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The main point of successful manufacture of metallic composites by direct bonding of dissimilar materials is achieving a homogeneous interface bonding. Two different types of deformation techniques for fabrication of metal composites were investigated. The first one was developed on the basis of high pressure torsion associated with a high energy impact on the material where part of energy involved can be dissipated via non equilibrium phase transition realization. This deformation due to high shear deformations allows not only to form a nanostructure, but also to bond dissimilar metals. Moreover, this method allows for a relatively short time and in a number of compounds to receive in one step at room temperature monolithic composites of sufficient size to certify the structure and properties. The second technique is diffusion bonding which integrate one material with the other by pressure under high temperature. In order to clarify the bonding mechanism by plastic deformation of dissimilar materials, the microstructural and some mechanical properties were studied in the processed samples.

Changes in the structure of coarse-grained nickel due to ultrasonic treatment (UST) are studied. For this purpose, commercially pure nickel was annealed at a temperature of 1300 °C for 1 h to obtain a coarse-grained structure with a low dislocation density. On the electrolytically polished surface of the sample, a 2 × 2 mm2 sized area was marked and investigated by EBSD analysis. Further, the sample was subjected to UST with an oscillating stress amplitude of 20 MPa, and the marked area was polished and scanned again. X-ray diffraction analysis and microhardness measurements were carried out before and after UST as well. It is shown that UST leads to dislocation generation, an increase in the fraction of low-angle boundaries, changes in the shape of twins and grain boundaries. The dependence of structural parameters and microhardness on the UST amplitude is also described.

Commercially pure nickel was processed by equal channel angular pressing (ECAP) through route Bc for 12 passes to obtain an ultrafine grained (UFG) microstructure and further subjected to an ultrasonic treatment (UST) which introduced a maximum amplitude equivalent to anormal stresses of 100 MPa in a steady state regime. It was observed that the microstructure of UFG Ni can be differently altered depending on the position in the sample, i.e. on the amplitude of the ultrasonic wave. The microstructural analysis demonstrated a lower dislocation density (due to the activation of recovery) at the cross section of the specimen subjected to UST at an amplitude equal to 0.8 of the maximum one, plus enhanced hardening in the cross section at the maximum amplitude.

Grain refinement is an effective approach to improve mechanical properties of conventionally-manufactured high entropy alloys (HEAs). Additive manufacturing of HEAs is a new materials challenge and increasing reports are available for exploring the optimal processing parameters and post-manufacturing treatments to advance the physical and mechanical properties of additively-manufactured (AM) HEAs. At the current stage of the development of AM HEAs, it is necessary to investigate the significance of grain refinement on their mechanical properties and structures. In the present study, a CoCrFeNi HEA is manufactured by a laser powder-bed fusion technique using pre-alloyed HEA powders on which grain refinement was conducted by high-pressure torsion for up to 8 turns under 6 GPa at room temperature. The results from nanoindentation and Vickers microhardness testing demonstrate high strain hardening capability and increased plasticity, thus potentially high ductility, in the nanostructured AM CoCrFeNi HEA. X-ray diffraction analysis demonstrates the structural evolution with decreasing crystallite size, increasing microstrain and expanding lattice parameter with grain refinement in the HEA. The structural changes justify the estimation by nanoindentation of the rate-controlling mechanism of the grain boundary-mediated dislocation activity for the nanostructured AM HEA. This study provides advantages of nanostructuring for current developments in the AM technology of HEAs.

The ultrafine-grained microstructures, mechanical properties and electrical conductivity of a Cu–0.3%Cr–0.5%Zr alloy subjected to equal channel angular pressing (ECAP) at a temperature of 400 °C to a total strain of 1, 2, and 4 were investigated. The ultrafine-grained microstructure resulting from progressive increase in the misorientations of strain-induced low-angle boundaries during the multiple ECAP process is considered as a type of continuous dynamic recrystallization. The multiple ECAP process resulted in substantial strengthening of the alloy. The yield stress of CuCrZr alloy in the initial solution treated condition (ST) increased from 65 MPa to 476 MPa after four ECAP passes at 400 °C. For the aged condition (AT), the yield stress increased from 170 MPa to 511 MPa after four passes. The strengthening was attributed to the grain refinement and high dislocation densities evolved via large strain deformation. Bacon–Kocks–Scattergood modification of the Orowan model is sufficient for acceptable description of the precipitation hardening of AT specimens during ECAP processing; this finding is in excellent agreement with the experimental data. The discrepancy between the experiment and model for ST specimens disappears after taking into consideration additional precipitating of the supersaturated solid solution during preheating and ECAP processing.

