Search

Advanced structural materials are expected to display significantly improved mechanical properties and this may be achieved, at least in part, by refining the grain size to the submicrometer or the nanocrystalline range. This report provides a detailed summary of the role of grain size in the mechanical properties of metals. The effect of grain size on the high temperature behavior and the development of superplasticity is illustrated using deformation mechanism maps and the development of exceptional strength through grain refinement hardening at low temperatures is also discussed. It is shown that the deformation mechanism of grain boundary sliding, as developed theoretically, can be used to effectively predict both the high and low temperature behavior of metals having different grain sizes. This analysis explains the increase in strain rate sensitivity in ultrafine-grained metals with low and moderate melting points and the ability to increase both the strength and ductility of these materials to thereby overcome the strength-ductility paradox. The recent development of hybrid materials is also reviewed and it is demonstrated that, although these hybrids have received only limited attention to date, they provide a potential for making significant advances in the production of new structural materials.

Magnesium alloys are of considerable current interest for use as degradable implants due to their unique properties including biodegradability, biocompatibility, low density and adequate mechanical properties. Nevertheless, there is a need to further improve these properties either by alloying or through the use of appropriate processing. Among the different biodegradable Mg alloys now in use, the Mg‒Zn series are of special interest and have been the subject of many research investigations. This is primarily because Zn is an essential element for the human body in addition to its positive effects in improving the mechanical strength and lowering the degradation rate of the implant. The properties of Mg‒Zn alloys may be further improved both through the addition of third and fourth alloying elements such as Ca, Ag, Sn or Sr and/or by thermo-mechanical processing where the latter is more environmentally and economically favorable. In practice, procedures based on the application of severe plastic deformation (SPD) are especially suited to produce fine-grained microstructures with improved mechanical, degradation and cell behavior. Equal-channel angular pressing (ECAP) is a popular SPD technique that has the capability of producing bulk materials that are sufficiently large for use as typical implants. Accordingly, this review is designed to provide a comprehensive summary of the research that has been undertaken on ECAP-processed biodegradable Mg‒Zn alloys.

Experiments were conducted to assess the effect of processing temperature on the hardness evolution, microstructure and the flow properties of an annealed Al–3Mg alloy processed by high-pressure torsion (HPT) at either 300 or 450 K. The results demonstrate that HPT processing at room temperature (RT) leads to higher microhardness values with a more uniform hardness distribution in the Al alloy compared with processing at 450 K. After 10 HPT turns at RT, the microstructure displayed a more intense grain refinement, a higher dislocation density and smaller Al3Mg2 precipitates than after HPT at 450 K. Accordingly, the metal processed at RT showed enhanced strength at RT and consistently exhibited superplastic flow during deformation at 523 K, with tensile elongations of ∼530–650% for strain rates from 10−3 to 10−2s−1. Conversely, a maximum elongation of ∼110% was recorded at 523 K in the alloy processed by HPT at 450 K. This behaviour is attributed to an enhanced thermal stability in the metal processed at RT compared with HPT at 450 K where deformation at 523 K led to the onset of abnormal grain coarsening due to a more disperse distribution of grain sizes and the partial dissolution and coalescence of Al3Mg2 precipitates after HPT processing.

An analysis of the processes of plastic deformation and acoustic relaxation in a high-entropy alloy, Al0.5CoCrCuFeNi, was carried out. The following were established: dominant dislocation defects; types of barriers that prevent the movement of dislocations; mechanisms of thermally activated movement of various elements of dislocation lines through barriers at room and low temperatures. Based on modern dislocation theory, quantitative estimates were obtained for the most important characteristics of dislocations and their interaction with barriers.

The CoCrFeNiMn multicomponent is one of the most widely and best studied of all high-entropy alloys (HEAs) since its exceptional properties suggest potential uses as a structural material in many industrial applications. In practice, a good balance between strength and ductility is one of the most important challenges in developing alloys in modern materials science and engineering. There have been many attempts to overcome the strength-ductility trade-off in the CoCrFeNiMn alloy due to its relatively low strength. These attempts include adding a minor sixth element, such as oxide or carbide particles, and also developing thermomechanical processing for grain refinement or precipitation to produce a tailored microstructure with optimum mechanical properties. This review describes the deformation mechanisms and the microstructural evolutions in this alloy during plastic deformation under different conditions of temperature, strain and strain rate where these are important for the processing and manufacturing of the alloy. In addition, this study provides a perspective on the behavior of the alloy under high temperature exposure and also under cyclic loading which is generally the most important factor in industrial applications. This understanding demonstrates that there is a very large potential for the future enhancement and optimization of this and other comparable HEAs.

