High-pressure Torsion Processing Research Articles

Coupling of surface and volume behaviors to understand the generation of TTS in pure magnesium proceeded by HPT

This study analyzes the formation of tribologically transformed structures (TTS) in pure magnesium (Mg) using high-pressure torsion (HPT) processing. Generally, studies conducted in such conditions do not focus on surface behaviors. The correlation between the friction and wear phenomena at the surface and the microstructural changes was investigated to supplement the knowledge on TTS formation during the first stage of rotation. An RHEOS apparatus was used to test the samples with an average grain size of 70 μm under a mean pressure of 1 GPa and a rotation speed of 0.5 rpm. The samples were conducted in an unconstrained setup at room temperature. Surface and microstructure changes were examined using optical microscopy, scanning electron microscopy (SEM), and the focused ion beam (FIB). Observation of surfaces shows that friction between the anvils and the surfaces of the sample was set to satisfy the sticking condition. Three different zones in surface contact are identified: the centre zone, the adhesion/sliding zones, and the edge zone, which generate consequently different behaviors. It was found that 0.5 turns of HPT produced a significant refinement in the grain size of the processed Mg. The TTS were considered a zone with a fine microstructure, where the initial grain size was reduced to the range of 300 nm to 1000 nm. The results show that TTS produced in these conditions are not homogenous. The deformation occurs differently, so the TTS were less or more refined. According to the different observations, a scenario of surface degradation was established. The accommodation mechanisms considered are the rupture and shear modes, which occurred, respectively, in the first material and the third body.

Ultrastrong Nanocrystalline Inconel 718 Fabricated by Powder High‐Pressure Torsion and Annealing

In this study, bulk nanocrystalline material is fabricated from Inconel 718 powder through a process of powder high‐pressure torsion (HPT) and subsequent annealing heat treatment. Numerous dislocations generated during the powder HPT process successfully produced bulk Inconel 718 with nanocrystalline grains during the post‐annealing treatment. In addition, the Inconel 718 exhibited additional strengthening contributions such as dislocation strengthening by high pre‐existing dislocation density and precipitation strengthening from randomly distributed precipitates. As a result of these strengthening effects, the Inconel 718 showed ultrastrong yield strength of 1.40 GPa and ultimate tensile strength of 1.53 GPa. Although the elongation of 12.0% did not indicate good ductility, the proposed Inconel 718 boasted improved mechanical properties compared to previous studies of Inconel 718 fabricated by various processing methods. The prepared Inconel 718 and its fabrication technique suggest the potential for manufacturing ultrahigh‐strength bulk nanocrystalline materials not only for Inconel 718 but also for various metallic materials using powder metallurgy.

Achieving high-Tc superconductivity in Magnéli phase based on Ti oxides: prediction by machine learning and material synthesis by high-pressure torsion processing

Achieving high-Tc superconductivity in Magnéli phase based on Ti oxides: prediction by machine learning and material synthesis by high-pressure torsion processing

Effects of Severe Plastic Deformation and Subsequent Annealing on Microstructures of a Ni50.6Ti49.4 Shape Memory Alloy

High-pressure torsion (HPT) was applied to the Ni50.6Ti49.4 (at. %) alloy ingot up to 1/4, 2, 16, 32 and 48 turns under a pressure of 6.0 GPa. The samples were examined by X-ray diffraction (XRD), transmission electron microscope (TEM) and microhardness measurement. The results indicate that martensitic transformation and formation of amorphous phase occurred during the HPT process. As the HPT turns increased, more amorphous phase formed. The fraction of amorphization was analyzed based on the X-ray results. The microhardness increased with the HPT turns, which may be related to strain-induced martensite transformation, formation of the amorphous phase, increased dislocation densities and grain refinement. Differential scanning calorimetry (DSC) test revealed that shape memory alloys can be produced by HPT and post-HPT annealing from a NiTi ingot.

Improving thermoelectric properties of Bi2Te3 by straining under high pressure: Experiment and DFT calculation

Bi2Te3 intermetallic compounds show high thermoelectric performance near room temperature and are widely used for cooling and power generation applications. It is still necessary to improve its dimensionless figure of merit zT for large-scale industrial use. In this study, high-pressure torsion processing is applied to Bi2Te3 powder samples to investigate the crystal structure and enhance their thermoelectric properties. The processed Bi2Te3 samples show reduced cell volume, higher electrical conductivity, enhanced Seebeck coefficient and higher zT value compared with the original powder sample, even if the grain refinement is not remarkable. First-principles calculations are also used to investigate the electronic structure and thermoelectric properties to clarify the influence of high-pressure torsion processing.