This study demonstrates the evolution of microstructure and mechanical properties of a hypoeutectic Al–7Si–0.4Mg (A356) alloy processed by accumulative back extrusion (ABE) at temperatures ranging from 200 to 500 °C. ABE processing is one of the new severe plastic deformation techniques enabling one to produce relatively large ultrafine-grained materials in a cylindrical shape. One complete pass of ABE was estimated to introduce a reasonably homogeneous effective strain of ~3 as calculated by finite element analysis. Microstructural observation showed that globular α-Al primary phase was subdivided into fine substructures and Si particles having a fibrous shape were fragmented and spheroidized within the eutectic constituent through ABE processing. There was no evidence of homogeneous distribution of the fine Si particles in the α-Al phase after ABE. Mechanical testing at room temperature showed that both yield strength and ultimate tensile strength of the A356 alloy dramatically increased through ABE, especially at lower processing temperatures, as compared with the as-cast condition whereas there was no significant reduction in ductility at all processing temperatures. The experimental results were discussed with emphasis on the microstructure evolution involving dynamic recrystallization and deformation behavior including strengthening mechanisms and strain hardening in the Al–Si alloy.

Although superplasticity has intensively been studied for half century, few observations have been reported for pure metals due to fast grain growth at temperatures required for superplasticity. With developing of nanocrystalline materials, there was a hope that superplasticity could be obtained in a number of pure metals. Indeed, low temperature superplasticity in pure nickel was reported in pioneering work in 1999, later superplastic feature of nanonickel was attributed to sulfur presence in grain boundaries. Recently, it was concluded that superplasticity it is not related to the presence of sulfur at grain boundaries or a liquid phase at grain boundaries. Thereby, the phenomenon of superplasticity in pure metals is still far away for our understanding and it requires future work. This report is devoted to reassessment of superplastic behavior of nanonickel and it provides new results on enhanced plasticity of pure nickel processed by HPT consolidation of rapid quenched ribbons.

Bulk nickel-graphene (Ni-Gr) composites are in the high demand for most of the engineering applications. Despite many efforts made, the main problem remains the homogeneous dispersion of graphene into the Ni matrix without destroying the structural integrity of Gr. The present paper addresses the fabrication, structural features and mechanical characteristics of bulk Ni-Gr composites, manufactured from nanosized nickel powder and exfoliated graphite platelets (GP) via modified powder metallurgy procedure including micromechanical GP splitting. The combination of XRD, SEM, Raman spectroscopy and EBSD analysis revealed homogeneous distribution of Gr in Ni matrix and the significant grain refinement observed for Ni-0.1 wt% Gr composite. Bimodal grain size distributions were obtained for all samples studied, and the mean grain size depends on Gr content. Raman spectroscopy proved the complete GP-to-graphene conversion for composite with 0.1 wt% Gr. Bulk nickel specimen and composites exhibit ∼3 times higher hardness and moderate tensile strength values compared to Ni, manufactured from micron-sized powders.

Multilayer laminates comprising thirteen layers of Ti–6Al–4V alloy were produced by diffusion bonding. The influence of the processing conditions on the microstructure and mechanical properties of laminates were determined. The multilayer laminates were tested at room and liquid-nitrogen temperatures under impact bending tests with recording of dynamic loading diagrams. The influence of the interfaces pores on the total energy of fracture, the energy of crack initiation and the energy of crack propagation was analyzed. The impact strength value of the laminate decreased by 1.7 times for the crack divider orientation, and it increased by 14 times for the crack arrester orientation with the relative length of pores increasing from 1 to 30%.

An investigation has been conducted on AZ91 magnesium alloy processed in high-pressure torsion (HPT) at 296, 423 and 473 K for different numbers of turns. The microstructure has altered significantly after processing at all processing temperatures. Extensive grain refinement has been observed in the alloy processed at 296 K with apparent grain sizes reduced down to 35 nm. Segmentation of coarse grains by twinning has been observed in the alloy processed at 423 K and 473 K with average apparent grain sizes of 180 nm and 250 nm. Substantial homogeneity in microhardness has been observed in the alloy processed at 296 K compared to that found at 423 K and 473 K. The ultrafine-grained AZ91 alloy exhibited a significant dependence of the yield strength on grain size as shown by the microhardness measurements, and it obeys the expected Hall–Petch relationship. The alloying elements, fraction of nano-sized particles of β-phase, and the dominance of basal slip and pyramidal modes have additional effects on the strengthening of the alloy processed at 296 K.

