High-pressure Torsion Straining Research Articles

Amorphous-crystalline Al-based laminates with enhanced plasticity produced by high pressure torsion

The consolidated disks consisted of two melt-spun Al86Ni9Gd5 amorphous ribbons inserted between two-layered stacks of Al foils were obtained by using unconstrained high-pressure torsion straining. Structure and mechanical properties of the Al-covered samples were studied by scanning electron microscopy, three-point bending testing and microhardness measurements and compared with those of consolidated disks consisted of two amorphous ribbons and of 12 layers of Al foils. It was established that the Al-covered disks had superior plasticity (3.6%) and intermediate average microhardness (1.95 GPa) compared with those of the uncovered and Al consolidated disks. The mixing of the Al and amorphous alloy was directly observed in the Al-covered disks which resulted in ductilisation and large variations of microhardness on surface and inside of the samples.

Promotion Effect of Hydrogen on Grain Refinement in Pure Fe by High-Pressure Torsion-Straining

Promotion Effect of Hydrogen on Grain Refinement in Pure Fe by High-Pressure Torsion-Straining

Role of higher-order effects in spin-misalignment small-angle neutron scattering of high-pressure torsion nickel

Magnetic-field-dependent unpolarized small-angle neutron scattering (SANS) experiments demonstrate that high-pressure torsion (HPT) straining induces spin misalignments in pure Ni, which persist in magnetic fields up to 4 T. The spin-misalignment scattering patterns are elongated perpendicular to the applied magnetic field due to an unusual predominant longitudinal $sin^2(\theta)$-type angular anisotropy. Such a contribution cannot be explained by the conventional second order (in spin misalignment amplitude) micromagnetic SANS theory in the approach-to-saturation regime, nor can its magnitude relative to the other features of the cross sections by the third order micromagnetic SANS theory. This indicates that the high-density of crystal defects induced via HPT straining in Ni makes such higher-order effects in the micromagnetic SANS cross sections observable.

Anomalous magnetic anisotropy and magnetic nanostructure in pure Fe induced by high-pressure torsion straining

This paper shows the formation of nanosized spin misalignment in pure Fe processed via high-pressure torsion straining.

Novel black bismuth oxide (Bi2O3) with enhanced photocurrent generation, produced by high-pressure torsion straining

Bismuth oxide (Bi2O3), a yellow semiconductor ceramic, is considered as an advanced photoactive energy material, but its activity still needs further improvement for practical applications. In this study, black Bi2O3 with nanocrystalline structure and large fraction of oxygen vacancies is synthesized by mechanical straining via the high-pressure torsion (HPT) method. The black oxide exhibits enhanced light absorbance under both UV and visible lights. It shows a bipolar photocurrent behavior with up to six times higher photocurrent density compared with the unprocessed yellow Bi2O3. This study shows the high potential of severely-strained black Bi2O3 for photovoltaic, photoconductivity and photocatalytic applications.

INDENTATION SIZE EFFECT IN HIGH PRESSURE TORSION PROCESSED HIGH ENTROPY ALLOY

High entropy alloy HfNbTaTiZr in as cast conditions and after high pressure torsion straining was characterized by nanoindentation. The length-scale dependent material response (indentation size effect) was characterized by indentation at various indentation depths. Hardness dependence on the characteristic length (depth of penetration) indicated decomposition of disordered high entropy alloy in the as cast sample, which probably occurred during slow cooling after casting. Subsequent severe plastic deformation by high pressure torsion led on the other hand to the short-range disorder of (originally partially decomposed as cast) structure. Further hardening was generated during high pressure torsion by the mechanisms of grain refinement and increasing dislocation density.

FCC phase formation in immiscible Mg–Hf (magnesium–hafnium) system by high-pressure torsion

Magnesium and hafnium, two hydride-forming and biocompatible metals with hexagonal close-packed crystal structures, are thermodynamically immiscible even in the liquid form. In this study, these two elements were mechanically mixed by high-pressure torsion straining, and a new FCC (face-centered cubic) phase was formed although these two elements do not form the FCC phase even under high pressure or at high temperature. Microstructural examination by scanning-transmission electron microscopy combined with an ASTAR automatic crystal orientation and phase mapping technique confirmed that the FCC phase was stabilized mainly in the Hf-rich nanograins with localized supersaturation. Attempts to control the phase transformations under a hydrogen atmosphere to produce ternary magnesium–hafnium hydrides for hydrogen storage applications were unsuccessful; however, the material exhibited enhanced hardness to an acceptable level for some biomedical applications.