Improving density and strength-to-ductility ratio of a 3D-printed Al–Si alloy by high-pressure torsion

Good combination of strength and ductility in metallic materials is always desired. To this end, this study assesses the combination of two modern manufacturing processes, namely additive manufacturing (AM) and severe plastic deformation, for an AlSi11Cu alloy. Laser powder bed fusion (L-PBF) produced an alloy with spherical pores with an average size of 42 μm, representing a volume fraction lower than 0.15%. At the mesoscale, the alloy showed a cellular microstructure made up of Al cells and Si-rich boundaries with an average size of 0.69 µm, which were broken down by the high-pressure torsion (HPT) process into ultrafine particles smaller than 0.41 µm. The HPT process transformed the columnar grains of the as-built material into ultrafine-grained grains around the disk edges, while the central zone conserved the as-built characteristics for a number of HPT turns smaller than ¼. HPT processing at room and warm temperatures gave rise to strength–ductility improvements with yield strengths and elongations larger than 400 MPa and 10%, respectively. The good strength–ductility trade-off was related to the porosity decrease, the breakdown of the interconnected network into particles of ultrafine size, the grain size reduction due to the dislocation density increase, and the formation of precipitates and Si-rich particles of different sizes. Thus, AM and HPT improved the grain boundary and precipitation strengthening, giving rise to an Al–Si alloy with superior mechanical properties.Graphical abstract

Nanostructuring of Additively Manufactured 316L Stainless Steel Using High-Pressure Torsion Technique: An X-ray Line Profile Analysis Study.

Experiments were conducted to reveal the nanostructure evolution in additively manufactured (AMed) 316L stainless steel due to severe plastic deformation (SPD). SPD-processing was carried out using the high-pressure torsion (HPT) technique. HPT was performed on four different states of 316L: the as-built material and specimens heat-treated at 400, 800 and 1100 °C after AM-processing. The motivation for the extension of this research to the annealed states is that heat treatment is a usual step after 3D printing in order to reduce the internal stresses formed during AM-processing. The nanostructure was studied by X-ray line profile analysis (XLPA), which was completed by crystallographic texture measurements. It was found that the as-built 316L sample contained a considerable density of dislocations (1015 m-2), which decreased to about half the original density due to the heat treatments at 800 and 1100 °C. The hardness varied accordingly during annealing. Despite this difference caused by annealing, HPT processing led to a similar evolution of the microstructure by increasing the strain for the samples with and without annealing. The saturation values of the crystallite size, dislocation density and twin fault probability were about 20 nm, 3 × 1016 m-2 and 3%, respectively, while the maximum achievable hardness was ~6000 MPa. The initial <100> and <110> textures for the as-built and the annealed samples were changed to <111> due to HPT processing.

Recrystallization of bulk nanostructured magnesium alloy AZ31 after severe plastic deformation: an in situ diffraction study

The magnesium alloy AZ31, which has undergone high-pressure torsion processing, was subjected to in situ annealing microbeam synchrotron high-energy X-ray diffraction and compared to the as-received rolled sheet material that was investigated through in situ neutron diffraction. While the latter only exhibits thermal expansion and minor recovery, the nanostructured specimen displays a complex evolution, including recovery, strong recrystallization, phase transformations, and various regimes of grain growth. Nanometer-scale grain sizes, determined using Williamson–Hall analysis, exhibit seamless growth, aligning with the transition to larger grains, as assessed through the occupancy of single-grain reflections on the diffraction rings. The study uncovers strain anomalies resulting from thermal expansion, segregation of Al atoms, and the kinetics of vacancy creation and annihilation. Notably, a substantial number of excess vacancies were generated through high-pressure torsion and maintained for driving the recrystallization and forming highly activated volumes for diffusion and phase precipitation during heating. The unsystematic scatter observed in the Williamson–Hall plot indicates high dislocation densities following severe plastic deformation, which significantly decrease during recrystallization. Subsequently, dislocations reappear during grain growth, likely in response to torque gradients in larger grains. It is worth noting that the characteristics of unsystematic scatter differ for dislocations created at high and low temperatures, underscoring the strong temperature dependence of slip system activation.Graphical

High-Pressure Torsion for Highly-Strained and High-Entropy Photocatalysts

Nowadays, the environmental crisis caused by using fossil fuels and CO2 emissions has become a universal concern in people’s life. Photocatalysis is a promising clean technology to produce hydrogen fuel, convert harmful components such as CO2, and degrade pollutants like dyes in water. There are various strategies to improve the efficiency of photocatalysis so that it can be used instead of conventional methods; however, the low efficiency of the process has remained a big drawback. In recent years, high-pressure torsion (HPT), as a severe plastic deformation (SPD) method, has shown extremely high potential as an effective strategy to improve the activity of conventional photocatalysts and synthesize new and highly efficient photocatalysts. This method can successfully improve the activity by increasing the light absorbance, narrowing the bandgap, aligning the band structure, decreasing the electron-hole recombination, and accelerating the electron-hole separation by introducing large lattice strain, oxygen vacancies, nitrogen vacancies, high-pressure phases, heterojunctions, and high-entropy ceramics. This study reviews the recent findings on the improvement of the efficiency of photocatalysts by HPT processing and discusses the parameters that lead to these improvements.