Abstract A stable { $$\stackrel{\mathrm{-}}{1}{\text{10}}$$ 1 - 10 } single component sharp texture was obtained during ambient temperature tube High-Pressure Shearing (t-HPS) of 99.999% purity aluminum. It is shown that the grain size and the grain aspect ratio saturate at ~ 8 μm and ~ 1.6, respectively, at an equivalent strain of ~ 30 and the high-angle grain boundary fraction continues to decrease after this saturation even to equivalent strains exceeding ~ 200. The { $$\stackrel{\mathrm{-}}{1}{\text{10}}$$ 1 - 10 } texture emerges at an equivalent strain of ~ 6 to 9 with the completion of recrystallization and develops gradually as a sole component sharp texture with increasing intensity upon further processing. This component is a stable orientation in t-HPS processing although it was not previously observed experimentally as a shear texture. Thus, t-HPS processing provides a new and effective experimental tool for simple shear testing that is distinctly different from earlier shear strain methods such as torsional processing.

It was shown recently that the grain size contribution to the flow stress and strain rate sensitivity of pure metals having different structures can be estimated by a model of deformation for grain boundary sliding. The present research extends this earlier study by estimating the grain size contribution to the flow stress in the CrMnCoFeNi high entropy alloy. This alloy has attracted significant attention in recent years due to its remarkable mechanical properties which include a higher strength compared to f.c.c. pure metals due to a significant contribution from solid solution strengthening. The present work demonstrates that the flow stress of the CrMnCoFeNi alloy can be readily estimated from the sum of the contributions from solid solution and grain size strengthening. There are some unique experimental trends observed in this alloy and these provide supporting evidence for the assumption that the grain size strengthening is thermally-activated. The flow stress, strain rate sensitivity, activation volume and activation energy are predicted for different grain sizes and testing conditions as a function of the fundamental properties of this alloy and the results show excellent agreement with the reported experimental data.

Heterostructured materials afford a new way to improve the mechanical properties, which has become vital in both materials science and engineering applications. In the present research, Cu/Nb multilayer composites with layer thicknesses from the micrometer to nanometer were fabricated by accumulative roll bonding and the microstructure and mechanical properties of the Cu/Nb multilayer composites were then investigated. The yield strength and ultimate tensile strength of these composites increase with decreasing layer thickness. Moreover, the relationship between yield strength and (layer thickness)−1/2 approximately accords with the conventional Hall-Petch equation but with a decrease in the Hall-Petch slope when the layer thickness decreases from the micrometer to nanometer scales. The deformation microstructure of these Cu/Nb multilayer composites clearly exhibit dislocations glide in the layers, which reduces the stacking of dislocations at the Cu–Nb interface and thereby weakens the strengthening effect of the interface.

Biodegradable magnesium (Mg) alloys exhibit great potential for use as temporary structures in tissue engineering applications. Such degradable implants require no secondary surgery for their removal. In addition, their comparable mechanical properties with the human bone, together with excellent biocompatibility, make them a suitable candidate for fracture treatments. Nevertheless, some challenges remain. Fast degradation of the Mg-based alloys in physiological environments leads to a loss of the mechanical support that is needed for complete tissue healing and also to the accumulation of hydrogen gas bubbles at the interface of the implant and tissue. Among different methods used to improve the performance of the biodegradable Mg alloys to address these challenges, it appears that heat treatment is the most effective way to modify the microstructure and thus the corrosion behavior and mechanical properties without changing the composition or shape of the alloys. A desirable combination of corrosion and mechanical properties can be obtained through a precise control of the heat treatment parameters. In this report, the effects of different heat treatments (T4 and T6) on the microstructure, corrosion behavior, and mechanical properties of some of the most important heat-treatable biodegradable Mg alloys (Mg-Zn, Mg-Gd, Mg-Y, Mg-Nd, Mg-Al and Mg-Ag) are examined as well as new perspectives to enhance their clinical implementation.

