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The past decade was marked by significant advances in the development of severe plastic deformation (SPD) techniques to achieve new and superior properties in various materials. This review examines the achievements in these areas of study and explores promising trends in further research and development. SPD processing provides strong grain refinement at the nanoscale and produces very high dislocation and point defect densities as well as unusual phase transformations associated with particle dissolution, precipitation, or amorphization. Such SPD-induced nanostructural features strongly influence deformation and transport mechanisms and can substantially enhance the performance of advanced materials. Exploiting this knowledge, we discuss the concept of nanostructural design of metals and alloys for multifunctional properties such as high strength and electrical conductivity, superplasticity, increased radiation, and corrosion tolerance. Special emphasis is placed on advanced metallic biomaterials that promote innovative applications in medicine.

High-purity Ni was processed by high-pressure torsion (HPT) at room temperature under an imposed pressure of 6.0 GPa and a rotation rate of 1 rpm through 1/4 to 10 turns, and samples were then examined using Electron Back-Scattered Diffraction (EBSD) and microhardness measurements. The results show that the grain size and low-angle grain boundaries (LAGBs) gradually decrease with the growth of HPT revolutions while the microhardness values gradually increase. After 10 turns of HPT processing, ultrafine-grained (UFG) pure Ni with a reasonable microhardness value and microstructure homogeneity can be achieved across the disk, thereby giving great potential for applications in micro-forming. A grain refinement model for severe plastic deformation (SPD) of pure Ni is proposed.

The evolutions of microstructure and texture and the corrosion behaviour of low light rare-earth containing Mg-1.4Nd and low heavy rare-earth containing Mg-0.6Gd and Mg-0.4Dy (wt.%) were evaluated and compared after processing by high-pressure torsion (HPT) and isochronal annealing at 250 and 450 °C for 1 h using electron backscatter diffraction (EBSD) and electrochemical tests in a 3.5% (wt.%) NaCl solution. The EBSD results show that dynamic recrystallisation (DRX) was restricted in the Mg-1.4Nd alloy which led to a heterogenous deformation microstructure whereas the Mg-0.6Gd and Mg-0.4Dy alloys exhibited a homogenous deformation microstructure formed mostly of equiaxed dynamically recrystallised DRX grains. The HPT processing caused the development of a deviated basal texture in the three alloys. A good thermal stability of the three alloys was noticed after annealing at 250 °C. By contrast, annealing at 450 °C led to a homogenous equiaxed microstructure and weakening of texture for the Mg-1.4Nd alloy and a heterogenous bimodal microstructure with a stable basal texture for the Mg-0.6Gd and Mg-0.4Dy alloys. The HPT-processed Mg–RE alloys exhibited an improved corrosion resistance due to grain refinement. Thereafter, the corrosion resistance of the Mg-0.6Gd and Mg-0.4Dy alloys decreased with increasing annealing temperature due to an increase in grain size while the corrosion resistance of the Mg-1.4Nd alloy was improved after annealing at 450 °C due to precipitation and texture weakening.

Superplasticity refers to the ability of some materials to pull out to tensile elongations of 400% or more when the strain rate sensitivity is ~0.5. The first report of true superplastic flow was published in 1934 in experiments conducted in England. However, this remarkable result attracted little interest among western scientific researchers and the result remained a laboratory curiosity for many years. Later, following extensive research on superplasticity in the Soviet Union, interest developed in the west, and superplasticity became a topic of extensive scientific research. This research was further enhanced with the demonstration that the application of severe plastic deformation provided an opportunity for achieving grain refinement to the submicrometer or even the nanometer level, and these small grains were especially attractive for achieving good superplastic properties. It is now recognized that superplastic alloys provide an excellent forming capability, especially in making high quality curved parts that are not easily fabricated using more conventional processes. This has led to the development of a large superplastic forming industry that currently processes many thousands of tons of sheet metals. This report traces these developments with an emphasis on the scientific principles behind the occurrence of superplastic flow.