Disks of commercial Al-1050 and ZK60A alloys were stacked together and then processed by conventional high-pressure torsion (HPT) through 1 and 5 turns at room temperature to investigate the synthesis of an Al–Mg alloy system. Measurements of microhardness and observations of the microstructures and local compositions after processing through 5 turns revealed the formation of an ultrafine multi-layered structure in the central region of the disk but with an intermetallic ?-Al3Mg2 phase in the form of nano-layers in the nanostructured Al matrix near the edge of the disk. The activation energy for diffusion bonding of the Al and Mg phases was estimated and it is shown that this value is low and consistent with surface diffusion due to the very high density of vacancy-type defects introduced by HPT processing. The results demonstrate a significant potential for making use of HPT processing in the preparation of new alloy systems.

An investigation was conducted to evaluate the dependence on grain size of the compressive deformation of commercial purity (CP) Ti. Tests were performed at room temperature using grain sizes from coarse-grained CG (20 ?m) to ultrafine-grained UFG (500 nm) and nanocrystalline NC (90 nm) with testing strain rates in the range from 0.01 to 10 s?1. The results show the flow stress and the strain rate sensitivity of CP Ti increase with decreasing grain size. Work hardening dominates at all strain rates in CG Ti but it balances with flow softening at 0.01 and 0.1 s?1 in UFG and NC Ti and there is obvious flow softening in these two materials at 10 s?1.

An investigation has been conducted on the tensile properties of a fine–grained AZ91 magnesium alloy processed at room temperature by high pressure torsion (HPT). Tensile testing was carried out at 423 K, 473 K and 573 K using strain rates from 1×10–1 s–1 to 1×10–4 s–1 for samples processed in HPT for N = 1, 3, 5 and 10 turns. After testing was completed, the microstructures were investigated by scanning electron microscopy and energy dispersive spectroscopy. The alloy processed at room temperature in HPT exhibited excellent superplastic behaviour with elongations higher than elongations reported previously for fine–grained AZ91 alloy produced by other severe plastic deformation processes, e.g. HPT, ECAP and EX–ECAP. A maximum elongation of 1308 % was achieved at a testing temperature of 573 K using a strain rate of 1×10–4 s–1, which is the highest value of elongation reported to date in this alloy. Excellent high–strain rate superplasticity (HSRSP) was achieved with maximum elongations of 590 % and 860 % at temperatures of 473 K and 573 K, respectively, using a strain rate of 1×10–2 s–1. The alloy exhibited low–temperature superplasticity (LTSP) with maximum elongations of 660 % and 760 % at a temperature of 423 K and using strain rates of 1×10–3 s–1 and 1×10–4 s–1, respectively. Grain–boundary sliding (GBS) was identified as the deformation mechanism during HSRSP, and the glide–dislocation creep accommodated by GBS dominated during LTSP. Grain–boundary sliding accommodated with diffusion creep was the deformation mechanism at high test temperature and slow strain rates. An enhanced thermal stability of the microstructure consisting of fine equiaxed grains during deformation at elevated temperature was attributed to the extremely fine grains produced in HPT at room temperature, a high volume fraction of nano ?–particles, and the formation of ?–phase filaments.

Commercial purity (CP) Ti was processed by a two-step procedure of ECAP and drawing to give a nanocrystalline (NC) grain size of ~90 nm. Samples were tested in compression at a strain rate of 10 s −1 over the temperature range of 298−673 K. The results show the high stored energy and high interface energy promote the process of static recrystallization (SRX) when the NC Ti is heated above 573 K. Migration dynamic recrystallization (m-DRX) operates during compressive deformation at temperatures of 298−573 K giving a homogeneous microstructure. The compressive yield strength and peak stress at 298 K were measured as 1230 and 1330 MPa, respectively. When the test temperature is increased from 298 to 673 K, the yield strength and peak stress decrease and the work hardening rate and dynamic softening rate are also reduced.

The strengthening of metals is essentially controlled by the microstructures of the metal solids and it is well understood that smaller grain sizes lead to higher hardness and increased strength. Nevertheless, true bulk nanostructured materials are difficult to produce using established engineering techniques, especially when considering the practical and societal needs of materials selection. Lightweight Al and Mg are conventional metals having excellent physico-chemical and mechanical properties and with good strength/weight ratios in the finished products. However, the fabrication of high-strength metals consisting of these elements, using mechanical alloying and milling and cladding-type metal working, generally involves long-term processing conducted under extreme conditions using special facilities. The present study demonstrates the very rapid synthesis of a metal matrix nanocomposite (MMNC) of the Al–Mg system which was achieved by stacking metal disks of the two pure metals and processing by high-pressure torsion at ambient temperature for 10 turns. An exceptionally high hardness was achieved, similar to many steels, through rapid stress-induced diffusion of Mg and the simultaneous formation of intermetallic nano-layers and a nanostructured intermetallic compound with a supersaturated solid solution. This unexpected result suggests a potential for simply and expeditiously fabricating a wide range of MMNCs.

