High-pressure Torsion Research Articles

Deformation mechanism and microstructure evolution during superplastic deformation of friction stir processed CoCrFeNiMn high entropy alloy

Since superplastic formation offers a significant advantage in preparing the complex structure parts of HEAs, the study of their superplastic behavior is extremely important. Using friction stir processing (FSP) with an improved convex hemispherical shape tool, this study reported for the first time an equiaxed ultrafine-grained CoCrFeNiMn HEA (489 nm) with a maximum elongation of 870% at 675 °C and 3 × 10−4 s−1. This superplastic property is much larger than ever reported in the CoCrFeNiMn HEA, which is even larger than that of the nano-sized CoCrFeNiMn HEA (10 nm) prepared by the high-pressure torsion. A high proportion of high angle grain boundaries and twin boundaries, a hard Cr-rich phase, and the sluggish diffusion effect of the CoCrFeNiMn HEA were primarily responsible for the exceptional superplasticity. Additionally, this study provides the first elucidation of the mechanism underlying Cr precipitation and Cr-rich phase growth, as well as the function of low-energy annealed twins during superplastic deformation. The diffusion of Cr along dislocations and grain boundaries facilitated grain boundary sliding as the predominant deformation mechanism. This study elucidates the superplastic deformation mechanism and microstructure evolution, thereby furnishing theoretical guidance for the practical implementation of superplastic forming of complex components of HEAs. Additionally, it presents an efficient method for fabricating such components.

Recent Developments in the Use of High Pressures for the Production of Nanostructured Materials

The use of pressure to achieve superior properties in metals, especially for use in a range of weaponry, has a long history dating back to the artisans of ancient China and many parts of Asia. Nevertheless, scientific principles were not introduced until the classic experiments conducted by Nobel Laureate Professor Percy Bridgman at Harvard University almost 100 years ago and these experiments led directly to the development of metal processing through the procedure now known as high‐pressure torsion (HPT). This review provides a brief overview of the historical evolution of pressure as a convenient tool in metal‐working and then summarizes the major features associated with the processing of metals by HPT and, more recently, the torsional straining of stacked disks in HPT to produce a range of hybrid materials. Finally, there is a brief report on the development of the relatively new processing procedure of tube high‐pressure shearing (t‐HPS) which may be used for the fabrication of metal matrix composites.

Shear straining promoted structural transition in bulk metallic glasses

Metallic glasses (MGs) have unusual structural characteristics and resulting unique properties, but effectively modulating their structure is difficult. The recently achieved two-way structural tuning (ordering-disordering) between distinct states in MGs offers an opportunity to overcome this challenge. However, pressures up to tens of GPa were required for the process, which limits sample sizes at the micrometer scale and challenges further comprehensive research. Here, by introducing concurrent shear straining with compressive force using high-pressure torsion, the pressure to trigger the disordering transition is significantly reduced to a few GPa. This enables the acquisition of millimeter-sized samples for comprehensive study, including synchrotron x-ray diffraction and small angle x-ray scatting, nanoindentation, electrical resistivity, and differential scanning calorimetry. It is evidenced that the two-way tuning of MG can be achieved both structurally and thermodynamically, which could help deepen our understanding of the mechanism behind the two-way tuning and promote exploration of distinct MGs.

Quantitative study of solid-state mechanical mixing by high-pressure torsion based on image processing

Mechanical alloying through the process of severe plastic deformation can significantly enhance the mechanical properties of dissimilar metals. However, uneven mixing of these materials can adversely affect their mechanical performance and may even lead to material failure. To quantitatively assess the degree of mixing between dissimilar materials, a mechanical mixing efficiency index (η) has been introduced. This index is calculated accurately by combining an improved Non-dominated Sorting Genetic Algorithm II (NSGA-II) with computer image processing techniques. The study focuses on the Al-Zn mechanical mixture, establishing a parametric correlation between the mechanical mixing efficiency index and the microstructural evolution of mechanical mixing. The findings indicate that when the η value falls below 0.27865, the material transitions from the mechanical mixing stage to the chemical fusion stage. This research facilitates the controllable preparation of mechanical alloying, providing theoretical support and practical evidence for the process.

Influence of Severe Plastic Deformation on the Magnetic Properties of Sm–Co Permanent Magnets

High pressure torsion (HPT) is presented as a new fabrication route to produce bulk Sm–Co magnets with a strongly refined microstructure down to the nanometer regime. The initial powders, based on the compositions SmCo5, Sm2Co7 and Sm2Co17, are compacted and subsequently deformed by HPT. The microstructural evolution in dependence on the applied deformation parameters is characterized by electron microscopy and the effect of HPT on the phase stability is monitored by synchrotron X‐ray diffraction. An increasing amount of applied strain leads to a stronger reduction in grain size while strain localization counteracts a homogeneous microstructural refinement. The positive effect of elevated deformation temperatures is demonstrated for Sm2Co17, which promotes homogeneous grain refinement, but causes strain‐induced phase transformations at the same time, strongly affecting the magnetic behavior. Superconducting quantum interference device magnetometry is used to characterize the magnetic properties after HPT deformation, which indicates the formation of a magnetic texture depending on the respective phase.