Processing magnesium alloy by High-Pressure Torsion (HPT) is a technique used successfully to refine the grains of an alloy to the submicrometer and nanometer scale to produce an ultrafine grained microstructure. Grain refinement can improve the mechanical properties of magnesium alloys as well as enhancing its ductility and providing a potential for exhibiting superplastic behaviour at elevated temperature. Research was conducted to process the AZ80 magnesium alloy by HPT at room temperature for different numbers of turns with the microstructures before and after HPT critically investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Vickers microhardness (Hv) tests. Subsequently, tensile specimens were cut from the processed disks and pulled in tension to failure at temperatures of 473, 523 and 573 K and at strain rates in the range from 1.4 × 10 4 to 1.4 × 10-¬1 s-1. The introduction of superplasticity in the AZ80 magnesium alloy when processing by HPT was demonstrated for the first time with a maximum elongation of 645% at a testing temperature of 573 K. There was also evidence for low temperature superplasticity with an elongation of 423% at 473 K. The dominant mechanism for superplastic flow was grain boundary sliding with a strain rate sensitivity of m ≈ 0.5 and an activation energy of Q ≈ 73 kJ mol-1.

The processing of bulk metals through the application of severe plastic deformation (SPD), using procedures such as equal-channel angular pressing (ECAP) and high-pressure torsion (HPT), is now well established for the fabrication of materials with exceptionally small grain sizes, usually in the submicrometer range and often having grain sizes at the nanometer level. These grain sizes cannot be achieved using thermo-mechanical processing or any conventional processing techniques. Recently, these procedures have been further developed to process alternative advanced materials. For example, by stacking separate disks within the HPT facility for the synthesis of bulk nanocrystalline metastable alloys where it is possible to achieve exceptionally high hardness, or by pressing powders or metallic particles in order to obtain new and novel nanocomposites exhibiting unusual properties.

Superplasticity denotes the ability of a limited number of materials to achieve exceptionally high tensile elongations of at least 400%. Experiments show that the Al-Mg-Sc alloys provide excellent capabilities for achieving superplastic flow and also they can be formed easily in biaxial superplastic forming operations. It is important, therefore, to examine the superplastic flow mechanism when the alloy is prepared using different procedures. This report examines the superplastic characteristics of these alloys after preparation without subjecting to any severe plastic deformation (SPD), after processing using the two SPD procedures of equal-channel angular pressing (ECAP) and high-pressure torsion (HPT) and after processing using the alternative procedure of friction stir processing (FSP). The results are compared using each technique and they are examined with reference to a theoretical model that was developed specifically for superplastic flow in conventional alloys.

Processing through the application of high-pressure torsion (HPT) provides significant grain refinement in bulk metals at room temperature. These ultrafine-grained (UFG) materials after HPT generally demonstrate exceptional mechanical properties. Recent reports demonstrated the bulk-state reactions for mechanical bonding of dissimilar lightweight metal disks to synthesize hybrid alloy systems by utilizing conventional HPT processing. Accordingly, the present report provides a comprehensive summary of the recent work on processing of several UFG hybrid alloy systems including Al-Mg and Al-Cu by HPT under 6.0 GPa at room temperature and a special emphasis was placed on understanding the evolution of hardness. This study demonstrates a significant opportunity for the application of HPT for a possible contribution to current enhancements in diffusion bonding, welding and mechanical joining technologies as well as to an introduction of hybrid engineering nanomaterials.

High-pressure torsion (HPT) was conducted under 6.0 GPa on commercial purity titanium up to 10 turns. An ultrafine-grained (UFG) pure Ti with an average grain size of ~96 nm was obtained. The thermal properties of these samples were studied by using differential scanning calorimeter (DSC) which allowed the quantitative determination of the evolution of stored energy, the recrystallization temperatures, the activation energy involved in the recrystallization of the material and the evolution of the recrystallized fraction with temperature. The results show that the stored energy increases, beyond which the stored energy seems to level off to a saturated value with increase of HPT up to 5 turns. An average activation energy of about 101 kJ/mol for the recrystallization of 5 turns samples was determined. Also, the thermal stability of the grains of the 5 turns samples with subsequent heat treatments were investigated by microstructural analysis and Vickers microhardness measurements. It is shown that the average grain size remains below 246 nm when the annealing temperature is below 500 °C, and the size of the grains increases significantly for samples at the annealing temperature of 600 °C.

An aluminum silicon-based alloy Al-7wt%Si-2wt%Fe, was processed by severe plastic deformation technique in high-pressure torsion (HPT) at room temperature under a pressure of 6.0 GPa and rotational speed of 1.0 rpm with various numbers of turns up to five. Microstructure evolution, especially iron-containing intermetallic phases, was observed using an optical microscope and a scanning electron microscope (SEM). The microstructure results demonstrate that the large strains introduced by HPT at ambient temperature cause fragmentation of iron-intermetallic particles. The degree of fragmentation increases with increasing numbers of turns so that the intermetallic particles decreased in size with increasing imposed strain. In addition, the wear properties were evaluated using ball-on-disc dry sliding testing for both the as-cast material and the alloy processed by HPT using micro-tribometer UMT-2 (CETR Co., USA) following the ASTM G99-05 (2010) standard. The wear tests were conducted on the surface of the samples at 1.5 mm from the disc center under a normal load of 5 N with a rotational speed of 60 rpm and sliding time of 10 min. The friction coefficient and wear volume loss were examined to evaluate the effect of HPT on wear resistance. The results show that the samples processed by HPT have lower average values for the COF and wear volume loss than that of unprocessed samples.