Research was conducted to investigate the potential for consolidating titanium powder using high-pressure torsion (HPT) at room temperature. The nanostructured samples processed by HPT were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The results show there is a significant refinement of the Ti powder and it consolidates into bulk nanostructured titanium with a mean grain size estimated by TEM as ~200–300 nm and a mean crystallite size measured by XRD as ~20–30 nm. Microhardness measurements and tensile testing show high strength and low ductility after consolidation under a pressure of 6.0 GPa for 5 revolutions. Additional short annealing at a temperature of 300 °C for 10 min leads to a significant enhancement in ductility while maintaining the high strength.

The ultrafine-grained microstructures and functional properties, including wear resistance for dry sliding and electrical conductivity, were investigated in a Cu–0.3 %Cr–0.5 %Zr alloy processed by equal-channel angular pressing (ECAP) at a temperature of 400 °C to total strains of 1, 2, and 4. Severe plastic deformation by ECAP to a total strain of ~1 led to a significant decrease in the wear resistance because of the rapid surface damage to both solution-treated (ST) and aged (AT) samples where a high density of dislocations was arranged in stochastic low-angle subboundaries by brittle fracture. Further deformation by subsequent ECAP passes promoted the subdivision of the shear bands by geometrically necessary boundaries. Correspondingly, the number of fine crystallites outlined by high-angle boundaries increased, and the wear rate decreased. After four ECAP passes, the wear rate decreased to the level of the initial state of the alloy and equaled 1.68 × 10−5 mm3/(N m) and 1.40 × 10−5 mm3/(N m) for ST and AT samples, respectively. The results demonstrate that the damage mechanism is the controlling factor for wear resistance of Cu–Cr–Zr bronze hardened by intense plastic straining.

Commercial purity titanium was processed by equal-channel angular pressing (ECAP) for 8 passes and then subjected to dynamic compressive testing using a split-Hopkinson pressure bar (SHPB) facility with an imposed strain rate of ~4000 s−1 and testing temperatures from 288 to 673 K. The results show that ECAP produces an average grain size of ~0.3 μm in transverse sections, but grains which are elongated in longitudinal sections. During dynamic compressive testing at temperatures ranging from 288 to 473 K, the grain shapes and sizes remain unchanged in the transverse sections, but the elongated shapes in the longitudinal sections evolve into polygons due to cell dislocation evolution. At 673 K, the grains become equiaxed with an average size of ~1.8 μm thereby demonstrating the occurrence of dynamic recrystallization. It is shown that the flow stresses decrease with increasing temperature from 288 to 673 K, and there is also a reduction in the rate of strain hardening.

The microstructure evolution and the deformation behavior of a Cu-0.3%Cr-0.5%Zr alloy subjected to multidirectional forging at a temperature of 673 K under a strain rate of about 10-3 s-1 were studied. Following a rapid increase in the flow stress during straining to about 1, the strain hardening gradually decreases, leading to a steady-state flow behavior at total strain above 2. The multidirectional forging led to the development of ultrafine grained microstructures with mean grain sizes of 0.9 μm and 0.64 μm in the solution treated and aged samples, respectively. The presence of second phase precipitates promoted the grain refinement. After processing to a total strain of 4, the fractions of ultrafine grains (D < 2 μm) comprised 0.36 and 0.59 in the solution treated and aged samples, respectively.

High-pressure torsion (HPT) is recognized as the most effective method for producing ultrafine-grained and even nanocrystalline structures in metallic systems. Although there are many reports on microstructural refinement of pure metals and metallic alloys, several important problems remain unresolved. For example, more information is needed on the homogeneity of the processed microstructure and on the heat release and temperature rise during HPT processing. Recently, there were reports of hardening in HPT-processed pure metals under conditions of self-annealing. This report presents new experimental data on the relaxation processes in HPT copper and aluminium processed at room temperature under loads of 6.0 and 1.0 GPa for zero and one turn. The experimental results were obtained using X-ray diffraction and by measuring the average Vickers microhardness after ageing.

Ultrafine-grained (UFG) pure Ti processed by equal-channel angular pressing was subjected to dynamic compression at a stain rate of ∼4000 s−1 over a wide temperature range. It is shown that the structure in longitudinal sections evolves from elongated shapes to polygons in dynamic testing at 93–473 K and to equiaxed grains at 673 K which grow to ∼1.8 μm. The deformation mechanisms of UFG pure Ti are discussed with reference to its crystallographic texture.