Tuneable Magneto-Resistance by Severe Plastic Deformation

Bulk metallic samples were synthesized from different binary powder mixtures consisting of elemental Cu, Co, and Fe using severe plastic deformation. Small particles of the ferromagnetic phase originate in the conductive Cu phase, either by incomplete dissolution or by segregation phenomena during the deformation process. These small particles are known to give rise to granular giant magneto-resistance. Taking advantage of the simple production process, it is possible to perform a systematic study on the influence of processing parameters and material compositions on the magneto-resistance. Furthermore, it is feasible to tune the magneto-resistive behavior as a function of the specimens’ chemical composition. It was found that specimens of low ferromagnetic content show an almost isotropic drop in resistance in a magnetic field. With increasing ferromagnetic content, percolating ferromagnetic phases cause an anisotropy of the magneto-resistance. By changing the parameters of the high pressure torsion process, i.e., sample size, deformation temperature, and strain rate, it is possible to tailor the magnitude of giant magneto-resistance. A decrease in room temperature resistivity of ~3.5% was found for a bulk specimen containing an approximately equiatomic fraction of Co and Cu.

Photocatalytic activity of aluminum oxide by oxygen vacancy generation using high-pressure torsion straining

Alumina is an insulator oxide with a large abundance in the earth's crust, but it shows no photocatalytic activity due to its large bandgap (9 eV). Based on first-principles calculations, the Al2O3 bandgap can be reduced below 3 eV, if large oxygen vacancies are generated. Here, Al2O3 with large oxygen vacancy concentrations and ~2.5 eV optical bandgap is produced by severe plastic deformation via the high-pressure torsion (HPT) method. The material exhibits photocatalytic dye degradation with a reasonable rate compared with TiO2 photocatalysis. This finding introduces a simple but effective approach to produce new nature-friendly photocatalysts even from insulator oxides.

Effect of High‐Pressure Torsion on Hardness and Electrical Resistivity of Commercially Pure Cu

Commercially pure Cu is processed through high‐pressure torsion (HPT) at 6 GPa up to 50 turns and stored under the ambient condition for 21 months. Subsequently, microhardness and electrical resistivity of the Cu samples are measured and correlated with the microstructure. Grain size monotonically decreases with the number of HPT turns and becomes saturated at ≈300 nm after an equivalent strain of ≈40. Considerable fractions of low‐angle grain and twin boundaries are observed in the samples processed through HPT to low strains; however, their fractions decrease with increasing HPT turns. Consistently, although the hardness of the processed samples is greater than that of the annealed coarse‐grained sample, it first decreases with HPT straining and subsequently increases to a saturated value of ≈160 HV. In contrast, the electrical resistivity of Cu first increases with HPT processing, then decreases and finally becomes saturated at a value of ≈22 nΩ m, which is slightly higher than the least value obtained in this study. The obtained results are compared with the literature on HPT processing of Cu, and the usefulness of the ambient ageing for optimizing the hardness‐conductivity relationship is discussed.

Miniature tensile testing of SPD processed fine-grained aluminum

Manufacturing of ultrafine-grained (UFG) or nanocrystalline (NC) metals via a single step top-down approach imposing severe plastic deformation (SPD) is one of the most promising ways to achieve superior properties such as high strength and superplastic forming capability. Nonetheless, the lack of relevant data on their post-SPD performance in different test environments makes it difficult to fully understand their mechanical behavior. While characterizing the tensile behavior, almost all of the previous reports are limited to the discussion on the plastic performance of the material in terms of elongation to failure and corresponding strength, with only a few studies discussing the effect of grain fragmentation on work hardening response of the material. In the present work, a comprehensive analysis is presented in terms of the uniform and post-necking mechanical behavior of the ultrafine-grained material. Commercially pure aluminum is subjected to high pressure torsion (HPT) deformation with strains ranging from very low levels (γ ≈ 2.1) to values well in the saturation regime (γ ≈ 25.1). When tested in uniaxial tension, the strength increases monotonously. The uniform elongation improves with the imposed HPT strain, though remains lower than the value of the initial material. Based on the slopes of the stress-strain curve, three distinct zones are identified, i.e. uniform deformation, post-necking-1, and post-necking-2. With accumulating SPD deformation, the material shows enhanced pre-necking strength and ductility; while post-necking material fails early and at lower strength levels. The post-necking response is observed to be highly microstructure dependent: a lower grain size augments the resistance for micro-crack propagation and thus the ductility, however, once initiated, a crack propagates much faster in fine-grained than in coarse-grained HPT processed material.