Commercial purity titanium (CP-Ti) and a Ti-6Al-4V alloy (Ti64) were processed by high-pressure torsion (HPT) for 10 and 20 turns. The HPT processing produced a nanostructured microstructure and a significant strength enhancement in the CP-Ti and Ti64 samples. After 20 turns, the samples of HPT-processed CP-Ti and Ti64 were subjected to laser surface treatments in an air atmosphere using different scanning speeds and laser powers. The surface roughness of the laser-modified samples increased with increasing laser power and this produced hydrophilicity due to a lower contact angle. After a holding time of 27 days, these samples underwent a hydrophilic-to-hydrophobic transformation as the contact angle increased from 13° to as much as 120° for the CP-Ti sample, and for the Ti64 sample the contact angle increased from 10° to 126°. In addition, the laser surface modification process was carried out with different atmospheres (air, vacuum and O2) on heat-treated but unstrained CP-Ti and Ti64 samples and the contact angle changed due to the surface element content. Thus, as the carbon content increased from 28% to 47% in CP-Ti in a vacuum environment, the surface contact angle increased from 22° to 140°. When a laser surface modification process is conducted under oxygen-less conditions, it is concluded that the contact angle increases rapidly in order to control the hydrophobic properties of Ti and the Ti alloy.

An investigation was conducted to evaluate the effect of dynamic plastic deformation (DPD) and post-DPD annealing on the microstructural evolution and mechanical properties of a tempered low carbon-low alloy steel. The results showed that ultrafine-grained structures consisting of elongated martensitic laths and sub-grains are achieved after DPD processing. The amounts and sizes of carbides in the steels, identified as (Fe, Cr, Mn, Mo)3C, decreased markedly with DPD straining due to their fragmentation and dissolution but the strength increased and the plasticity showed a slight decrease with increasing DPD strain. A simultaneous improvement in the strength and plasticity was obtained at strains below ~0.8. This increase in strength by ~30-60% after DPD processing by comparison with the as-received state is attributed primarily to grain boundary strengthening and dislocation strengthening and the good plasticity is due to the occurrence of more active sliding systems and a reduction in the stress concentration during loading because of a decrease in the amounts of M3C distributed along the interfaces. After post-DPD annealing both the strength and the plasticity improved by comparison with the as-received steel in the tempered state. Strengths higher by ~15-35% than the as-received condition were attributed to a combination of grain boundary strengthening, dislocation strengthening and precipitation strengthening derived from a re-precipitation of fine and dispersed carbides. The dislocation recovery occurring during annealing led to a decrease in strength compared with the strength before annealing. The incremental increase in plasticity after annealing is ascribed to dislocation recovery and the inhibition in micro-crack propagation due to an even distribution of fine carbides within the matrix

The occurrence of superplastic flow in metals has a long history but it is only over the last three or four decades that it was recognized that this process provides an opportunity for fabricating complex parts, especially curved panels, that may be used in a wide range of industrial applications. In practice, this use is dependent upon the high strain rate sensitivity of ~0.5 which is an inherent feature of true superplastic flow but in practice excellent forming may be achieved also through the use of metals deforming within the range of dislocation glide where the strain rate sensitivity is close to 0.3. New possibilities have arisen over the last two decades with the demonstrations that exceptionally refined microstructures, usually within the submicrometer or even the nanometer range, may be prepared from a wide range of commercial alloys through the application of severe plastic deformation in which the material is subjected to a very high strain without any significant changes in the overall dimensions of the sample. This presentation examines these historical developments and describes the new processing procedures that provide new opportunities within the field of superplastic forming.

The effect of the grain size on the mechanical properties of metallic materials has been a topic of significant interest for researchers and industry. For many decades a relationship defining the mechanical strength proportional to the inverse of the square root of the grain size has been widely accepted despite some reports of deviations from this behavior. Nevertheless, the initial explanations for this relationship, based mainly on the activation of slip systems by dislocation pile-ups at grain boundaries, have provided essentially no predictive capability. Here we show that a physically-based model for grain boundary sliding predicts, in excellent agreement with experimental data, the flow stress for plastic deformation for a broad range of materials using the fundamental properties of each material over a wide range of grain sizes and testing conditions. This mechanism also successfully predicts the reported enhanced strain rate sensitivity in ultrafine and nanocrystalline materials at different temperatures.

