Turns Of High-pressure Torsion Research Articles

Effect of high-pressure torsion on the hydrogen evolution performances of a melt-spun amorphous Fe73.5Si13.5B9Cu1Nb3 alloy

In this work, a melt-spun amorphous Fe73.5Si13.5B9Cu1Nb3 alloy is subjected to high-pressure torsion (HPT) treatment under 6 GPa in order to enhance the alkaline hydrogen evolution reaction (HER) performance. Different HPT turns, including 1, 2, 5 and 10, are selected and the samples are marked as HPT-1N, HPT-2N, HPT-5N and HPT-10 N, respectively. The HPT treatment can lead to a significant structural change such as partial crystallization and defect generation on the Fe-based amorphous alloy with a considerable increase in the HER electrocatalytic activity. The HPT-5N sample shows the best catalytic performance, with an overpotential of only 174 mV at 10 mA cm−2 current density in 1 M KOH, which is 244 mV lower than that of the melt-spun alloy at the same condition. Moreover, it was established that the HPT-treated amorphous alloy shows good stability during the HER cycling process.

Nano-indentation nanohardness and elastic modulus evolution of molybdenum processed by high-pressure torsion

In this paper, high-pressure torsion (HPT) was conducted on pure molybdenum (Mo) (99.9 wt.%) under the 6 GPa pressure at room temperature with an angular velocity of 1 rev min−1. The samples processed by 1, 2, 5 turns of HPT were obtained. After 5 revolutions of HPT, the grains were significantly refined to 0.77 ± 0.17 µm and the main textures are {012}<121> and {001}<110>. The dislocation density of the sample after 2 turns of HPT deformation reaches to the maximum value. The elastic modulus of pure molybdenum after 5 turns of HPT has a great decrease. It is found that texture structures, dislocations and non-equilibrium grain boundaries can influence the elastic modulus evolution, and the latter two are the main factors.

The effect of high-pressure torsion on the microstructure and outstanding pseudoelasticity of a ternary Fe–Ni–Mn shape memory alloy

Abstract Experiments were conducted to examine the effect of high-pressure torsion (HPT) processing on the microstructure and pseudoelastic behavior of a ternary Fe–10Ni–7Mn (wt.%) shape memory alloy in both the solution-annealed and intercritically-annealed conditions. X-ray diffraction (XRD) patterns and electron backscatter diffraction (EBSD) analyses showed that the initial microstructure of the alloy in the solution-annealed condition was a fully lath α′-martensite which partially transformed to a strain-induced austenite (α′→γ) by HPT processing. Also, the austenite formed in the dual phase (α′+γ) specimens after intercritical annealing treatment at 600 °C for 7.2 ks underwent a γ→e→α′ transformation during subsequent HPT processing such that a multi-phase microstructure was formed consisting of α′-martensite, austenite and e-martensite. The HPT processing led to a significant increase in the microhardness value to ~690 Hv due to a high density of dislocations and the associated grain refinement of the microstructure. Cyclic loading-unloading tensile tests at room temperature revealed a strain hysteresis and pseudoelastic behavior in the HPT-processed specimens with different initial microstructures. Outstanding pseudoelasticity values of about 67% and 75% were obtained at the fourteenth loading-unloading cycle after 20 HPT turns in the solution-annealed and intercritically-annealed specimens, respectively.

A stored energy analysis of grains with shear texture orientations in Cu-Ni-Si and Fe-Ni alloys processed by high-pressure torsion

Experiments were conducted to evaluate the evolution of the stored energy in grains with shear texture orientations A1* {111}<1̅1̅2>, A2* {111}<12̅1>, A {111}<11̅0>, A̅ {111}<01̅1>, B {112}<11̅0>, B̅ {112}<11̅0>and C {100}<110>for Cu-2.5Ni-0.6Si and Fe-36Ni (wt%) alloys after high-pressure torsion (HPT) processing up to 10 turns at ambient temperature using a Kernel Average Misorientation (KAM) approach. A typical stable shear texture developed in the Cu-2.5Ni-0.6Si alloy immediately after 1 turn whereas there was a continuous transformation of texture in the Fe-36Ni alloy up to 10 turns. The results show that HPT processing produces similar stored energies of ~35 J/mol and ~24 J/mol but with different shear texture components for the Cu-2.5Ni-0.6Si and the Fe-36Ni alloy, respectively. The stored energy in all shear components for the Cu-2.5Ni-0.6Si alloy increases with increasing HPT processing up to 1 turn and then slightly decreases through 10 turns. By contrast, the stored energy of the Fe-36Ni alloy continuously decreases with increasing numbers of HPT turns. These evolutions are examined with reference to the initial textures, dynamic recrystallization, grain refinement mechanisms and differences in the stacking fault energies.