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 CO2 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

Research was conducted to investigate the effects of high-pressure torsion (HPT) and post-deformation annealing (PDA) on the microstructure evolution and mechanical behavior of an Fe-9.6Ni-7.1Mn (at.%) steel with an initial lath martensitic microstructure. The experimental results showed that HPT processing led to the formation of an ultrafine grain martensitic microstructure accompanied by small amounts of strain-induced austenite. Phase analysis and microstructural examination confirmed that during PDA at 600 °C a large fraction of fine and coaxial austenite grains was introduced in the microstructure by diffusionless shear mechanism whereas its volume fraction at ambient temperature was drastically decreased by increasing the annealing time. Also, the grain size was reduced from a value of about 5.2 μm in the solution-treated specimen to ultrafine values of about 570 and 280 nm for the martensite and austenitic phases, respectively, after PDA for 7.2 ks. PDA yielded an outstandingly good combination of an ultimate tensile strength (∼1340 MPa) and fracture strain (∼11.9%) in comparison to the solution-annealed condition which can be attributed to the finer grain size and the presence of shear-formed austenite in the microstructure. In addition, the fracture mode changed from a fully ductile nature in the solution-treated specimen to a combination of ductile and brittle nature after applying the HPT and then returned again to a ductile behavior after PDA.

Ultrafine-grained materials display almost no strain hardening, an enhanced strain rate sensitivity and grain boundary offsets during plastic deformation. It is expected that dislocation climb is active in order to enable prompt recovery. The present analysis proposes a deformation mechanism that includes these effects and follows from the mechanism for high temperature grain boundary sliding. This mechanism predicts the relationship between strain rate, flow stress, grain size, temperature and basic material properties such as the Burgers vector modulus, the shear modulus and the grain boundary diffusion coefficient. The model may be used to estimate the final grain size achieved by severe plastic deformation and the strain rate sensitivity. An analysis shows that the predicted behavior agrees with the data from multiple experimental investigations and provides a good estimate of the Hall–Petch slope for different materials which includes breakdown and inverse Hall–Petch behavior under some conditions. The incorporation of a threshold stress provides an opportunity to predict the relationship between flow stress and grain size for a broad range of grain sizes, strain rates and temperatures. An excellent agreement is observed between the predictions of the model and experimental data for Al, Cu, Fe (α), Fe(γ), Mg, Ni, Ti and Zn.

In order to investigate the effect of grain boundaries on the mechanical response in the micrometer and submicrometer levels, complementary experiments and molecular dynamics simulations were conducted on a model bcc metal, tantalum. Microscale pillar experiments (diameters of 1 and 2 μm) with a grain size of ~100–200 nm revealed a mechanical response characterized by a yield stress of ~1500 MPa. The hardening of the structure is reflected in the increase in the flow stress to 1700 MPa at a strain of ~0.35. Molecular dynamics simulations were conducted for nanocrystalline tantalum with grain sizes in the range of 20–50 nm and pillar diameters in the same range. The yield stress was approximately 6000 MPa for all specimens and the maximum of the stress–strain curves occurred at a strain of 0.07. Beyond that strain, the material softened because of its inability to store dislocations. The experimental results did not show a significant size dependence of yield stress on pillar diameter (equal to 1 and 2 um), which is attributed to the high ratio between pillar diameter and grain size (~10–20). This behavior is quite different from that in monocrystalline specimens where dislocation ‘starvation’ leads to a significant size dependence of strength. The ultrafine grains exhibit clear ‘pancaking’ upon being plastically deformed, with an increase in dislocation density. The plastic deformation is much more localized for the single crystals than for the nanocrystalline specimens, an observation made in both modeling and experiments. In the molecular dynamics simulations, the ratio of pillar diameter (20–50 nm) to grain size was in the range 0.2–2, and a much greater dependence of yield stress to pillar diameter was observed. A critical result from this work is the demonstration that the important parameter in establishing the overall deformation is the ratio between the grain size and pillar diameter; it governs the deformation mode, as well as surface sources and sinks, which are only important when the grain size is of the same order as the pillar diameter.

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.

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.