There is a great interest in improving mechanical testing of small samples produced in the laboratory. Plane strain compression is an effective test in which the workpiece is a thin sheet. This provides great potential for testing samples produced by high-pressure torsion. Thus, a custom tool was designed with the aim to test 10 mm diameter discs processed by this technique. Finite element analysis is used to evaluate the deformation zone, stress and strain distribution, and the accuracy in the estimation of stress–strain curves. Pure magnesium and a magnesium alloy processed by high-pressure torsion are tested using this custom-made tool. The trends observed in strength and ductility agree with trends reported in the literature for these materials.

Severe plastic deformation (SPD) is effective in producing bulk ultrafine-grained and nanostructured materials with large densities of lattice defects. This field, also known as NanoSPD, experienced a significant progress within the past two decades. Beside classic SPD methods such as high-pressure torsion, equal-channel angular pressing, accumulative roll-bonding, twist extrusion, and multi-directional forging, various continuous techniques were introduced to produce upscaled samples. Moreover, numerous alloys, glasses, semiconductors, ceramics, polymers, and their composites were processed. The SPD methods were used to synthesize new materials or to stabilize metastable phases with advanced mechanical and functional properties. High strength combined with high ductility, low/room-temperature superplasticity, creep resistance, hydrogen storage, photocatalytic hydrogen production, photocatalytic CO 2 conversion, superconductivity, thermoelectric performance, radiation resistance, corrosion resistance, and biocompatibility are some highlighted properties of SPD-processed materials. This article reviews recent advances in the NanoSPD field and provides a brief history regarding its progress from the ancient times to modernity. Abbreviations: ARB: Accumulative Roll-Bonding; BCC: Body-Centered Cubic; DAC: Diamond Anvil Cell; EBSD: Electron Backscatter Diffraction; ECAP: Equal-Channel Angular Pressing (Extrusion); FCC: Face-Centered Cubic; FEM: Finite Element Method; FSP: Friction Stir Processing; HCP: Hexagonal Close-Packed; HPT: High-Pressure Torsion; HPTT: High-Pressure Tube Twisting; MDF: Multi-Directional (-Axial) Forging; NanoSPD: Nanomaterials by Severe Plastic Deformation; SDAC: Shear (Rotational) Diamond Anvil Cell; SEM: Scanning Electron Microscopy; SMAT: Surface Mechanical Attrition Treatment; SPD: Severe Plastic Deformation; TE: Twist Extrusion; TEM: Transmission Electron Microscopy; UFG: Ultrafine Grained.

This study demonstrates an approach of utilizing high-pressure torsion (HPT) to fabricate a novel hybrid material by the direct bonding of Al and Ti disks at room temperature under a compressive pressure of 6.0 GPa and with increasing numbers of HPT turns up to 50. Detailed structural observations revealed the formation of a multi-layered nanostructure in the edge regions of the disks with a grain size of ~30 nm. X-ray diffraction (XRD) and selected area electron diffraction (SAED) confirmed the presence of three intermetallic compounds, AlTi, Al3Ti and Ti3Al, in the layered structures. Processing by HPT led to the formation of a hybrid nanocomposite with exceptional hardness (over 300 Hv) in the edge regions of the disks. Special emphasis was placed on understanding the evolution of hardness in the hybrid material. The investigation demonstrates a significant opportunity for using HPT processing to deepen the knowledge on diffusion bonding and mechanical joining technologies as well as for fabricating new and valuable hybrid nanomaterials.

Research was conducted on the microstructural evolution and ensuing mechanical response from high-pressure torsion (HPT) processing of Ti-6Al-4V alloy in the as-cast and β-forged conditions with and without 0.1 wt.% boron addition. The boron addition produces refinement of the prior β grains and the (α+β) colonies and introduces an additional TiB phase but this affects the deformation response and the microstructural evolution only at low strains of 0.5 to 5 rotations. In the initial condition the orientation of the (α+β) colonies significantly affects the deformation response and leads to differences in substructure formation in both the as-cast and β-forged conditions. This orientation dependence counts on the initial microstructural differences between the unmodified and the boron modified alloys. At higher strains, there is a similar deformation response and microstructure evolution all the alloys. The hardness variation with equivalent strain is similar for the unmodified and boron modified alloys in as-cast and β-forged conditions and represents various deformation regimes in HPT-processing. Strength modelling confirms a simultaneous contribution from microstructural refinement and increased dislocation density towards the hardness increment during HPT processing. Overall, the as-cast and β-forged Ti-6Al-4V-0.1B alloys possess identical deformation response to the β-forged unmodified Ti-6Al-4V alloy in the initial and intermediate stages. At high levels of straining, all alloys respond in an equivalent manner, thus ruling out any possible effects from additional TiB phase or microstructural refinement for the boron-modified alloys.