Phase evolution and mechanical properties of an intercritically-annealed Fe–10Ni–7Mn (wt. %) martensitic steel severely deformed by high-pressure torsion

Experiments were conducted on an Fe–10Ni–7Mn (wt. %) martensitic steel processed by high-pressure torsion (HPT) after intercritical annealing in the dual phase (α+γ) region at 600 °C for 2 h. Microstructural examination and phase analysis using electron backscatter diffraction (EBSD) confirmed that post-HPT processing led to a phase transformation of austenite to α′-martensite (γ→α′ and γ→ε→α′) such that after 20 turns of HPT the microstructure included multi-phases of α′-martensite, austenite and ε-martensite. The HPT processing also produced a significant reduction in the α′-martensite and austenite grain sizes from initial values of ~3.8 and ~3.9 μm to ~225 and ~60 nm after 20 HPT turns, respectively, while the ε-martensite final grain size was ~44 nm. Micro-hardness measurements and tensile testing showed a significant increase in hardness and in the ultimate tensile strength to about 680 Hv and 2235 MPa after 20 turns, respectively. Finally, the fracture surfaces of samples after HPT processing showed a combination of dimples, cracks and shear bands which confirmed a transition in fracture behavior from ductile to a mixture of a ductile and brittle mode.

Effect of severe plastic deformations on structure features and mechanical behavior of Al4Ca intermetallic in Al-18% Ca alloy

The influence of severe plastic deformation by high-pressure torsion (HPT) on the phase composition, structure, and properties of Al-18% Ca alloy containing the Al4Ca intermetallic was studied. It was established that even after 5 turns of HPT, the additional hardening of the alloy does not exceed 30–40 HV, and the Al4Ca intermetallic is noticeably plastically deformed, which is clearly seen from the change in the microstructure; its fine structure after 5 turns of HPT is polygonized with a large number of defects in the crystal structure, the size of subgrains is predominantly 10–50 nm. Using X-ray phase analysis, it was found that calcium does not dissolve in aluminum solid solution even after 5 turns of HPT.

Influence of high-pressure torsion on microstructure, hardness and shear strength of AM60 magnesium alloy

The evolution of the microstructure, crystallographic texture and hardness of an AM60 magnesium alloy was studied in the center and the edge parts of disks processed by high-pressure torsion (HPT) technique at room temperature. In addition, the mechanical properties were also tested by shear punch test (SPT). The number of HPT turns varied between ½ and 10. It was found that the grain size of the initial extruded material (~16 μm) decreased to ~2 and ~0.8 μm at the disk center and edge, respectively, even after ½ turn of HPT. Ten turns of HPT resulted in further grain refinement to ~1 and ~0.23 μm at the disk center and edge, respectively. The dislocation density saturated even after ½ turn with the value of ~11 × 1014 m−2. The maximum value of the hardness was ~1300 MPa that was measured at the edge of the disk deformed for ½ turn. Despite the practically unchanged grain size and dislocation density, the hardness decreased between 3 and 10 turns which can be explained by texture softening. A good agreement was observed between the yield strength estimated as one-third of the hardness and the values calculated from the dislocation density, grain size and texture. The difference between the microstructure and hardness obtained for AM60 samples processed by HPT and equal channel angular pressing (ECAP) is discussed.