Differential scanning calorimetry (DSC) was used to evaluate the recrystallization temperature and activation energy for an Mg-1.43Nd (wt.%) alloy after severe plastic deformation by high-pressure torsion (HPT) at room temperature up to 10 turns. The recrystallization kinetics were determined from DSC analysis. The results show that the recrystallization temperature increases with increasing heating rate and decreases with increasing numbers of HPT turns. Severe plastic deformation by HPT significantly reduces the recrystallization temperature. The estimated activation energy for recrystallization was in the range of ∼ 84−89 kJ mol−1.

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.

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.

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.

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.

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.

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.

A three-dimensional finite element method (3D FEM) simulation was carried out using ABAQUS/Explicit software to simulate multi-pass processing by equal-channel angular pressing (ECAP) of a circular cross-sectional workpiece of a Cu-Zr alloy. The effective plastic strain distribution, the strain homogeneity and the occurrence of a steady-state zone in the workpiece were investigated during ECAP processing for up to eight passes. The simulation results show that a strain inhomogeneity was developed in ECAP after one pass due to the formation of a corner gap in the outer corner of the die. The calculations show that the average effective plastic strain and the degree of homogeneity both increase with the number of ECAP passes. Based on the coefficient of variance, a steady-state zone was identified in the middle section of the ECAP workpiece, and this was numerically evaluated as extending over a length of approximately 40 mm along the longitudinal axis for the Cu-Zr alloy.

Tube high-pressure shearing (t-HPS) processing was performed on a eutectic Bi–Sn (57/43) alloy for 0.25, 1, 5 and 20 turns. The selected samples were stored at room temperature for up to 56 days to examine the strain weakening and self-annealing behavior of the alloy. The results showed that t-HPS processing gradually refined the microstructure and led to decreasing of microhardness, but microhardness increased slowly during the subsequent storage at room temperature. Shear localization of the eutectic structure during t-HPS processing was observed as large amounts of narrow dense lamellar zones were visible in the deformed microstructures. The Bi–Sn (57/43) alloy processed by t-HPS exhibited significantly enhanced superplastic properties with elongations up to >1800% in a sample after t-HPS processing for 20 turns. This high elongation is attributed to the breaking of the lamellar structure and the very small grain size.

An Al-3%Mg-0.2%Sc alloy was solution treated and subjected to 10 turns of high-pressure torsion (HPT). Thereafter, the HPT-processed material was annealed for 1 hour at temperatures ranging from 423 to 773 K and its mechanical properties and microstructural evolution were examined using microhardness measurements and EBSD analysis. The results demonstrate that the Al-Mg-Sc alloy exhibits an average microhardness of ~190 Hv and an average grain size of ~140 nm immediately after HPT processing and also after further annealing at 423 K. Conversely, it is shown that annealing at temperatures above 473 K leads to a substantial decrease in the hardness values as well as a sharp increase in the grain size of the material previously processed by HPT. In addition, detailed EBSD analysis revealed the formation of a bi-modal distribution of grains after annealing at temperatures from 623 to 773 K, and this becomes more uniform with increasing temperatures.

The influence of Mo alloying on the thermal stability of grain size, dislocation density and hardness of ultrafine-grained (UFG) Ni alloys was studied. The UFG microstructure in alloys with low (â¼0.3Â at.%) and high (â¼5Â at.%) Mo contents was achieved by high-pressure torsion (HPT) performed for 20 turns at room temperature. The thermal stability of the two alloys was studied by calorimetry. A Curie-transition from ferromagnetic to paramagnetic state was not found for the Niâ5% Mo alloy due to the high Mo content. It was found that heating at a rate of 40Â K/min up to â¼850Â K resulted in a complete recovery and recrystallization of the UFG microstructure in the alloy with 0.3% Mo. The same annealing for Niâ5% Mo led only to a moderate reduction of the dislocation density and the grain size remained in the UFG regime. Therefore, the higher Mo content yielded a much better thermal stability of the Ni alloy. The influence of the change of the microstructure during annealing on the hardness is discussed. Keywords: High-pressure torsion, NiâMo alloys, Dislocations, Grain size, Hardness, Thermal stability