The influences of annealing temperature on the wear properties and electrical conductivity of Cu were studied after processing by high-pressure torsion (HPT). The annealing of Cu specimens processed by HPT leads to an increase in electroconductivity and a decrease in the wear rate. It is apparent that a nanotribolayer at the surface induced during wear sliding plays a more significant role than the ultrafine-grained structure. A slight increase was observed in the microhardness of HPT copper specimens upon annealing at a relatively low temperature (100 °C), and this is most likely due to a change in texture. The annealing leads to an increase in the Taylor factor by ~5 %, which is in good agreement with the increase in the microhardness level which is also by ~5 %. It is apparent that low-temperature annealing of HPT copper may produce optimal properties of the specimens including high strength and electroconductivity with a lower wear rate.

In this work we demonstrate that pure compression of the single phase bcc Mg–12 wt.% Li and Mg–20 wt.% Li alloys in a high-pressure torsion system induces the precipitation of hexagonal close-packed (hcp) Mg and hcp Li-rich phases, which remain stable under ambient conditions. This transformation is enhanced with increasing pressure. Shearing under pressure of the multiphase microstructure results in the dissolution of the metastable hcp phases and the subsequent growth of the bcc matrix grains.

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.

Severe plastic deformation methods include equal-channel angular pressing/extrusion, high-pressure torsion, and plane strain machining. These methods are extremely effective in producing bulk microstructure refinement and are generally initiated at a low homologous temperature. The resulting deformation-induced microstructures exhibit progressively refined cellular dislocation structures during the initial stages of straining that give way to refined, equiaxed grain structures at larger strains. Often, grain refinement appears to saturate but frequently coarsening is observed at the largest strains after a minimum in grain size is attained during SPD. Here, we summarize results on grain refinement by these processing methods and provide an analysis that incorporates adiabatic heating to explain the progressive refinement to intermediate strains and that may be followed either by an apparent saturation in grain refinement or by grain coarsening at the largest strains. This analysis is consistent with continuous dynamic recrystallization in the absence of the formation and long-range migration of high-angle boundaries.

The technique of high pressure torsion (HPT) has been widely used to refine the microstructure of many metallic materials, especially pure metals and disordered alloys. Comparatively fewer studies have, however, been carried out in intermetallics. γ-TiAl alloys are envisioned as high potential materials to replace Ni superalloys in some turbine components due to their good performance at high temperatures and light weight. Exploring the potential beneficial effects of severe plastic deformation techniques in these materials is now timely. In this work, a γ-TiAl alloy with a lamellar microstructure has been processed by HPT using pressures ranging from 1 to 6 GPa and 0 to 5 anvil turns at room temperature. Significant refinement of the microstructure via twin formation, bending of the lamella and the accumulation of a high dislocation density upon the application of shear give rise to a drastic hardness increase.

Pure Zn was processed by high pressure torsion (HPT) at room temperature in order to evaluate the effect of this severe plastic deformation (SPD) technique on materials processed at high homologous temperatures. HPT leads to the development of coarse grains with orientations belonging to a basal fiber by discontinuous dynamic recrystallization (DDRX). The strength of the texture increases dramatically with the applied shear strain in such a way that, after 5 turns, the volume fraction of oriented material is 85%. The samples processed to large strains possess high yield strength along the in-plane directions due to the reduced activity of basal slip and the operation of non-basal systems. The microhardness values nevertheless remain similar to those of the annealed material up to equivalent strains of 50 and then they decrease gradually with increasing strain due to grain growth. The microstructural development of pure Zn after HPT is critically influenced by the initial microstructure, as well as by the temperature increase during processing which is highly dependent on the rotational speed of the anvils.

Ultrafine-grained and even nanostructured materials can be fabricated using severe plastic deformation to ultra-high strains in equal-channel angular pressing (ECAP), high-pressure torsion (HPT), machining and their combinations, such as machining of ECAP specimens, HPT of ECAP billets and HPT of machining chips. This report presents recent results of investigations of the microstructures and microtextures of pure copper, nickel and aluminium subjected to different deformation processes to ultimately high imposed strains. A comparison of the microstructure, dislocation density and microhardness developed during combinations of different strain paths is performed. All characteristics were analysed by X-ray, transmission and scanning electron microscopy, and electron backscatter diffraction (EBSD). The influence of different processing routes is discussed in terms of the accumulated strain and microstructure refinement. The saturation in grain refinement is examined with reference to the recovery taking place during ultra-high strain deformation. A phenomenological model based on the Voce equation is applied for fitting parameters based on the experimental data and this is suggested for a prediction of microhardness evolution for pure metals (Ag, Au) and Cu-based (Zn, Al) alloys.