Strengthening by grain refinement via the Hall–Petch mechanism and softening by nanograin formation via the inverse Hall–Petch mechanism have been the subject of argument for decades, particularly for ultrafine-grained materials. In this study, the Hall–Petch relationship is examined for ultrafine-grained magnesium, aluminum, copper, and iron produced by severe plastic deformation in the literature. Magnesium, aluminum, copper, and their alloys follow the Hall–Petch relationship with a low slope, but an up-break appears when the grain sizes are reduced below 500–1000 nm. This extra strengthening, which is mainly due to the enhanced contribution of dislocations, is followed by a down-break for grain sizes smaller than 70–150 nm due to the diminution of the dislocation contribution and an enhancement of thermally-activated phenomena. For pure iron with a lower dislocation mobility, the Hall–Petch breaks are not evident, but the strength at the nanometer grain size range is lower than the expected Hall–Petch trend in the submicrometer range. The strength of nanograined iron can be increased to the expected trend by stabilizing grain boundaries via impurity atoms. Detailed analyses of the data confirm that grain refinement to the nanometer level is not necessarily a solution to achieve extra strengthening, but other strategies such as microstructural stabilization by segregation or precipitation are required.

Experimental data show that the conventional Hall-Petch relationship cannot be maintained in its original form for metals having submicrometer structures. We now propose a dislocation model which modifies the Hall-Patch relationship to provide a uniform description of the grain size strengthening of submicron-structured face-centered cubic (f.c.c.) metals and solid solution alloys.

The high pressure torsion (HPT) process was performed on a Bi-Sn eutectic alloy up to 10 revolutions and the materials were stored at room temperature for durations up to 91 days in order to investigate the strain weakening and self-annealing behaviour of the alloy. The results show that HPT processing leads to significant grain refinement and enhanced superplasticity in the alloy. When the material is stored at room temperature, the grain size increases and the microhardness also increases producing a near linear relationship between grain size and microhardncess. This effect is attributed to the enhanced creep behaviour of the Bi-Sn alloy after HPT processing.

High-pressure torsion (HPT) was used to evaluate the effect of straining on the electrochemical properties of a ZK60 magnesium alloy. The samples were processed using HPT up to 20 turns and the electrochemical responses were examined through polarization, EIS, Mott–Schottky and hydrogen evolution tests. Electron back-scatter diffraction (EBSD) studies showed more homogeneity with finer average grain sizes accompanied by the evolution of a nobler texture when increasing the numbers of HPT turns. Ultimately, this led to improved corrosion behavior for the HPT-processed disks. Post corrosion observations revealed improved protective layers after higher turns of HPT.

In general, the process of superplastic deformation is regarded as steady-state so that the flow stress is given as a function of the strain rate only, thereby emphasizing the significance of the strain rate sensitivity and its determining methods. In this work, in addition to the important role of the strain rate sensitivity, it is shown that it is necessary also to consider the stability criteria for real, stable superplastic deformation through other factors such as work hardening. A possible scenario is proposed to describe the process whereby the work hardening rate may stabilize the deformation process when a perturbation occurs in the cross-section of the sample. The assumption of a work hardening effect is confirmed by its application for interpretation of the systematic deviations observed between the strain rate sensitivities determined experimentally using different experimental methods.

High-pressure torsion (HPT) processing was applied to an Fe-10Ni-7Mn (wt.%) martensitic steel at room temperature and the grain size was reduced from an initial value of ~5.5 μm to an ultrafine value of ~185 nm for the ferritic phase and around 30 nm for the austenitic phase after 20 HPT turns. The microstructure and mechanical properties of the as-processed material were evaluated using X-ray diffraction (XRD), electron backscatter diffraction (EBSD), field emission scanning electron microscopy (FESEM), microhardness measurements and tensile testing. In addition, annealing of an as-processed specimen was analyzed by differential scanning calorimetry (DSC). The results show that HPT processing increases the hardness and ultimate tensile strength to ~690 Hv and ~2230 MPa, respectively, but the ductility is decreased from ~16.5% initially to ~6.4% and ~3.1% after 10 and 20 turns, respectively. The hardness distributions and EBSD images show that a reasonably homogeneous microstructure is formed when applying a sufficient level of pressure and torsional strain. The DSC results demonstrate that processing by HPT reduces the start and finish temperatures of the reverse transformation of martensite to austenite and there is continuous re-crystallization after the recovery process.