The Microstructural Evolution and Corrosion Behavior of Zn-Mg Alloys and Hybrids Processed Using High-Pressure Torsion.

Zinc (Zn) alloys, particularly those incorporating magnesium (Mg), have been explored as potential bioabsorbable metals. However, there is a continued need to enhance the corrosion characteristics of Zn-Mg alloys to fulfill the requirements for biodegradable implants. This work involves a corrosion behavior comparison between severe-plastic-deformation (SPD) processed cast Zn-Mg alloys and their hybrid counterparts, having equivalent nominal compositions. The SPD processing technique used was high-pressure torsion (HPT), and the corrosion behavior was studied as a function of the number of turns (1, 5, 15) for the Zn-3Mg (wt.%) alloy and hybrid and as a function of composition (Mg contents of 3, 10, 30 wt.%) for the hybrid after 15 turns. The results indicated that HPT led to multimodal grain size distributions of ultrafine Mg-rich grains containing MgZn2 and Mg2Zn11 nanoscale intermetallics in a matrix of coarser dislocation-free Zn-rich grains. A greater number of turns resulted in greater corrosion resistance because of the formation of the intermetallic phases. The HPT hybrid was more corrosion resistant than its alloy counterpart because it tended to form the intermetallics more readily than the alloy due to the inhomogeneous conditions of the materials before the HPT processing as well as the non-equilibrium conditions imposed during the HPT processing. The HPT hybrids with greater Mg contents were less corrosion resistant because the addition of Mg led to less noble behavior.

High-pressure torsion effect on microstructural and hardness properties of Magnesium with Silicon Carbide nanoparticles

Without a doubt, lightweight materials of high strength are in high demand in the automotive, aerospace, biomedical, and other industries that require such materials. Processing or manufacturing such materials has been a vital topic in contemporary research, as well as material development in the industry. A possible solution for the processing of lightweight materials of high strength is to target lightweight materials by nature such as magnesium and improve their mechanical properties such as stiffness, strength, and hardness. The aforementioned properties are sometimes achieved by processing soft and light materials through High-Pressure Torsion. In this work, Magnesium with Silicon Carbide nanoparticles (Mg-SiC) was strengthened and hardened through the High-Pressure Torsion (HPT) processing technique. The samples were compressed with a pressure of 6.0 GPa and twisted at the rotating speed of 1 rpm with varying numbers of turns N = 0, N = 1, N = 5 and N = 10 at a temperature of 23°C. The processed samples were prepared for the experimental investigation of microstructural characterization and hardness test examinations. Microstructural results showed that grain refinements of material can be achieved through HPT processing methods, which reduced the average grain sizes of unprocessed (N = 0) Mg alloy samples from 149.9 µm to 27.1 µm after processing ten turns. However, hardness test results do not indicate any significant improvement after one HPT processing turn although homogeneity is attained at five processing turns within the nanocomposites.

Effect of High-Pressure Torsion on Phase Formation and Mechanical Properties of a High-Entropy TiZrHfMoCrCo Alloy.

This investigation delved into the alterations in the mechanical properties of a TiZrHfMoCrCo high-entropy alloy due to phase transformations induced by high-pressure torsion (HPT). The alloy's genesis involved levitation melting within an argon atmosphere, presenting two distinct states for analysis: the initial, post-manufacturing state and the state subsequent to HPT treatment. The original alloy featured a composition comprising a singular A2 phase with a bcc lattice and two Laves phases, C15 and C14. The HPT process triggered significant phase modifications: a retention of one C15 Laves phase and decomposition of the bcc phase into two distinct phases exhibiting different bcc lattice parameters. The HPT-induced effect prominently manifests as strong grain refinement. However, scanning electron microscopy (SEM) observations unveiled persistent inhomogeneities at a micron scale both before and after HPT treatment. Thus, grain refinement occurs separately within each of the bcc and Laves phases, visible in the light, dark, and gray areas in SEM images, while mixing does not occur on the scale of several microns. The examination of Ti, Cr, Co, Zr, Mo, and Hf via X-ray absorption spectroscopy (EXAFS) at specific K-edges and L3-edge revealed that the HPT treatment conserves the local atomic environment of metal atoms, albeit with a slight elevation in static disorder. Assessments through microhardness and three-point bending tests demonstrated the material's inherent hardness and brittleness. The microhardness, standing at a substantial value of 600 HV, displayed negligible augmentation post-HPT. However, the microhardness of individual phases exhibited a notable alteration, nearly doubling in magnitude.