Ultrafine-grained, and even nanostructured materials can be manufactured by ultra-high straining by equal-channel angular pressing (ECAP), high-pressure torsion (HPT), by machining, and through combinations, such as machining of ECAP specimens, HPT plus ECAP, and HPT of machining chips. This report describes the results of investigations of the microstructure and microtexture of pure aluminium and copper subjected to different deformation processes to high imposed strains. The microstructures, dislocation densities, and microhardness developed during combinations of different strain paths were investigated and all characteristics were analyzed by X-ray, transmission and scanning electron microscopy, and by orientation imaging microscopy. The influence of different processing routes is examined in terms of the accumulated strain and microstructure refinement. A saturation in grain refinement is also considered with reference to the occurrence of recovery during ultra-high strain deformation.

A fine-grained magnesium ZK60 alloy was used to study texture evolution during processing by equalchannel angular pressing. Experiments were conducted by pressing the alloy through dies having angles of 90◦, 110◦ and 135◦ and the textures were predicted using a visco-plastic self-consistent model. It is shown that the development of texture depends upon the channel angle within the die. Calculations

The microstructure and microhardness of copper subjected to large strains either using one or a combination of severe plastic deformation (SPD) processing techniques was evaluated. The individual SPD techniques used include equal-channel angular pressing (ECAP), high-pressure torsion (HPT), and chip formation during machining (M). Microstructural characterization using orientation imaging microscopy provided detailed information on the grain sizes and misorientation statistics after different processing routes. Vickers indentation analysis was used to evaluate the hardness of the deformed samples. The results show that excellent microstructures and properties are achieved when these three processes are used in combination, including grain sizes in the range of ~0.2–0.3 μm and hardness values up to >1,900 MPa.

High-pressure torsion (HPT) refers to the processing of metals whereby samples are subjected to a compressive force and concurrent torsional straining. Although the fundamental principles of this procedure were first proposed more than 60 years ago, processing by HPT became of major importance only within the last 20 years when it was recognized that this metal forming process provides an opportunity for achieving exceptional grain refinement, often to the nanometer level, and exceptionally high strength. This review summarizes the background and basic principles of processing by HPT and then outlines the most significant recent developments reported for materials processed by HPT. It is demonstrated that HPT processing leads to an excellent value for the strength of the material, reasonable microstructural homogeneity if the processing is continued through a sufficient number of torsional revolutions and there is a potential for achieving a capability for various attractive features including superplastic forming and hydrogen storage. The review also describes very recent developments including the application of HPT processing to bulk and ring samples and the use of HPT for the consolidation of powders.

High-pressure torsion (HPT) is an important processing technique in which a disk is subjected to a high pressure with concurrent torsional straining. In principle at least, the imposed strain is zero at the center of the disk and a maximum at the outer edge. This difference suggests, therefore, that materials processed by HPT will exhibit considerable inhomogeneity. This paper describes the results obtained in a series of experiments which were designed to evaluate the evolution of homogeneity during the processing of two materials by HPT. It is demonstrated that it is possible to achieve a reasonable level of homogeneity in both materials but there are important differences which reflect the dependence of the microstructure on the occurrence of dynamic recovery.

A plate of as-cast NiAl bronze (NAB) material was sectioned from a large casting. A six-pass fusion weld overlay was placed in a machined groove; a portion of the weld reinforcement was removed by milling and a single friction stir processing (FSP) pass was conducted in a direction transverse to the axis of and over the weld overlay. A procedure was developed for machining of miniature tensile samples and the distributions of strength and ductility were evaluated for the fusion weld metal; for the stir zone (SZ) produced by the friction stir processing; and for a region wherein friction stir processing had taken place over the fusion weld. A region of low ductility in the heat affected zone (HAZ) of the fusion weld and in the thermomechanically affected zone (TMAZ) of friction stir processed material was attributed to partial reversion of an equilibrium lamellar eutectoid constituent upon local heating above ∼800 °C and formation of non-equilibrium transformation products upon subsequent cooling. The adverse effect on ductility is worse in the heat affected zone of the fusion weld than in the thermomechanically affected zone of friction stir processing due to the lower heat input of the latter process. The implications of this work to engineering applications of friction stir processing are discussed.