Low-temperature anatase-to-rutile phase transformation and unusual grain coarsening in titanium oxide nanopowders by high-pressure torsion straining

Simultaneous effect of pressure, temperature and plastic strain on structural evolution of titania ceramic nanopowder was investigated using the high-pressure torsion (HPT) method. Unlike static annealing, in which the anatase-to-rutile phase transformation occurred at 1173 K, the transformation was observed at low temperatures as 473–573 K by HPT processing. The formation of rutile phase accompanied with dynamic recrystallization and an unusual grain growth, indicating that severe plastic deformation at low homologous temperatures does not necessarily result in grain refinement of metastable nanoceramics. Besides the coarsened grains of rutile phase, a high-pressure TiO2-II columbite phase was also detected in small nanograins.

Microstructure and phase transformations of silica glass and vanadium oxide by severe plastic deformation via high-pressure torsion straining

Plastic deformation of oxides is generally difficult due to their hard and brittle nature resulting from their covalent or ionic bondings. In this study, severe plastic deformation was successfully introduced in SiO2 glass and crystalline VO2 by high-pressure torsion (HPT). SiO2 glass transformed to a denser amorphous phase without showing any crystallization after processing in the temperature range of 300–723 K. Crystalline VO2 transformed from the coarse-grained monoclinic phase to a nanograined triclinic phase with insulating electrical properties by plastic strain effect. These findings indicate the potential of HPT process in tuning the crystal structure of optical and electrical oxides.

Evolution of long-period stacking ordered structure and hardness of Mg-8.2Gd-3.8Y-1.0Zn-0.4Zr alloy during processing by high pressure torsion

High pressure torsion (HPT) was performed at room temperature on a Mg-8.2Gd-3.8Y-1.0Zn-0.4Zr (wt%) alloy containing long period stacking ordered (LPSO) phase with a 6.0 GPa pressure. The microstructure evolution and hardening mechanisms were analyzed. TEM shows that, with increasing HPT strain, the LPSO lamellar-shaped and block-shaped particles experience kink bending, fragmentation and dissolution; and eventually a supersaturated solid solution with nanosized grains is obtained. The decomposition of LPSO phase at room temperature is attributed to the high defect concentrations generated in the LPSO lamellae and blocks, and the Mg-rich phase. With equivalent strains increasing to ~6.6 (16 HPT revolutions), an exceptional grain refinement to 52 ± 2 nm is achieved, and the hardness is enhanced to 128 ± 2 HV. A quantitative model shows the hardness increase is due to the combined effects of nanosized grains, high dislocation density and dissolved alloying elements. XRD line broadening analysis, thermodynamic modelling software and elemental mapping are used to support the mechanistic interpretations.

Mechanical properties and microstructure of magnesium alloy Mg22Gd processed by severe plastic deformation

Mg22Gd alloy was processed by high pressure torsion (HPT) at room temperature and the pressure of 2 GPa. A series of specimens with different number of rotations N (N = 0-15) was prepared from the initial coarse grained as cast material. Mechanical properties were investigated by microhardness mapping. The microhardness was found to increase with increasing strain imposed by HPT and tend to saturate at about HV = 145. The microstructure (phase morphology and composition, etc.) evolution with strain was investigated by scanning electron microscopy and EDS. High Gd content in the alloy resulted in the precipitation of stable Mg5Gd phase. This phase exhibited apparently higher hardness than the magnesium matrix. During straining the phase was continuously fragmented and only tiny particles were found in heavily strained material. Electron back scatter diffraction (EBSD) and automated crystallographic orientation mapping in transmission electron (ACOM-TEM) were employed to characterize the fragmentation of the grain structure. HPT was found to result in strong grain refinement by the factor of approximately 1000. The dislocation density was determined by positron annihilation spectroscopy. Significant twinning was found in the initial stages of HPT straining. At high strains twin formation was suppressed and only dislocation storage in the material occurs.

Concurrent precipitation and associated texture evolution in AA 6082 alloy during high pressure torsion (HPT) processing

The effect of severe plastic straining by HPT processing on the mechanical properties and the microstructure evolution of a commercial AA 6082 alloy was studied. The HPT processing was carried out by torsion of 0.5 rotation per minute under 2GPa load, for up to 5 turns. The material was solutionized before HPT. Mechanical homogeneity is achieved after 3 turns with a saturation hardness of ~ 165HV and further increased to ~ 175HV after 5 turns, while the alloy showed ~ 68HV before HPT straining. The increase in hardness during HPT has been attributed to concurrent precipitation during HPT. The development of an ultrafine grained (UFG) microstructure was analyzed by investigating samples prepared from both the centre and the edge of the HPT discs after 1, 3 and 5 turns using TEM. In case of 1 turn, subgrain formation occurred at the edges of the disc while at the centre dislocation cell structure formation was observed. Subgrains are formed at the centre and the edge after 3 HPT turns with an average diameter of ~ 300nm that further decreased to ~ 200nm after 5 HPT turns. Texture evolution after 1 turn consists of formation of A*2, C, A and A- components. However, with increasing straining, these components remain stable and no new component arises. This has been attributed to continuous dynamic recrystallisation as the dominating mechanism of grain refinement that can occur in this alloy under the given severe plastic straining.