Mechanical alloying and phase transition of immiscible Al/Zn system during high-pressure torsion

Two half-disk samples of pure Al and pure Zn were mechanically alloyed via high-pressure torsion (HPT) processing, followed by post-deformation annealing (PDA). The microstructure evolution of the Al–Zn alloy was studied by scanning electron microscopy, transmission electron microscopy and molecular dynamics (MD) simulations. The results indicated that the HPT-processed Al/Zn assembly was presented as a mixture of nanocrystalline and amorphous phases. The deformation-induced special orientation of (0001)Al//(111)Zn facilitated the interatomic diffusion, and the dislocation density reached 2.17×1017 m−2 under the pinning effect of high solid solubility. Nanocrystalline, high diffusion degree, and high local dislocation density may primarily accounted for the crystalline-to-amorphous transformation in Al–Zn alloy. Moreover, the results indicated a bimodal grain size distribution of 150–250 nm and 500–900 nm, and Zn atoms were enriched at the grain boundaries, upon subsequent PDA. Under the effect of this special heterogeneous microstructure, the prepared alloy exhibited an excellent plasticity with 160% of tensile elongation.

Study of microstructural evolution, mechanical properties and plastic deformation behavior of Mg-Gd-Y-Zn-Zr alloy prepared by high-pressure torsion

The microstructure evolution and mechanical properties of Mg-9Gd-3.8Y-1.9Zn-0.5Zr alloy after high-pressure torsion (HPT) were analyzed. It was found that the dislocation density increased and gradually reached saturation as the number of torsion turns increased. Its microhardness value increases with increasing strain. The microhardness of the alloy was increased to 128 HV after HPT for 15 turns and the grain size was refined from the initial ∼110 μm to ∼63 nm. The strength enhancement mainly originates from fine-grained strengthening, with a contribution of about 42 %. Additional contributions are provided by dislocation strengthening, second phase strengthening, and stacking faults strengthening. The plastic deformation of the material during the HPT process is coordinated by the interaction of multiple slip and {101‾1} compression deformation twinning.

Design and Analysis of High-Pressure Torsion Process Using Finite Element Method

High Pressure Torsion (HPT) is a procedure that is frequently used to change the structure and characteristics of materials by using severe plastic deformation. It is widely believed that astraightforward shear causes deformation during HPT. In order to create nanocrystalline structures in metals and alloys as well as to cold weld powders, HPT is utilized. The flow kinematics and stress condition of a material under deformation dictate the HPT treatment’s outcome. Recent studies have revealed that the actual plastic flow during HPT might deviate greatly from the straightforward concept. Based on a modification of the traditional high pressure torsion technique of severe plastic deformation, a new approach called High Pressure Torsion Extrusion (HPTE) will be proposed. A specimen will be extruded via sectional containers that will be spinning in relation to one another during HPTE. The area where the containers converge on the specimen will be exposed to shear deformation. The findings of the FEM inquiry will be presented in this report

An EBSD analysis of a commercial immiscible Cu43%Cr alloy after high-pressure torsion processing and annealing

ABSTRACT The influence of high-pressure torsion (HPT) and annealing on microstructure, texture and thermal stability of an immiscible composite Cu43%Cr alloy was studied using electron backscatter diffraction, X-ray diffraction and microhardness measurements. As-received alloy samples were subjected to HPT and subsequent annealing treatment in the range of 210–850°C for 1 h in order to develop ultrafine-grained (UFG) microstructures and highlight their thermal stability. The Cu and Cr grains were refined to ∼0.45 and ∼0.39 µm, respectively and exhibited equiaxed morphology. The crystallographic texture was of shear type in both Cu and Cr with the domination of C and F orientations, respectively. The UFG microstructure and texture were retained in the Cu43%Cr alloy up to 850°C. The global results show that the immiscible composite Cu43%Cr alloy exhibits a high thermal stability up to 850°C. The evolution of the microstructure, texture and thermal stability of the UFG Cu43%Cr alloy was compared to published data and available models.