The mechanical behavior of commercial purity titanium with a nanocrystalline (NC) grain size was investigated using split Hopkinson pressure bar tests at high strain rates and over a range of temperatures. The study was accompanied by detailed microstructural investigations before and after compression testing. The results show that rotary dynamic recrystallization operates during compressive deformation at strain rates of ~3000 and ~4500 s?1 at temperatures from 298 to 573 K but cells form at 673 K. The dynamic mechanical behavior of NC Ti shows a strong dependence on temperature and strain rate such that the flow stress and the strain hardening rate both increase with increasing strain and decreasing temperature. A constitutive equation is derived to relate the flow stress to the temperature, strain rate and true strain and to predict the yield strength and the peak stress of NC Ti subjected to dynamic deformation at elevated temperatures.

Tantalum, a common refractory metal with body-centred cubic (BCC) crystalline structure, was processed by high-pressure torsion (HPT) at room temperature through different numbers of rotations. Significant grain refinement and high strength were achieved with a reduction in grain size from ∼60 μm to ∼160 nm and an increase in strength from ∼200 to >1300 MPa. Hardness measurements revealed a high level of homogeneity after 10 turns of HPT but the hardness after 10 turns was slightly lower than after 5 turns indicating the occurrence of some recovery. Tensile testing at a strain rate of 1.0×10−3 s−1 gave high strengths of ∼1200 MPa but little or no ductility after processing through 1, 5 and 10 turns. The introduction of a short-term (15 min) anneal immediately after HPT processing led to significant ductility in all samples and a reasonable level of strength at ∼800 MPa.

Pure nickel and commercially pure (CP) aluminium were selected as model fcc materials for a detailed investigation of the experimental parameters influencing grain refinement and evolution of microstructure and microtexture during processing by high-pressure torsion (HPT). Samples were examined after HPT using microhardness measurements, transmission electron microscopy and orientation imaging microscopy. Processing by HPT produces a grain size of ∼170 nm in pure Ni and ∼1 μm in CP aluminium. It is shown that homogeneous and equiaxed microstructures can be attained throughout the samples of nickel when using applied pressures of at least ∼6 GPa after 5 whole revolution. In CP aluminium, a homogeneous and equiaxed microstructure was achieved after 2 whole revolutions under an applied pressure of 1 GPa. For these conditions, the distributions of grain boundary misorientations are similar in the centre and at the periphery of the samples. It is shown that simple shear texture develops in fcc metals subjected to high-pressure torsion. Some grain growth was detected at the periphery of the Al disk after 8 revolutions. The factors influencing the development of homogeneous microstructures in processing by HPT are discussed.

The development of shear textures and band-like features in the microstructure of commercially pure (CP) aluminum have been studied as a function of backpressure and die relief angle during equal-channel angular pressing (ECAP) through a die with a 90° die channel angle. Microtexture data were acquired by orientation imaging microscopy (OIM) following one pass and four repetitive passes by route A. In a 90° die with zero relief angle, the microtexture data indicate that ECAP involves simple shear on a shear plane and in a shear direction that are rotated away from the plane of the die channel intersection toward the die exit channel. This rotation reflects the spreading of shear deformation through a fan-shaped region around the plane of the die channel intersection. The superposition of backpressure suppresses this spreading and the texture data indicate that deformation is confined to the plane of the die channel intersection, while the introduction of a relief angle at the outer corner of the die channel intersection promotes spreading and leads to further rotation of the shear plane and shear direction toward the axis of the die exit channel. Splitting of shear texture components is observed after the initial ECAP pass. Such splitting appears in the microstructure as band-like features separated by sub-grains. Band-like features became apparent in the microstructure after the initial pass and persisted through four ECAP passes by route A; these features become more prominent with increasing die relief angle.

Microtexture and microstructure evolution during repetitive equal-channel angular pressing (ECAP) of pure aluminum through a 90° die was evaluated by orientation imaging microscopy (OIM) and transmission electron microscopy (TEM). Billet distortion appears to conform to the idealized ECAP model. After the initial pass, the textures were inhomogeneous but one or more shear-texture components and long-range lattice rotations were apparent. Following repetitive ECAP, the textures became more homogeneous but still included either two or three distinct shear-texture orientations. The OIM and TEM data revealed meso-scale deformation bands that were inclined at about 26° to the axis of the as-pressed samples and that involved alternation of lattice orientations between distinct shear-texture orientations. The band interfaces were of high disorientation (40–62.8°) and were distinct boundaries in TEM. The evolution of the band structures during repetitive ECAP accounts for an increasing population of high-angle boundaries in repetitively processed materials.