Low-carbon steel AISI 1020 was subjected to high-pressure torsion (HPT) with 6.0Â GPa pressure through 1/4â5 turns. The microstructures of the samples in each turn were studied by means of X-ray diffraction (XRD) analyzing the changes in micro-strain, crystallite size and lattice parameter. Vickers testing was utilized to study the microhardness behavior of the samples subjected to HPT processing. The morphology evolution of the samples and especially the changes in ferrite and pearlite structures were studied for different numbers of turns using scanning electron microscopy (SEM). Keywords: 1020 steel, High pressure torsion, Hardness

An Alâ3% Mgâ0.2% Sc alloy was solution treated and processed through 10 turns of high-pressure torsion (HPT) at 450Â K. Afterwards, the HPT-processed alloy was annealed for 1Â h at temperatures ranging from 423 to 773Â K and its mechanical properties and microstructural evolution were examined using microhardness measurements and electron backscattered diffraction (EBSD) analysis. The results demonstrate that HPT processing at an elevated temperature leads to a more uniform microhardness distribution and to an early saturation in the hardness values in the Al alloy compared with high-pressure torsion at room temperature. In addition, detailed EBSD analysis conducted on the HPT-processed samples immediately after annealing revealed that the AlâMgâSc alloy subjected to HPT processing at 450Â K exhibits superior thermal stability by comparison with the same material subjected to HPT at 300Â K. Keywords: Aluminium alloys, HallâPetch relationship, Hardness, High-pressure torsion, Severe plastic deformation, Thermal stability

The processing of bulk metals through the application of severe plastic deformation leads to significant grain refinement and a consequent strengthening of the material. High-pressure torsion (HPT) generally refers to the processing of disk samples and this technique is especially effective in producing extremely small grains. Recently, new experiments were conducted in which disks of two different alloys, based on aluminum and magnesium, were stacked together and then processed by HPT for up to 20 turns at room temperature. Analysis after processing revealed the formation of a multi-layered structure in the central region of the disks but with a true nanoscale microstructure containing different types of intermetallic compounds within an Al matrix leading to the formation of metal matrix nanocomposites at the disk edges. Measurements showed a lowering of density at the disk edges, thereby confirming the potential for using HPT to fabricate materials with exceptionally high strength-to-weight ratios.

Discs of an extruded AZ80 magnesium alloy were processed by high-pressure torsion (HPT) using 6.0 GPa up to 10 turns at different temperatures (296 K and 473 K). The disc surfaces and cross-sectional planes were examined before and after processing using scanning electron microscopy (SEM) and Vickers microhardness (Hv). The microhardness results at the surface show differences in the strength of the material as a function of distance from the disc centres up to saturation, as well as a function of distance from the bottom to the surface in the cross-sectional plane. This study analyses the effect of processing temperature on the evolution of microhardness in the AZ80 magnesium alloy processed by high-pressure torsion. Keywords: Hardness, High-pressure torsion, Magnesium alloy, Severe plastic deformation

Early published results have demonstrated that high purity Cu and a Pbâ62% Sn alloy exhibit very different behaviour during high-pressure torsion (HPT) processing at room temperature and subsequent room temperature storage. High purity Cu showed strain hardening behaviour with a refined grain structure during HPT processing whereas a Pbâ62% Sn alloy displayed a strain weakening behaviour because the hardness values after HPT processing were significantly lower than in the initial as-cast condition even though the grain size was reduced. During room temperature storage after HPT processing, high purity Cu with lower numbers of rotations softened with the time of storage due to local recrystallization and abnormal grain growth whereas the Pbâ62% Sn alloy hardened with the time of storage accompanied by grain growth. Through comparisons and analysis, it is shown that the low absolute melting point and the high homologous temperature at room temperature in the Pbâ62% Sn alloy contribute to the increase in hardness with coarsening grain size during room temperature storage. Keywords: High-pressure torsion, High purity Cu, Pbâ62%Sn alloy, Self-annealing

The energy stored in severely deformed ultrafine-grained (UFG) 316L stainless steel was investigated by differential scanning calorimetry (DSC). A sample was processed by high-pressure torsion (HPT) for N = 10 turns. In the DSC thermogram, two peaks were observed. The first peak was exothermic and related to the annihilation of vacancies and dislocations. During this recovery, the phase composition and the average grain size were practically unchanged. The energy stored in dislocations was calculated and compared with the heat released in the exothermic DSC peak. The difference was related to the annihilation of vacancy-like defects with a concentration of â¼5.2 Ã 10â4. The second DSC peak was endothermic which was caused by a reversion of αâ²-martensite into γ-austenite, however in this temperature range dislocation annihilation and a moderate grain growth also occurred. The specific energy of the reverse martensitic phase transformation was determined as about â11.7 J/g. Keywords: High-pressure torsion, Stored energy, Stainless steel, Phase transformation, Thermal stability

