosen: 33 und 52 μm) nach Kompressionskriechen bei 1200°C unter Spannungen zwischen 34,4 und 103,3 MN m(-2) gemessen.

The magnitude of grain-boundary sliding in 6 specimens of polycrystalline MgO of 2 grain sizes (33 and 52 μm) was measured by interferometry after compressive creep at 1200°C uner stresses from 34.4 to 103.3 MN m-2. Grain-boundary sliding increases in importance with decreasing stress and/or grain size, although the largest strain contributed by sliding under these experimental conditions was only 20% of the total strain.

For NaCl-KCl solid solutions, the shape of creep curves was consistent with that anticipated for class II alloy behavior. For class II alloys the stress component is approximately 3, there is little or no instantaneous strain on application of load, the creep curve exhibits little or no primary creep, the steady-state creep rate appears to be independent of the stacking-fault energy of the alloy, and subgrains either are not formed during creep, or, if formed, are not very extensive. (WDM)