Correlation of structure development and electric characteristics within high pressure torsion processed copper

AbstractCopper of a high purity features excellent electric conductivity, but generally very low mechanical properties. Nevertheless, optimized deformation/thermomechanical treatment can introduce favorable combinations of both. The presented study characterizes the correlation of microstructure development and electric properties within copper processed by the severe plastic deformation method of high pressure torsion, the primary advantage of which is that it enables to achieve grains with the sizes in the ultra‐fine, or even nano scales. The study investigates structure development during progressive deformation. In other words, samples processed by single and double high pressure torsion revolutions were evaluated from the viewpoints of grain sizes and grain boundaries, and the results were correlated with the experimentally measured electric conductivity. The single high pressure torsion revolution contributed to grain size decrease, while the structure after double revolution exhibited very fine grains, especially at the sample periphery featuring the highest imposed strain. Both the samples also exhibited increases in microhardness (especially after double revolution), and electric conductivity higher than 100 % IACS. The results confirmed that copper conductors featuring enhanced mechanical properties and favorable electric conductivity can be manufactured by severe plastic deformation.

Effect of annealing temperature on microstructure and properties of stainless steel rings after high-pressure torsion

Effect of annealing temperature on microstructure and properties of stainless steel rings after high-pressure torsion

Development of Fe‐10% (HA/β‐Tricalcium Phosphate) Composite via Solid‐State Manufacturing Route and Investigation of Material Properties for Biodegradable Implant Applications

Day‐by‐day biodegradable alloys and composites are getting more attention. Besides their competing material properties with traditional permanent implants, their harmless degradation in the body eliminates the need for a second surgery to remove the implant. Beyond other biodegradable materials such as Mg and Zn, Fe is known with high strength with low degradation rate. However there is still need to improve its biocompatibility. Herein, a Fe‐based composite is developed via solid‐state manufacturing route. 10% HA/β‐TCP is selected as an additive which is a biocompatible ceramic material and the powder mix is ball‐milled. Afterward, the powders are consolidated via high‐pressure torsion (HPT) with 1–5–10 revolutions at room temperature. Crack‐free structures are obtained even after 1 HTP rotation. Fe is in lamellar form around HA‐βTCP particles. With the increase of HPT rotation numbers, lath thickness decreased. After 10 HPT rotations, 24% enhancement in density is observed that points to a more condensed structure. Transmission electron microscopy observations show significant grain refinement after 1 HPT rotation, and Fe grain size remains constant (≈300 nm) up to 10 turns. Ultimate tensile strength increases while degradation rate decreases after 5 HPT rotations. This study provides the potential of enhanced Fe‐based biomaterials for thinner and smaller implant designs.

Processing of high-strength thermal-resistant Al-2.2% cerium-1.3% lanthanum alloy rods with high electric conductivity by High Pressure Torsion Extrusion

A novel severe plastic deformation (SPD) process of High Pressure Torsion Extrusion (HPTE) was applied to the rods of the Al-2.2 wt.% Ce-1.3 wt.% La (Al–3.5RE) alloy. The microstructure, microhardness, the mechanical strength, thermal stability, and electrical conductivity of the alloy after HPTE and subsequent annealing have been investigated. It was demonstrated that HPTE processing can simultaneously increase yield strength from 127 to 225 MPa and electrical conductivity from 54.7% IACS to 55.7% IACS in this alloy. Such a remarkable combination of properties was achieved thanks to significant refinement of microstructure constituents: grain size of Al matrix was reduced down to 0.9 µm and initially continuous network of Al11RE3 phase was broken to micrometer- and nanometer-sized particles. Furthermore, the yield strength of the HPTE-processed Al–3.5 RE alloy remains stable at 230 °C for 1 h due to micrometer- and nanometer-sized particles that pin the grain boundaries. Therefore, HPTE processing of Al–RE alloys has a high application potential in the electric power industry.Graphical abstract

Influence of the Temperature of High Pressure Torsion Deformation on the Recrystallization Kinetics of Iron with a Submicrocrystalline Structure

Influence of the Temperature of High Pressure Torsion Deformation on the Recrystallization Kinetics of Iron with a Submicrocrystalline Structure

A new compact symmetric shear diamond anvil cell for in situ high-pressure-torsion studies.

