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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.

Abstract: Additive Manufacturing (AM) is considered one of the key technologies for realizing Industry 4.0. There are numerous stages in the end-to-end AM process, including component design, material design, build, and so on. An enormous amount of data is generated along the end-to-end AM process that can be acquired from the 3D printer in real-time, micro-characterization studies, and process plan details, among others. For instance, these data can be employed to predict the printed components’ quality and, at the same time, proactively adapt the 3D printer parameters to achieve better quality. This end-to-end AM process can be mapped onto the digital thread. The current article elaborates on a conceptual framework to acquire the data from various sources associated with the end-to-end AM process and realize monitoring and control of the end-to-end AM process, leading to an intelligent AM process.

An equiatomic CoCrFeNiMn high-entropy alloy was processed by severe plastic deformation followed by post-deformation annealing over a range of temperatures and times leading to a wide range of grain sizes from ~0.05 to ~70 μm. The results demonstrate there is a sharp evolution in grain size and hardness after annealing above 800 °C due to coarsening facilitated by the dissolution of precipitates together with a high rate of diffusion at high temperatures. Grain growth behavior revealed an incremental low value grain growth exponent with increasing annealing temperature together with a high value activation energy for grain growth of ~440 kJ mol-1. A critical grain size of ~2 µm is proposed in which deformation-induced twinning is suppressed during plastic deformation. Nevertheless, slip and deformation-induced twinning are deformation mechanisms occurring in samples with grain sizes above this critical value. A model is presented for engineering the grain size by controlling the annealing parameters in the fine grain size range to benefit from the advantages of deformation-induced twining in the CoCrFeNiMn alloy.

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.

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

An Al 7050-10 vol.% Al2O3 nanocomposite was produced by cold consolidation of Al 7050 gas-atomized powder with alumina particles by means of severe plastic deformation using high-pressure torsion (HPT). The processing was conducted at room temperature under a pressure of 6.0 GPa, for 50 revolutions with a rotation speed of 2 rpm. The purpose of this work was to consolidate at room temperature one high strength Al alloy with Al2O3 particles without any prior or post sintering. In order to characterize the product, the disk microstructure was examined after consolidation using scanning and transmission electron microscopy and X-ray diffraction was used to determine the presence of different phases. Energy-dispersive X-ray spectroscopy (EDS) was undertaken to provide chemical analysis of the phases and hardness tests were performed to check the strength. The results show that the alumina particles in the periphery of the disk are more finely dispersed than in the central regions. The average grain size of the Al 7050 matrix ranged from ∼50 to ∼200 nm and the hardness measurements varied from ∼237 Hv to ∼307 Hv with disk edges having higher hardness values than in the central area.

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.

A cast Bi–Sn eutectic alloy was processed by high-pressure torsion (HPT) at room temperature and stored at room temperature for durations of up to 91 days in order to investigate the effect of self-annealing. The HPT processing produces grain refinement but hardness measurements show there is strain softening with lower microhardness values than in the initial as-cast material and with hardness values that decrease with increasing amounts of imposed torsional strain. This softening is attributed to a loss of precipitates within the Sn and Bi phases during the processing operation. In self-annealing at room temperature, the microhardness increases significantly due to reprecipitation and there is also a minor increase in grain size with increasing time of storage. It is demonstrated by tensile testing that the HPT-processed Bi–Sn alloy exhibits superplastic behavior with elongations of up to >1000% after storage for 35 days and with an associated strain rate sensitivity close to ∼0.5. Grain boundary sliding plays an important role in superplastic flow and it is shown that maximum sliding occurs on the Bi–Bi interfaces where this is consistent with estimates of the coefficients for grain boundary diffusion in the Bi–Sn alloy.

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.

Nanocrystalline (NC) metals are attractive due to their outstanding mechanical, physical and chemical properties. However, they are generally metastable and prone to undergo grain coarsening, which severely impedes their engineering application, particularly at elevated temperatures. A strategy to counteract this phenomenon lies in decreasing the energetic driving force for grain growth, through grain boundary segregation according to the Gibbs adsorption isotherm. The natural thought of ‘more segregation is better’, which was exercised over decades to solve this problem, however, does not work because interfaces themselves are thermodynamic entities, with only limited solute decoration tolerance. Herein, we propose to solve this long-standing problem via introducing multi-component co-segregated interfaces. This concept allows us to maximize the solute decoration of interfaces and minimize their energy, without triggering formation of new phases. This is enabled by extending the high entropy alloy (HEA) concept from the bulk to interfaces, a strategy we refer to as interfaces with higher entropy. This design concept is not transferred one-to-one but constrained by a few rules that reflect the nature of interfaces: use of several co-segregating elements with high mixing entropy; segregation remains in the dilute limit; and use of elements with high segregation coefficient. The latter criterion marks an important difference to the HEA concept as a high segregation coefficient guarantees that alloying elements are deposited only to those locations where they are needed, namely, to the interfaces, thus drastically reducing alloying costs. We applied this new design approach to NC-Nb and show that we can increase its stability from 873 to 1023 K with as little alloying as only 1 at.% of Ti, Ni, Co and Hf with equal fractions. The material maintains its nanocrystalline structure even after 2200 h at 973 K. This work thus offers a new paradigm for designing stable dilute NC materials for advanced engineering application.