The Microstructural Refinement of the A356 Alloy Using Semi-Solid and Severe Plastic-Deformation Processing

Improving the engineering properties of A356 alloy is an appealing option for the automotive industry. This study aimed at refining and redistributing Si particles and the eutectic phase by applying T6 heat treatment to a semi-solid A356 alloy, followed by severe plastic deformation (SPD). Using a cooling-slope technique, the as-cast and rheocast samples were subjected to heat treatment prior to being processed using equal-channel angular pressing (ECAP) and high-pressure torsion (HPT) at room temperature. The results show that the brittle Si particles were effectively fragmented and redistributed in the homogenous microstructure. The grain size reduced from 170 μm to 23 μm in the as-cast sample after combining heat treatment and the cooling-slope. This was followed by the ECAP sample after six passes through route A (where the sample is not rotated between each pass), while it reduced to 160 nm after five turns of the HPT process in a heat-treated cooling-slope sample. The hardness of the heat-treated cooling-slope casting samples increased with the ECAP process; there was an increase from 61 HV to 134 HV for the as-cast alloy after six passes through route A. The hardness of the heat-treated cooling-slope sample improved with the HPT process to 211 HV after five turns.

Effect of high-pressure torsion on first hydrogenation of Laves phase Ti0.5Zr0.5(Mn1-xFex)Cr1 (x = 0, 0.2 and 0.4) high entropy alloys

Laves phase high-entropy alloys are considered as good candidates for hydrogen storage applications. However, they usually suffer from poor first hydrogenation kinetics, the so-called activation process. In this paper, we attempt to solve the activation problem of the Ti0.5Zr0.5(Mn1-xFex)Cr1 (x = 0, 0.2 and 0.4) by high-pressure torsion (HPT). The HPT process was carried out under 6 GPa pressure for 5 revolutions in air on samples synthesized by arc melting. The hydrogenation kinetics were measured using a Sievert’s type apparatus at room temperature under 2 MPa of hydrogen pressure. While the as-cast alloys become totally inert to hydrogen after air exposure, the HPT-processed samples absorb 1.6–1.8 wt% of hydrogen at room temperature in a few seconds even after air exposure for 2 months. The easy activation of alloys processed by HPT is due to the formation of lattice defects that act as nucleation points. These results confirm that HPT processing is an effective strategy to develop active hydrogen storage materials.

Microstructural evolution and intermetallic formation in Zn-3Mg (wt%) powder mixture processed by high-pressure torsion

Severe plastic deformation (SPD) techniques have been used extensively over the past 40 years for producing strong metals and alloys. High-pressure torsion (HPT) is one of the most promising SPD techniques for achieving high strength through nanoscale grain refinement and phase transformation. In this research, a mixture of pure zinc (Zn) and magnesium (Mg) powders, Zn-3Mg (wt%), was HPT-processed under a pressure of 6 GPa for 1, 5, 10, 20, and 30 turns at room temperature to achieve a high strength biodegradable material. In order to understand the effects of pre-consolidation on the resulting microstructure and hardness, HPT processing was performed on loose powders placed in the die and also on a pre-compacted powder mixture and the resulting HPT disks were characterized by X-ray diffraction, scanning electron microscopy, atom probe tomography, and Vickers microhardness. In both cases, the HPT disk microstructures contained nanoscale grains, and stable and metastable strain-induced intermetallics, but an unusual softening appeared at large shear strains. Grain size, grain morphology, and the formation of different intermetallics were analyzed to explain the unusual hardness distribution, and it was found that an inverse Hall-Petch relationship between hardness and grain size exists. It is suggested that thermally-activated phenomena such as grain boundary sliding contributed to the strain-induced softening of this nano-structured biomaterial due to its low melting point. The current results are compared with those for HPT-processed cast alloys and hybrids of the same composition.

Using direct high-pressure torsion synthesis to produce aluminium matrix nanocomposites reinforced with carbon nanotubes

Aluminium matrix nanocomposites reinforced with carbon nanotubes were fabricated in a new way by direct synthesis using high-pressure torsion (HPT). Aluminium of 99.99 % and 99.5 % purities were used as matrix materials with carbon nanotubes in amounts of 0.5 and 1 wt% as reinforcement. The HPT processing led to extensive grain size refinement which was significantly higher than for pure metals and to a relatively uniform distribution of the fillers. The grain size of the matrix was smaller for Al99.5 compared to Al99.99 while the particle spatial distribution was more homogenous for the Al99.99 matrix. This was attributed to a lower hardness and higher plasticity of Al 99.99 alloy. The addition of carbon nanotubes also improved the thermal stability of the ultrafine-grained structure, especially if homogenously distributed as for the Al99.99 matrix nanocomposites.