Texture evolution and deformation - induced amorphization in high pressure torsion of W

Severe plastic deformation is a convenient method to prepare amorphous materials directly. To study the mechanism and critical conditions of deformation-induced amorphization, high pressure torsion (HPT) experiments of 1, 2, 5 and 10 turns subjected to sintered W were performed under 823 K at 1.5 GPa. The microstructures of sintered and HPT-processed W were observed by electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). In HPT, the angle between the (110) crystal plane of grains and the tangential direction decreased with more turns of HPT and the Schmid factor value of the texture with (110) was the biggest during HPT. It is implied that the slip systems in {110} were the easiest to be activated. Therefore, the number of dislocations in (110) crystal plane was the largest. Likewise, the dislocations on the grain boundaries increased with more turns of HPT. It is concluded that the grains with (110) orientation were more likely to transform into the amorphous structure and grain boundaries were the first area to occur deformation-induced amorphization. Deformation-induced amorphization mainly depends on three factors: temperature, local dislocation density and grain size. The critical local dislocation density is ∼6 × 1016 m−2 and the critical grain size is ∼62.4 nm for W in HPT under 823 K.

Fabrication and characterization of nanostructured immiscible Cu–Ta alloys processed by high-pressure torsion

Nanostructured Cu–Ta alloys show great potential as high strength nanocrystalline materials due to their excellent mechanical properties and limited grain growth at high temperatures. This report describes the fabrication of nanostructured immiscible Cu–Ta alloys in bulk by high-pressure torsion (HPT) using a stack of Cu/Ta/Cu discs at room temperature. A microstructural study after HPT processing showed that the internal Ta layer breaks into small individual flakes which distribute uniformly over the Cu matrix through increases in the numbers of HPT turns. There is solid-state diffusion between the Cu and Ta when the HPT processing increases to 100 turns due to microstructural refinement and increasing crystalline defects. After processing through 150 turns, a composite microstructure of two phases is formed including supersaturated Cu–Ta solid solutions (Cu 81 Ta 19 and Ta 78 Cu 22 alloys) with a crystallite size of ∼35–45 nm. This fine microstructure produces exceptional mechanical properties including a high hardness of over 350 Hv corresponding to ∼3.43 GPa, a tensile strength of ∼1300 MPa and a tensile elongation of about 40%. • Immiscible Cu–Ta alloys successfully fabricated from Cu/Ta/Cu stacked discs by HPT. • The microstructures contain Cu-rich and Ta-rich layers, and Ta-rich clusters. • The HPT-processed Cu–Ta alloy has excellent tensile strength and elongation.

Interface structures in Al-Nb2O5 nanocomposites processed by high-pressure torsion at room temperature

Extremely thin Nb2O5 nanowires and Al powder were successfully consolidated at room temperature by using high-pressure torsion (HPT), producing a novel metal matrix nanocomposite with exceptional mechanical properties. It is shown that minor additions of Nb2O5 increase sharply the hardness of commercially pure Al. For instance, hardness of over 180 Hv was developed at the edge of samples with 10% nanowires and processed through 10 turns of HPT. This is markedly higher than any other value reported for pure aluminum matrix composites having this level of reinforcement phase. A detailed characterization of the interface structure using high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) revealed a pronounced grain refinement of the Al matrix at the nanoscale and the occurrence of the aluminothermic reduction of the part of Nb2O5. The latter led to: (i) the formation of Al2O3 nanolayer at the Al/Nb2O5 interface and (ii) the nanosegregation of metallic Nb (with few atomic layers) along grain boundaries and dislocations. The pronounced increase in hardness is attributed to the formation of this interface nanostructure.

Recrystallization in an Mg-Nd alloy processed by high-pressure torsion: a calorimetric analysis

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.

Magnesium-Based Bioactive Composites Processed at Room Temperature.

Hydroxyapatite and bioactive glass particles were added to pure magnesium and an AZ91 magnesium alloy and then consolidated into disc-shaped samples at room temperature using high-pressure torsion (HPT). The bioactive particles appeared well-dispersed in the metal matrix after multiple turns of HPT. Full consolidation was attained using pure magnesium, but the center of the AZ91 disc failed to fully consolidate even after 50 turns. The magnesium-hydroxyapatite composite displayed an ultimate tensile strength above 150 MPa, high cell viability, and a decreasing rate of corrosion during immersion in Hank’s solution. The composites produced with bioactive glass particles exhibited the formation of calcium phosphate after 2 h of immersion in Hank’s solution and there was rapid corrosion in these materials.