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.

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.

International audience; In the present study, the stored energy and activation energy for recrystallization were investigated for a Cu-Ni-Si alloy after high-pressure torsion processing for N = ½, 1, 5 and 10 turns at room temperature. The contributions of geometrically necessary dislocations (GNDs), statistically stored dislocations (SSDs) and vacancies to the stored energy were calculated through the Vickers microhardness measurements, kernel average misorientation (KAM) measurements and an analysis by differential scanning calorimetry (DSC). The results show that the total stored energy decreases rapidly after equivalent strain of εeq ~ 9 (N = 1 turn) and then saturates through εeq ~ 86 (N = 10 HPT turns) at ~70 J/mol. Concurrently, the local stored energy in GNDs and SSDs was found to depend strongly on the radial distance from the centre of the disc and increase with increasing equivalent strain at εeq ~ 16 and saturate with further straining. Accordingly, the results indicate that the GNDs and vacancies are responsible for the high stored energy in the initial stage of deformation at equivalent strain range of εeq = 8.6-16 (N = ½-1 turn) and thereafter their contribution decreases slightly due to the occurrence of dynamic recrystallization and the formation of fine grains. At the same time, the contribution of the SSDs is similar to that of the GNDs only in high strain deformation as at εeq = 49.3 (N = 5 turns) to accommodate the deformation process. The 2 recrystallization peak was detected in the range of 157-194 °C depending on the number of HPT turns and heating rate. An activation energy for recrystallization was estimated in the range of ~ 89.7-98.7 kJ/mol, thereby suggesting a poor thermal stability.

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.

Magnesium and its alloys have gained significant popularity due to their light weight and their potential for use as bioresorbable materials. However, their application is limited in practice due to their relatively poor corrosion resistance. Several methods are available for improving the corrosion resistance of Mg alloys for bio-applications such as using different coatings, alloying, and modifying the microstructural parameters such as the grain size and the crystallographic texture. This review provides a comprehensive summary of the effects of crystallographic texture and twinning, as one of the most important deformation mechanisms of Mg and Mg alloys, on the corrosion behavior. Regarding the crystallographic texture, it is shown that theoretically the basal planes should exhibit a lower corrosion rate but in some cases, such as when there is a galvanic effect or when corrosion films control the overall corrosion behavior, different results may take place. Also, there are contradictory results concerning the effect of twinning on the corrosion behavior. Thus, in some cases twinning may provide preferential sites for corrosion due to the higher energies of atoms located in the twin region by comparison with normal atomic positions in the crystalline lattice whereas there are also other examples where experiments show that twins produce more protective films than in the surrounding matrix.

Experiments were conducted to investigate, within the framework of a multiscale approach, the mechanical enhancement, deformation and damage behavior of copper–silicon carbide composites (Cu–SiC) fabricated by spark plasma sintering (SPS) and the combination of SPS with high-pressure torsion (HPT). The mechanical properties of the metal–matrix composites were determined at three different length scales corresponding to the macroscopic, micro- and nanoscale. Small punch testing was employed to evaluate the strength of composites at the macroscopic scale. Detailed analysis of microstructure evolution related to SPS and HPT, sample deformation and failure of fractured specimens was conducted using scanning and transmission electron microscopy. A microstructural study revealed changes in the damage behavior for samples processed by HPT and an explanation for this behavior was provided by mechanical testing performed at the micro- and nanoscale. The strength of copper samples and the metal–ceramic interface was determined by microtensile testing and the hardness of each composite component, corresponding to the metal matrix, metal–ceramic interface, and ceramic reinforcement, was measured using nano-indentation. The results confirm the advantageous effect of large plastic deformation on the mechanical properties of Cu–SiC composites and demonstrate the impact on these separate components on the deformation and damage type.

CoCrMo alloys are used in hip and knee replacements due to their excellent long-term survival rates. However, high failure rates have recently been observed associated with adverse tissue reactions. CoCrMo alloy surfaces undergo microstructural changes during wear, including the formation of ε-martensite and, occasionally, a nanocrystalline surface layer. It is not clear whether these changes are beneficial or detrimental to the performance of the component. Thus, high-pressure torsion (HPT) was employed to produce different grain sizes and crystallographic structures in a CoCrMo alloy and the corrosion and tribocorrosion behaviour were critically investigated as a function of grain size. The results reveal a degradation of the corrosion resistance for the HTP processed samples. The contributions of mechanical and corrosion material loss in tribocorrosion is also examined.