Magnesium and its alloys display interesting properties such as low density and biocompatibility but the lack of ductility and low strength compromise their performance in many applications. Fabrication of metal matrix composites may alleviate these challenges and improve overall performance. Magnesium matrix composites can be produced by cold consolidation of particles through high-pressure torsion and this paper summarizes recent findings in processing routes for the fabrication of composites, the microstructure developed, mechanical properties obtained and potential applications. It is shown that ductile materials can be mixed with magnesium by processing half samples placed side by side and hard and brittle materials can be incorporated by processing mixed particles. The distribution of phases may be controlled by the amount of rotation imposed during processing. Well-dispersed second phase particles within a continuous magnesium matrix can be obtained. Thus, it is possible to incorporate bioactive materials within a biodegradable magnesium matrix. Detailed characterization using transmission electron microscopy reveals the processing also refines the grain structure of the metallic matrix. The composites may display good ductility, improved strength and an ability to precipitate intermetallics through thermal treatment. The composites produced using high-pressure torsion have different potential applications including the development of bioactive and biodegradable biological implants.

Grain reduction in a widely used Ti-6Al-4V alloy increases its endurance limit at room temperature. In this work, the behavior of the ultrafine-grained alloy under cyclic load at the temperature of 625 K is considered. Research was conducted to examine the fatigue life in a low-cycle area of the ultrafine-grained Ti-6Al-4V alloy produced by equal-channel angular pressing. Tensile and fatigue testing of the Ti-6Al-4V samples with coarse-grained (CG) and ultrafine-grained (UFG) structures were carried out at 293 and 625 K. The alloy demonstrated an enhanced strength and fatigue life at both temperatures. The representative features of the microstructural evolution and the fracture features in the UFG and CG alloys after fatigue tests are described in detail. The prospects for the use of the UFG Ti-6Al-4V alloy for engineering applications, such as in the production of critical gas-turbine engine parts, is discussed.

The effect of high-pressure torsion (HPT) on the textural components and microstructural evolution of an Fe-10Ni-7Mn steel was investigated. Six turns of HPT, with a rotation rate of 1rpm and with a pressure of 6.0 GPa, was applied to a solution annealed steel. Microstructural and textural evolution, including dislocation characteristics and grain size/misorientation measurements, were accomplished using Electron Backscatter Diffraction (EBSD). The resultsshowed that a fully martensitic structure, containing a high density of dislocations, was developed during the solution annealing and subsequently the application of six turns of HPT processing converted the low-angle grain boundaries (LAGBs) of the solution annealed condition into highangle grain boundaries (HAGBs). Thus, the sub-grain boundaries established in the solutionannealed condition were altered into very sharp HAGBs. The grain size decreased from ~25 µm in the solution annealed condition to ~210 nm in the severely deformed condition after six turns of HPT and the yield strength, tensile strength and ductility were increasd from 765 MPa, 840 MPa and 15.2 % to 1890 MPa, 2230 MPa and 6.1 % in the HPT-processed sample. Applying six turnsof HPT developed a predominantly {111}//ND texture but the overal texture intensity of the HPTprocessed sample was weakened to a value of about one-quarter of the as-quenched solution treated sample. This was related to dynamic recrystallization and grain subdivision as well as a dislocationdensity reduction in the HPT-processed sample. The texture components of strong Copper ({112}), S ({123}) and some avarage intesnsity Cube {100} were observed in the {100} pole figure in the solution annealed condition and γ-fiber (//ND) and β-fibre were seen in this condition. However, {110} fibres mostly developed in the six-turn sample with texture components of E1 {011̅} < 111 >, E2 {01̅1} < 111 >, J1{01̅1} < 2̅11 >,J2 + {11̅0} < 1̅1̅2 >, D2 {1̅1̅2} < 111 > , D1 {112̅} < 111 > and F {110} < 001 >. In addition, the Cu{112}〈111〉 component was significantly diminished. This weakened Cu{112}〈111〉 will enhance the mechanical properties, particularly the formability and toughness, as previously reported by the current authors. It was found that applying severe plastic deformation (SPD) in the form of HPT reduces the effect of martensitic texture in the solution treated steel.