High pressure torsion provides an opportunity to process materials with low formability such as magnesium at room temperature. The present work shows the microstructure evolution in commercially pure magnesium processed using a pressure of 6.0 GPa up to 10 turns of rotation. The microstructure evolution is evaluated using electron microscopy and the hardness is determined using dynamic hardness testing. The results show that the grain refinement mechanism in this material differs from materials with b.c.c. and f.c.c. structures. The mechanism of grain refinement observed at high temperatures also applies at room temperature. The hardness distribution is heterogeneous along the longitudinal section of the discs and is not affected by the amount of deformation imposed to the material.

Equal-channel angular pressing provides an opportunity for refining the grain structure and introducing superplastic properties in magnesium alloys. This report describes the use of this processing technique with a ZK10 (Mg–1.0 wt.% Zn–0.26 wt.% Zr) alloy. The grain structure was successfully refined from ∼12.9 to ∼5.2 μm after 4 passes and superplastic elongations were observed when testing at low strain rates at temperatures of 473 and 523 K. An analysis shows that the superplastic behavior is consistent with the conventional theoretical model for superplastic flow and at higher stresses and strain rates there is a transition to control by a viscous glide process.

The effect of the increase in crystalline defects in the structure generated by high-pressure torsion (HPT) on the corrosion behavior of metallic materials is not well understood. This report evaluates the influence of HPT on the corrosion behavior of a solution treated Al-3wt% Mg-0.2wt% Sc alloy in a 3.5 wt./v.% NaCl solution. The electrochemical behavior of the alloy was evaluated using cyclic polarization, electrochemical impedance spectroscopy (EIS), Mott Schottky, X-ray photoelectron spectroscopy, and chronoamperometry testing. The passive current density decreased after HPT processing, indicating a more protective oxide layer was formed on the surface of the HPT-processed alloy. Mott Schottky analysis confirmed the higher protection efficiency of the passive layer on the HPT-processed alloy. The film formed in the solution-treated alloy was a p-type and an n-type semiconductor while the alloy processed by HPT exhibited only an n-type semiconductor behavior. EIS showed that corrosion resistance in a saline medium increased with HPT processing.

High-pressure torsion was used to process grade 2 titanium to 10 turns using different pressures up to a maximum of 8 GPa. X ray diffraction showed that the as-received material and the material processed at pressures of 2 GPa and 4 GPa were single phase but the material processed at 8 GPa contained both the h.c.p. and the simple hexagonal ω-phase. Nanoindentation was used to determine the mechanical behavior and the results show a pronounced increase in hardness due both to the severe plastic deformation processing and to the phase transformation at the highest pressure. A maximum hardness of ~4.7 GPa was attained.

An AZ80 magnesium alloy with an initial grain size of ∼25 μm and a hardness of Hv ≈ 63 was processed by high-pressure torsion (HPT) at room temperature for up to 10 turns under an imposed pressure of 6.0 GPa. After processing, the specimens were examined by optical microscopy and transmission electron microscopy and measurements were taken of the Vickers microhardness along diameters of the HPT discs. The results show that the grains are refined to ∼200 nm after 5 and 10 turns of HPT and the hardness increases to Hv ≈ 120 at an equivalent strain of ∼30. There is a saturation condition and no further hardening at additional equivalent strains up to >200.

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–Petch relationship.

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 10−5−10−1 s−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 µm 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 300–400% were observed at 623 K, attributed to a combination of grain-boundary-sliding and dislocation-climb mechanisms.