Superplasticity of nanostructured Ti-6Al-7Nb alloy with equiaxed and lamellar initial microstructures processed by High-Pressure Torsion

Microstructural modifications of a biomedical Ti-6Al-7Nb alloy were accomplished via heat treatment in 3 different quenching mediums and then processed by High-Pressure Torsion (HPT) at room temperature. The microstructure of the as-received condition is composed of an equiaxed duplex (α+β) structure. After the heat treatment, a combination of primary α phase and lamellar structures was obtained with an increasing fraction of the martensitic lamellar with increasing cooling rate. After HPT processing, refinement of the microstructures was observed for N=5 revolutions. Transmission electron microscopy (TEM) of the sample quenched in liquid nitrogen confirmed the nanostructure with grain sizes below 100 nm and high density of lattice defects after HPT processing for N=5 revolutions. High-temperature tensile tests were carried out at 800 °C with an initial strain rate of 2×10−3 s−1 on specimens with different combinations of heat treatment and HPT straining. The test in the as-received condition presented a maximum elongation to failure of ~400% after HPT processing for N=5 revolutions. The highest elongation to failure in the heat-treated samples was ~580% in the sample quenched in liquid nitrogen and processed for N=5 revolutions.

Microstructure and Mechanical Properties of Co21Cr22Cu22Fe21Ni14 Processed by High Pressure Torsion and Annealing

The strengthening mechanisms of Co21Cr22Cu22Fe21Ni14 multiple-principal element alloy processed by high pressure torsion (HPT) and annealing were examined. Two face-centered cubic (FCC) phases were observed in the as-cast alloy; one was a Cu-rich phase and the other was a Cu-lean one. In the HPT process, the microhardness increased from 190 HV to 470 HV at a strain of 157 due to strain hardening and grain refinement hardening. X-ray diffraction showed that the lattice parameters of the two FCC phases became closer to each other at higher HPT strain, indicating the alloying of Cu into the Cu-lean matrix. The HPT processed specimens were annealed at 500°C, 550°C, 600°C, and 650°C. The microhardness increased to 540 HV after annealing at temperatures lower than 650°C, whereas it decreased when the specimen was annealed at 650°C. The mean grain size of the specimens annealed at temperatures lower than 650°C was much smaller than 100 nm, and Cu-rich clusters with sizes ranging from 2 nm to 32 nm were distributed homogeneously. The reasons for the formation of the Cu-rich nano-clusters were discussed from a perspective of the positive mixing enthalpy of Cu in the alloy and thermalenergy for Cu diffusion at a given temperature. The dissolution and partitioning of two FCC phases played a key role in strengthening the Co21Cr22Cu22Fe21Ni14 system.

Sliding wear behavior of sub-microcrystalline pure iron produced by high-pressure torsion straining

Sliding wear behavior of sub-microcrystalline pure iron disks produced by high-pressure torsion (HPT) straining was studied using a ball-on-disk configuration, and the effects of the strain in the HPT process and of the duration of wear tests were investigated. It was found that the wear amount decreased with increasing number of turns during HPT, and that the specific wear rate of the initial stage was inversely proportional to the Vickers hardness of the disks. However, for long wear test times, the wear rate became very small for both non-deformed and HPT-processed specimens, because of transition from adhesive shearing wear to sliding process which was caused by (i) grain refinement at the wear-induced layer, (ii) submicron wear particles adhered to the worn surface, and (iii) surface flattening by local asperity deformation. There was no significant difference in microstructure and hardness beneath the worn surface between non-deformed and HPT-processed specimens after the wear tests. Thus HPT improved the wear resistance only at the early stage, and became less effective for long wear tests. This study is the first to show that submicron grains produced by HPT straining were further refined to a size of 100–150nm by friction shear straining while grain refinement of iron is saturated upon further straining in HPT processing.

Evolution of deformation texture of high-pressure ω-phases in pure Ti and Zr during high-pressure torsion straining

High-pressure torsion process is the only way to obtain bulk-shaped samples of nearly 100% of high-pressure ω-phase of pure Ti and Zr at ambient condition. The neutron diffraction experiment of HPT-processed pure Ti and Zr revealed that the deformation of high-pressure ω-phase mainly occurs at the prismatic plane, which is completely different as α-phase. This paper also reports the correlation between deformation textures and mechanical property of ω-phase obtained by HPT process.