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.

The high-pressure torsion processing technique was used to consolidate and process magnesium-based chips. Chips were prepared by machining commercially pure magnesium and a magnesium alloy AZ91 separately. Optical microscopy and microhardness measurements showed good consolidation of pure magnesium. The magnesium alloy continued to exhibit the boundaries between the chips even after 5 turns of HPT suggesting poor bonding. The results show that soft chips are easier to consolidate through HPT than harder alloys.

High-entropy alloys (HEAs) are currently attracting much interest because they offer unique properties and good ductility at low temperatures. These materials are of interest primarily because they contain five or more principal elements, with each element having a concentration between 5 and 35 at. %, and yet they have very simple structures based on solid solution phases. Superplasticity is defined formally as a tensile elongation without failure of at least 400% and very recent experiments have shown that the HEAs also have a potential for exhibiting superplastic ductilities when testing at elevated temperatures. Since superplasticity requires a very small grain size, typically

Bulk ultrafine-grained (UFG) materials usually show superior mechanical and physical properties. The development of micro-mechanical behavior is observed after significant changes in microstructure through high-pressure torsion (HPT) processing. This report summarizes recent results on the evolution of small-scale mechanical response examined by the nanoindentation technique on two UFG materials including a high-entropy alloy and an Al-Mg metal matrix nanocomposite processed by HPT. Special emphasis is placed on demonstrating the interrelationship of essential microstructural changes with increasing torsional strain and applying a post-deformation annealing treatment and the evolution of the micro-mechanical behavior in these UFG materials by estimating the strain rate sensitivity.

Oxygen-free copper of >99.95% purity was processed by equal-channel angular pressing at room temperature (RT) for up to 24 passes and then pulled to failure at RT using strain rates from 10−4 to 10−2 s−1. The results show that the microstrain increases with strain at the lower numbers of passes but decreases between 16 and 24 passes. Similar trends were found also for the dislocation density, the Vickers microhardness and the values of the measured yield stresses in tensile testing. X-ray diffraction measurements showed a minor increase in the crystallite size at the high strain imposed by processing through 24 passes. These results demonstrate the occurrence of dynamic recovery at the highest strain. In tensile testing at a strain rate of 10−3 s−1 the results gave a yield stress of ~391 MPa and an elongation to failure of 52% which is consistent with an earlier report using Cu of much higher purity but not consistent with an earlier report using Cu of the same purity.

The Pb-Sn alloy has a wide use in the electronic, energy storage and nuclear industries and a fine-grained Pb-Sn alloy may open up new possibilities for applications in these industries. In order to understand the behavior of grain refinement, a Pb-62% Sn eutectic alloy was processed by high-pressure torsion (HPT), stored at room temperature (RT) and then the microstructures of the alloy after HPT were repeatedly investigated during the course of self-annealing using electron backscatter diffraction, scanning electron microscopy and transmission electron microscopy. It is demonstrated that there is a large fraction of twin boundaries with a twin relationship of 62.8° in the microstructure of the initial as-cast condition. Due to the presence of the high imposed pressure, the mobility of Ʃ21 boundaries at 71° is greatly favoured during processing by HPT. After the high pressure is removed, the mobility of dislocation-twin boundaries near 62.8° is then favoured. Processing by HPT significantly increases the solubility of Sn in the Pb phase. The supersaturated state of Sn in Pb is not stable during self-annealing at RT and instead a decomposition of Sn from the Pb-rich phase is observed after storage for 16 days. The main mechanism for this decomposition is lattice diffusion.

The duplex stainless steel 2205, designated DSS2205 and having a duplex structure comprising ferrite and austenite phases, was processed by high-pressure torsion (HPT) and the microstructural and hardness evolutions were investigated after various HPT revolutions and at different positions within the specimens. The results show that the grain refinement induced by severe deformation processing is different in the ferrite and austenite phases such that the ferrite grains are refined via dislocation subdivision, whereas grain refinement in the austenite phase depends mainly on the interaction of dislocations and twin boundaries at relatively low strains. When the numbers of revolutions increases, the grain refinement in austenite restricts the occurrence of deformation twinning so that dislocation slip becomes dominant. During HPT processing, the effect of the phase boundaries on the mechanical properties of the alloy is very significant. The results show the average width between two adjacent phases and the hardness of the alloy are generally consistent with the classical Hall&ndash, Petch relationship.