Effect of Numbers of Turns of High‐Pressure Torsion on the Development of Exceptional Ductility in Pure Magnesium

The low ductility of magnesium at room temperature is usually attributed to an insufficient number of independent slip systems. Recent research has shown that refining the grain structure of pure magnesium promotes a breakdown in the Hall‐Petch relationship at low strain rates and may lead to the development of exceptional ductilities. This report describes the evolution of microstructure and the mechanical behaviour of pure magnesium using different amounts of imposed plastic deformation by high‐pressure torsion (HPT). It is shown that the initial coarse grains undergo twinning followed by a gradual grain refinement. The flow stress at low strain rates decreases as the grain size is reduced, thereby confirming an inverse Hall‐Petch behaviour. The elongation to failure increases with grain refinement and elongations over 300% are achieved after 1/2 turn of HPT. These experimental data agree with a model for the low temperature deformation behaviour of fine‐grained pure magnesium.

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.

Electrochemical behavior of a magnesium ZK60 alloy processed by high-pressure torsion

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

Phase Transformations in MgH2–TiH2 Hydrogen Storage System by High‐Pressure Torsion Process

Magnesium hydride (MgH2) and titanium hydride (TiH2) are two potential candidates for solid‐state hydrogen storage, but strong hydride formation energy in these hydrides undesirably results in their high dehydrogenation temperature. First‐principles calculations show that the metastable hydrides in the MgH2–TiH2 system have low hydrogen binding energy, which makes them more appropriate for low‐temperature hydrogen storage. In this study, severe plastic deformation (SPD) via the high‐pressure torsion (HPT) method is applied to the MgH2–TiH2 system to synthesize metastable hydrides. While MgH2 transforms to a high‐pressure orthorhombic γ phase, TiH2 does not exhibit any cubic‐to‐tetragonal phase transformation even by HPT processing at cryogenic temperature. Application of large strains by 400 HPT turns to the immiscible MgH2/TiH2 composite results in atomic‐scale mixing and formation of nanostructured ternary Mg–Ti–H hydride with the metastable FCC structure and lower dehydrogenation temperature than TiH2.

On the microstructure and mechanical properties of an Fe-10Ni-7Mn martensitic steel processed by high-pressure torsion

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

Processing of CP-Ti by high-pressure torsion and the effect of surface modification using a post-HPT laser treatment

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

Evaluating the textural and mechanical properties of an Mg-Dy alloy processed by high-pressure torsion

Samples of an Mg-0.41Dy (wt. %) alloy were severely deformed by high-pressure torsion (HPT) at room temperature up to 15 turns and the texture, microstructure and microhardness values in the centres, mid-radial points and edges of the HPT-deformed discs were investigated using X-ray diffraction, Electron BackScatter Diffraction and Vickers microhardness measurements. The textures in the centres of discs were characterized by a typical weak basal fiber whereas at both the mid-points and edges of the discs there was a strong basal texture where the c-axis of most grains was shifted 15° away from the shear direction. An almost homogeneous ultrafine-grained structure with a grain size of about 0.75 μm was achieved after 15 HPT turns. The microhardness values in these three positions increased with increasing numbers of turns, reached a maximum and then decreased to a lower steady-state level at large strains. In addition to dislocation and grain size hardening, the results show that texture strengthening contributes significantly to the rapid increase in hardening in the early stages of deformation.

Correlation between the thermal responses and microstructure patterns in aluminum‐based silicon carbide composites (SiCp/Al) consolidated by different high pressure torsion schemes

AbstractHigh pressure torsion of powder mixture is a novel strategy for manufacturing high‐quality aluminum‐based silicon carbide composite SiCp/Al billet used in electronic packaging, however, the correlation between thermal properties and the high pressure torsion process and the related mechanisms are still unclear. To this end, the variation rules of thermal expansion coefficient and thermal conductivity with high pressure torsion turns are studied and the corresponding microstructure morphology and dislocation patterns are observed by optical and transmission electron microscopes. The results show that thermal expansion coefficient decreases monotonically but thermal conductivity first increases then decreases with the increase of high pressure torsion turns. Moreover, the results of differential scanning calorimetry experiments indicate the decrease of recrystallization temperature with turns of high pressure torsion. The non‐monotonic variation of thermal conductivity is attributed to the combined effect of the improved relative mass density brought by the consolidation effect and the dislocation tanglement and pattern transformation (formation of in‐situ sub‐grains) during high pressure torsion process.