Nanocrystallization behavior of Al85Ce8Ni5Co2 amorphous ribbon induced by severe plastic deformation and by linear heating thermal treatment has been compared using differential scanning calorimetry, X-ray diffraction and transmission electron microscopy. Thermal treatment of the as-quenched ribbon results in the joint formation of several crystalline phases, whereas plastic deformation at room temperature induces only primary precipitation of a-Al nanocrystals. This is evidence for the athermal nature of the deformation induced crystallization process.

An investigation was conducted to evaluate the microstructural evolution occurring in disks of commercial purity aluminum processed by high-pressure torsion (HPT) under constrained conditions. Microhardness measurements were taken to assess the variation in hardness across the diameters of disks subjected to different imposed strains and the microstructures were observed at the edges and in the centers of the disks using transmission electron microscopy. The results show the microhardness is lower and there is less grain refinement in the central regions of the disks in the initial stages of torsional straining but the microstructures become reasonably homogeneous across the disks at high imposed strains.

The influence of strain path during equal channel angular pressing (ECAP) has been evaluated in pure aluminum by orientation imaging microscopy (OIM) and transmission electron microscopy (TEM). The material was examined after four pressing operations by route BC in a 90 ◦ die, or eight pressing operations by route BC in a 135 ◦ die. The von Mises equivalent strains were essentially the same for these two ECAP procedures. The microtexture data indicate that the distortion during ECAP corresponds to a simple shear in a direction approximately parallel to die-channel exit and on a plane perpendicular to the flow plane. For both procedures, the OIM data reveal prominent meso-scale band-like features. Lattice orientations in each band correspond to a texture orientation but the particular combinations of orientations depend upon ECAP die angle. High-angle boundaries in the structure correspond to interfaces between the bands. © 2005 Elsevier B.V. All rights reserved.

A detailed investigation was conducted to evaluate the microstructural characteristics in samples of pure nickel processed using three different procedures of severe plastic deformation (SPD): equal-channel angular pressing (ECAP), high-pressure torsion (HPT) and a combination of ECAP and HPT. Several different experimental techniques were employed to measure the grain size distributions, the textures, the distributions of the misorientation angles and the boundary surface energies in the as-processed materials. It is shown that a combination of ECAP and HPT leads both to a greater refinement in the microstructure and to a smaller fraction of boundaries having low angles of misorientation. The estimated boundary surface energies were higher than anticipated from data for coarse-grained materials and the difference is attributed to the non-equilibrium character of many of the interfaces after SPD processing.

Orientation imagining microscopy was used to measure the distributions of grain boundary misorientations in ultrafine-grained nickel processed by high-pressure torsion and equal-channel angular pressing. Both procedures give high fractions of high-angle boundaries but also higher fractions of low-angle boundaries than anticipated from a random distribution.

An as-cast Al–7 wt%Si alloy was subjected to processing by high-pressure torsion (HPT) at room temperature, through 1/4, 1/2, 1 and 5 turns at a pressure of 6 GPa and two rotation speeds, 0.1 and 1 rpm. Vickers microhardness was measured along diameters of HPT disk surfaces. The final hardness values were higher than in the initial as-cast condition and, unexpectedly, nearly constant under all different processing conditions, and along the disk diameter. The microstructure was characterised by optical and scanning electron microscopy. The as-cast microstructure comprises equiaxed primary α dendrite cells embedded in the Al–Si eutectic constituent. The morphology and distribution of the eutectic constituent in the HPT processed materials is used to delineate the shear strain, which was analysed in the cross-section planes of the disks. A high degree of homogeneity in the imposed shear strain throughout the samples was observed, being congruent with the ideal rigid-body torsion. In addition, the high compressive pressure applied, causing compressive strain prior to the torsional strain, is responsible for the deformation-induced precipitation of small Si particles and for the (sub)grain refinement in the primary Al constituent. The role of torsional strain is that of increasing monothonically the redistribution of the eutectic silicon and the misorientation of the (sub)grains, Financial support from MICINN (Projects MAT2009-14452 and MAT2012-38962)

The microhardness and microstructural evolution during high pressure torsion testing in samples of pure nickel were investigated. Results were obtained on Ni samples subjected to annealing prior to high pressure torsion. A detailed TEM and X ray study were used. During torsion, the final microstructure was independent of the grain size. It was demonstrated that the processing parameters of strain and applied pressure were important for microhardness refinement and increased microhardness during high pressure torsion.

In this study, superplasticity was obtained from electrodeposited nanocrystalline nickel at 350°C, which is 0.36 of the melting point T m . A maximum elongation of 895% was obtained at 0.4 T m , which is a large reduction in temperature and increase in elongation over the previously reported best result for nickel.