The presence of a gradient layer with an increased maximum microhardness and extended thickness is extremely attractive because it can enhance the service performances for surface-strengthened metallic materials. In this research, Cu-Al alloys with different Al contents and microstructures were processed by surface spinning strengthening (3S) and studies were conducted to examine the gradient microstructure and microhardness distributions of the 3S Cu-Al alloys. The experimental results show that each group of the 3S Cu-Al alloys having the same Al content has approximately the same maximum microhardness at the topmost surface layer and the maximum microhardness increases with an increase in the Al content. In addition, the thickness of the gradient layer in the 3S Cu-Al alloys increases with a decrease of yield strength and an increase in the work-hardening exponent, respectively. The relationship between the maximum microhardness and chemical composition which determines the Young's modulus and plastic deformation mode, and the relationship between the thickness of the gradient layer and the microstructure which governs the strength and work-hardening capacity, were both investigated. It is found that the maximum microhardness of the gradient layer depends mainly on the chemical composition; whereas the thickness of the gradient layer depends primarily on both the strength and work-hardening capacity which are closely related to the microstructure. By combining the compositional design and microstructure optimization of the materials, and improving the surface strengthening intensity, a gradient layer of the surface-strengthened materials with an enhanced maximum microhardness and an extended thickness may be achieved.

Nanostructuring of bulk metals is now well documented with the development of severe plastic deformation (SPD) for improving the physical and mechanical properties of engineering materials. Processing by high-pressure torsion (HPT), which was developed initially as a grain refinement technique, was extended recently to the mechanical bonding of dissimilar metals during nanostrcturing which generally involves significant microstructural heterogeneity. Here we introduce, for the first time, a bulk metastable Al-Mg supersaturated solid solution by the diffusion bonding of separate Al and Mg metal solids at room temperature using HPT. Exceptional hardness was achieved homogeneously throughout the metastable alloy with a record maximum supersaturated Mg content of ~38.5 at.% in the Al matrix having a grain size of ~35–40 nm. Our results demonstrate the synthesis of a bulk nanocrystalline metastable alloy with good microstructural stability at room temperature where such bulk solids are not yet reported for mechanical alloying by powder metallurgy.

A magnesium/aluminium composite was produced by room temperature consolidation through high-pressure torsion (HPT) processing. Half-discs of the pure metals were placed side-by-side and subjected to different numbers of turns. The initially reduced interface between the phases gradually increased with increasing rotation. The composite displayed a significant ductility even after 10 turns. The distribution of hardness in the HPT-processed discs was bi-modal in the early stages of processing. As the number of turns increased and the thickness of the phases decreased there was a noticeable increase in hardness. The hardness values of the composite further increased after thermal treatment due to the formation of intermetallics within the interface between the magnesium and aluminium-rich phases.

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.

Experiments were conducted to study the compressive mechanical properties of an Al-Zn-Mg alloy after solid solution treatment and equal-channel angular pressing (SS-ECAP) using strain rates ranging from 1.0 × 10−3 to 3.0 × 103 s−1. The results show that SS-ECAP processing enhances the compressive strength due to the high dislocation density, large numbers of fine precipitates and grain refinement. The alloy in both the peak-aged (as-received) and the SS-ECAP states shows a strain rate strengthening effect such that the strain rate sensitivity increases with increasing strain rate. The high volume fraction of fine precipitates in the SS-ECAP alloy decreases the strain rate sensitivity. The coarse precipitates in the peak-aged alloy are fragmented while their sizes increase in the SS-ECAP alloy due to dynamic precipitation assisted by the high density of dislocations during compressive testing. With increasing strain rate, the size of the precipitates further increases for the SS-ECAP alloy and this is influenced by accelerated dislocation motion. During compression, the T (Al20Cu2Mn3) and E (Al18Mg3Cr2) phases evolve into a new tetragonal phase containing Mg, Mn, Cr and Zn with Al.

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.

Commercial purity titanium (CP-Ti) was processed by high-pressure torsion (HPT) with various numbers of turns (N = 1, 10 and 20). The hardness of the CP-Ti increased with an increasing number of HPT turns due to grain refinement. Tensile testing showed that the PT-processed 10 turns sample had low ductility and high strength but the ductility may be improved through post-HPT short-term annealing at carefully selected temperatures. Some HPT-processed samples were laser surface-treated with different laser powers and scanning speeds.The surface roughness of the laser-textured samples increased with increasing laser power and led to a lower contact angle which signifies an increased hydrophilicity. After a holding time of 13 days, the samples underwent a hydrophilic-to-hydrophobic transformation as the contact angle increasedto as much as 129 degree. It is concluded that laser surface texture processes are capable of controlling the hydrophilic / hydrophobic properties of ultra-fine grained CP-Ti.

High pressure torsion offers unique conditions for the consolidation of metallic particles at room temperature owing to the high hydrostatic compressive stresses combined with the high shear strain. A Mg-Al2O3 composite was produced by consolidation of machining chips of pure magnesium with 10% in volume of alumina particles. The consolidation process was investigated by optical and scanning electron microscopy and X-ray microtomography. It is shown that shear deformation concentrates along thick alumina particle layers in the initial stage of deformation. A significant fraction of the hard phase particles are pushed into the outflow in quasi-constrained HPT and a homogeneous composite is achieved after significant straining. The composite exhibits a refined microstructure, a higher hardness and improved resistance against room temperature creep compared to pure magnesium.

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.