Severe plastic deformation is an attractive processing method for refining microstructures of metallic materials to have ultrafine grain sizes within the submicrometer or even the nanometer levels. Especially, it becomes generally known that processing of metals through the application of high-pressure torsion (HPT) provides the potential for achieving exceptional grain refinement in bulk disk metals. In the present study, a ZK60A magnesium alloy was processed by HPT at room temperature for a series of numbers of revolutions under a constant compressive pressure of 6.0 GPa. The change in texture was examined by X-ray diffraction (XRD) analysis and the evolution of hardness was evaluated using Vickers microhardness measurements to provide a comprehensive understanding of microstructural evolution. The XRD analysis showed the texture changed to weak and random in the early stage of HPT. The hardness results demonstrated that the hardness evolution with increasing equivalent strain follows the strain hardening model without microstructural recovery and the degree of hardenability was calculated as ∼0.07 which implies a high degree of strain hardening toward hardness homogeneity.

Titanium alloys, mainly Ti–6Al–4V, are commonly used in biomedical applications as orthopedic implants. Due to the potential toxic influence of V and Al cations on health, a new alloy composition, Ti–24Nb–4Zr–8Sn, was introduced. However, Ti–24Nb–4Zr–8Sn has a much lower tensile strength by comparison with the Ti–6Al–4V alloy. The aim of this research was to determine whether high-pressure torsion (HPT) can be an efficient method for obtaining the desired properties in the case of the Ti–24Nb–4Zr–8Sn β-titanium alloy. This paper presents an analysis of the microstructural and mechanical properties of the Ti–24Nb–4Zr–8Sn alloy processed by HPT with various processing parameters. The obtained microstructures were examined using transmission electron microscopy (TEM). Mechanical properties, such as hardness and tensile strength, were also measured. The study demonstrates that HPT of the Ti–24Nb–4Zr–8Sn alloy leads to a significant reduction of grain size and this grain refinement gives a major improvement in tensile strength and hardness.

A cold-rolled Ti-6Al-4V sheet was subjected to three different heat treatments prior to processing by high pressure torsion (HPT). Quantitative measurements revealed that the volume fractions were 70% equiaxed α phase and 30% lamellar (α + β) (Ti64-1), 47% equiaxed α phase and 53% lamellar (α + β) (Ti64-2) and 25% equiaxed α phase and 75% lamellar (α + β) (Ti64-3) and the grain sizes of the α phase were 7.0 ± 2 μm, 9.0 ± 1.5 μm and 9.5 ± 1.5 μm, respectively. The processing by HPT was performed at room temperature with a pressure of 6.0 GPa and a rotation speed of 1 rpm. Processing of the three heat treatment batches was conducted through a total number of revolutions, N, of 1/4, 1, 5, 10 and 20. The results show that the microhardness increases with increasing numbers of turns in HPT processing and for all three conditions stable microhardness values are reached after about 20 turns. The grain sizes after 20 turns of HPT were 115 ± 30 nm in Ti64-1, 85 ± 25 nm in Ti64-2 and 75 ± 15 nm in Ti64-3 so that the grain size decreases as the volume fraction of α phase decreases and the lamellar (α + β) increases.

High-pressure torsion (HPT) was used to impose severe plastic deformation on a magnesium alloy AZ31. The material was processed for 0.5, 1, 2, 3, 5 and 7 turns at room temperature under a pressure of 6.0 GPa. Samples were annealed for 1800 s at temperatures of 373 K, 423 K, 473 K, 573 K and 673 K. Microhardness tests and metallography were used to determine the evolution of strength and grain size as a function of the annealing temperature. The results show that recrystallization takes place at temperatures higher than 423 K. The annealing behavior is independent of the number of turns in HPT.

Stainless steel was selected to study the flow patterns developed with anvil misalignments of 100, 200 and 300 μm on the disc lower surfaces during processing by high-pressure torsion (HPT) through totals of up to 16 turns. A pair of anvils having a roughness of Ra ≈ 15 μm was utilized to investigate the flow pattern development. Discs subjected to only compression in HPT exhibit similar characteristics to the as-received material in the phase domains and there were no overall curvatures of the austenitic (γ) and ferritic (α) phases. Double-swirl flow patterns were not observed in the 1 turn sample but they appeared on the disc lower surfaces after 5 and 16 turns with all three-anvil alignment conditions. There was no significant difference in the double-swirl configuration size for the 5 and 16 turns samples with different amounts of anvil misalignments. These results have important implications for processing metals by HPT.