Among the severe plastic deformation processing (SPD) of materials, high-pressure torsion (HPT) is known to induce considerable amounts of plastic strain and crucial grain refinement up to the nanometer scale. There have been several reviews on the mechanical properties and/or microstructure correlations of SPD-processed materials published during the last two decades. By contrast, a review on the crystallographic texture evolution during HPT processing is not at present available. Consequently, this work was undertaken to review the texture evolution in Face-Centered Cubic (FCC), Body-Centered Cubic (BCC) and Hexagonal Close-Packed (HCP) crystal structure materials processed exclusively by HPT with a special emphasis on the influence of many intrinsic and extrinsic factors such as the imposed strain, the deformation temperature and the alloying elements.

Superplastic Pb-Sn alloys were produced via tube high-pressure shearing (t-HPS), in a single step starting from elemental solid bulks. A Pb-40 wt% Sn alloy showed an exceptional superplastic elongation as high as ∼1870% at a strain rate of 1.0 × 10 −3 s −1 at room temperature, thereby elevating the optimum strain rate for maximum elongation under these conditions by more than one order of magnitude over conventional cast Pb-Sn alloys. This unprecedented room temperature superplasticity is attributed to the equiaxed grains having uniform sizes of the order of one micrometer, and in particular to the well-mixed domains of Pb and Sn in nearly equal proportions. This microstructure cannot be attained in cast eutectic or hypoeutectic alloys through conventional thermomechanical processing, but instead it is a direct outcome of t-HPS-generated compositional patterning at room temperature.

The mechanical properties and structural stability of a high-pressure torsion (HPT)-induced bulk nanostructured metastable Al–Mg system were examined after natural aging at room temperature for 60 days. The sample demonstrated a high yield strength of 1.3–1.5 GPa with an excellent plasticity by achieving a high strain rate sensitivity of 0.036. The high hardness is attributed to the concurrent contributions of grain refinement and solid solution strengthening. An X-ray diffraction analysis revealed a high compositional microstrain of ~0.0202 due to the supersaturation of Mg in the Al matrix after processing. This microstrain increased to ~0.0274 after natural aging due to the heterogeneous distribution of supersaturated Mg solutes without any nucleation of a second phase, thereby demonstrating a reasonable structural stability.

Nanocomposites of Zr/Nb with exceptionally high hardness were fabricated successfully through the high-pressure torsion (HPT) processing of prepacked Nb/Zr/Nb sandwich samples at ambient temperature. The initial layers of Nb and Zr became fragmented during HPT processing with the formation of many fine-scale intermixed Zr/Nb layers. The intermixing of these Zr/Nb layers increased both with increasing HPT revolutions from 10 to 100 and with increasing radial positions on the disks. The Vickers microhardness, Hv, increased with increasing revolutions and with radial position reaching a maximum of ~700 Hv at the edge of the 100 turns sample. Exceptional grain refinement to the range of ~20–40 nm and the occurrence of twinning were associated with the HPT-processed Zr/Nb composites after 100 turns. These results suggest a potential route for fabricating high strength bulk Zr/Nb nanocomposites.

An AZ91 magnesium alloy (Mg-9%, Al-1% Zn) was processed by high-pressure torsion (HPT) after solution-heat treatment. Tensile tests were carried out at 423, 523, and 623 K in the strain rate range of 10&minus, 5&minus, 10&minus, 1 s&minus, 1 to evaluate the occurrence of superplasticity. Results showed that HPT processing refined the grain structure in the alloy, and grain sizes smaller than 10 &micro, m were retained up to 623 K. Superplastic elongations were observed at low strain rates at 423 K and at all strain rates at 523 K. An examination of the experiment data showed good agreement with the theoretical prediction for grain-boundary sliding, the rate-controlling mechanism for superplasticity. Elongations in the range of 300&ndash, 400% were observed at 623 K, attributed to a combination of grain-boundary-sliding and dislocation-climb mechanisms.

Molybdenum, a refractory metal with a body-centred cubic lattice, was processed by high-pressure torsion (HPT) at room temperature under an applied pressure of 6.0 GPa and torsional straining from 1/2 to 10 turns. The microstructures were characterized by electron back-scatter diffraction (EBSD) and transmission electron microscopy (TEM) and measurements were taken of the Vickers microhardness. The results show a gradual evolution of homogeneity in grain size and microhardness with increasing numbers of HPT turns with an average grain size of ~690 nm at the sample edge after 5 turns and a saturation microhardness of ~660 Hv. The grain size after 10 turns showed a slight decrease at the centre and the microhardness was slightly lower than after 5 turns due to the occurrence of limited dynamic recovery.