Experiments were conducted on two commercial alloys, a Cu–0.1%Zr alloy and an Al-7075 aluminum alloy, to investigate the significance of the saturation microstructure which is achieved after processing by high-pressure torsion (HPT). Samples were processed by HPT and also by a combination of equal-channel angular pressing (ECAP) followed by HPT. The results show that the saturation conditions are dependent upon the grain size in the material immediately prior to the HPT processing. Additional grain refinement may be achieved in HPT by initially processing the material to produce an ultrafine-grain size before conducting the processing by HPT.

The processing of metals through the application of high-pressure torsion (HPT) provides the potential for achieving exceptional grain refinement in bulk metals. Numerous reports are now available demonstrating the application of HPT to a range of pure metals and simple alloys. In practice, excellent grain refinement is achieved using this processing technique with the average grain size often reduced to the true nano-scale range. Contrary to the significant grain refinement achieved in metals during HPT, the models of the hardness evolution are very different depending upon the material properties. For a better understanding of the material characteristics after conventional HPT processing, this report demonstrates the hardness evolutions in simple metals including high-purity Al, commercial purity aluminum Al-1050, ZK60A magnesium alloy and Zn-22% Al eutectoid alloy after processing by HPT. Separate models of hardness evolution are described with increasing equivalent strain by HPT. Moreover, a new approach for the use of HPT is demonstrated by synthesizing an Al–Mg metal system by processing two separate commercial metals of Al-1050 and ZK60A through conventional HPT processing at room temperature.

Detailed EBSD analysis was performed on copper specimens processed by high-pressure torsion at P = 6 GPa for one whole turn and subsequently annealed at a temperature of 100 °C for 15, 30 and 60 min. The basic microstructural parameters (mean grain size, GB statistics, microtexture) were evaluated in the mid-radius areas of the HPT disks. Microhardness of all samples was measured across the two diameters and interlinked to the microstructures observed. Small but noticeable changes of microhardness in HPT copper after annealing were detected. The changes were interlinked to microstructural parameters acquired by EBSD. The relationships obtained are discussed in terms of the microstructure and microtexture evolution during low temperature annealing.

A low-carbon triple-alloyed steel was processed by high-pressure torsion at room temperature for up to 5 turns under a pressure of 6.0 GPa. Microhardness, scanning electron microscopy and X-ray diffraction were used to investigate the hardness and microstructural evolution of the steel. Values of the Vickers microhardness were recorded across the sample diameters. The results show that there is a gradual evolution in both the hardness and the microstructure with increasing numbers of turns. However, the microhardness does not become fully homogeneous across the sample diameter after five turns and there remain significantly lower values in the center of the disk. These results indicate that complete homogeneity across the disks for this steel requires applied pressures higher than 6.0 GPa and/or torsional straining through more than 5 turns.

The superplasticity of an extruded fine-grained Mg–9Gd–4Y–0.4Zr alloy was investigated by measuring the strain rate sensitivity using shear punch testing (SPT). Shear punch tests were conducted at shear strain rates in the range of 3 × 10−3–2 × 10−1 s−1 and at temperatures in the range of 573–773 K. The results indicate the strain rate sensitivity, m, increases from about 0.11 at 573 K to about 0.40 at 723 K and then decreases to 0.32 with a further increase in test temperature. A strain rate sensitivity of 0.40 and an activation energy of 140 kJ/mol are indicative of a superplastic deformation behavior dominated by grain boundary sliding accommodated by lattice diffusion at temperatures above 673 K.

Equal-channel angular pressing (ECAP) is widely recognized as an effective method for processing ultrafine-grained and even nanostructured materials. Important details on processes occurring in the die intersections can be obtained by mapping the microhardness and EBSD microstructures in partially-pressed aluminum and copper through the 90º die of ECAP. Precise measurements were made using grids of partially-pressed Al and Cu and detailed color maps were plotted and compared with EBSD maps. A narrow region along the bisector of two channels reflects altering in microhardness level of the material subjected to simple shear. The microstructural evolution suggests significant refining of the grain structure but there is noticeable inhomogeneity in the microhardness and microstructure for both materials. Factors contributing to the inhomogeneous hardness distribution include the coarse initial grain size, and inhomogeneous deformation across the plane of the die channel intersection due to die wall friction and the formation of a dead zone at the outer corner.