In situ studies under severe plastic deformation at high pressures, employing shear diamond anvil cells, have recently gained much interest in the high-pressure community owing to their potential applications in material processing methods, mechanochemistry, and geophysics. These studies, combined with multi-scale computational simulations, provide important insights into the transient hierarchical microstructural evolution, structural phase transitions, and orientation relationship between parent and daughter phases and help establish the kinetics of strain-induced phase transitions under severe plastic deformation. The existing SDACs are mostly used in axial x-ray diffraction geometry due to geometrical constraints providing less reliable information about stress states and texture. Their asymmetric design also poses serious limitations to high-pressure shear studies on single crystals. To overcome these limitations, a new compact symmetric shear diamond anvil cell has been designed and developed for in situ high-pressure torsion studies on materials. The symmetric angular opening and short working distance in this new design help obtain a more reliable crystallographic orientation distribution function and lattice strain states up to a large Q range. Here, we present the advantages of the symmetric design with a few demonstrative studies.

Digital Image Correlation Analysis of Uniform Deformation and Necking in Solid‐State Welded Nanocrystalline Aluminum via High‐Pressure Torsion

Solid‐state welding of Al 1043 sheets was achieved via high‐pressure torsion (HPT) processing to produce bulk nanostructured Al disks. A homogeneous nanostructure without segregation was observed, with grain sizes of ∽430‐470 nm. Miniature tensile testing, coupled with the digital image correlation (DIC) technique, was employed to determine the room‐temperature tensile deformation behavior, particularly the non‐uniform behavior with necking, of the HPT‐bonded ultrafine‐grained (UFG) aluminum, comparing it with annealed coarse‐grained counterpart. The HPT‐bonded UFG Al exhibited a large fraction of post‐necking strain, which is supported by the estimated high strain rate sensitivity value of m = 0.085, suggesting the delay of local necking leading to tensile fracture. Detailed DIC analysis revealed prolonged diffuse necking, thus delaying local necking, in the HPT‐bonded UFG Al, while the annealed samples show high fractions of local necking during the non‐uniform deformation. Moreover, the DIC data illustrated that local necking predominantly occurred at a limited neck zone, maintaining a plateau strain distribution at the out‐of‐neck zone throughout necking deformation towards tensile failure for both annealed and UFG aluminum. The DIC method offers an alternative means to demonstrate the transition in necking behaviors of materials by estimating the plastic lateral contraction exponent.This article is protected by copyright. All rights reserved.

Precipitate-mediated enhancement of mechanical and electrical properties in HPTE-processed Al–Mg–Si alloy

In this study the effect of high-pressure torsion extrusion (HPTE) and artificial aging (AA) on the strength and electrical conductivity of an Al–Mg–Si alloy was investigated. The resulting material shows remarkable improvement in ultimate strength reaching 380 MPa and in electrical conductivity with 53 % IACS (International Annealed Copper Standard), after post-processing aging at 160 °C for 10 h. X-ray diffraction analysis and electron backscatter diffraction were employed to analyze the structure of the alloy caused HPTE followed by AA. Transmission electron microscopy (TEM) and energy-dispersive X-ray (EDX) spectroscopy were used to study the morphology, elemental composition, and crystal structure of the secondary phases. The study confirms that Guinier-Preston (GP) zones and nano-scale precipitates created during the deformation and subsequent heat treatment are evenly distributed inside the grains. Chemical composition analysis of the precipitates indicated the presence of one main phase based on the Mg/Si ratio coexisting with a low volume fraction of other phases in this Al grade. High-resolution TEM revealed precipitates either with a partially ordered structure or a fully crystalline structure. Strength and electrical conductivity modeling were carried out based on the microstructural characterization results. The findings of this study indicate that HPTE together with thermal treatment has the potential as new approach for processing of commercially significant aluminum alloys.

Nanostructuring of Additively Manufactured 316L Stainless Steel via High-Pressure Torsion for Enhanced Strength and Corrosion Performance

Bulk nanostructured materials (BNM) are defined as metallic materials with grain sizes <100 nm and are known to possess superior mechanical and functional properties compared to those with grain sizes in the micron and millimeter ranges. In this study, high-pressure torsion (HPT), a nanostructuring approach that imposes compressive stress and extreme torsional strain on bulk materials is applied to an additively manufactured (AM) 316L stainless steel (316L SS) for 10 revolutions. The grain sizes, hardness, and corrosion performance before and after HPT are evaluated by using extensive microscopy techniques, Vickers hardness (HV) measurements, and electrochemical tests conducted in 3.5 wt.% NaCl solution, respectively. The results show that after 10 HPT revolutions, nano-sized grains (average: ~42 nm) are obtained, whereas a three-fold increase in HV values from ~220 HV to ~600 HV are observed. Furthermore, the corrosion performance is also significantly enhanced as indicated by the reduction in corrosion ratefrom 2.53 μm/year initially, to 0.48 μm/year after HPT processing. These results highlight the benefits of HPT in producing bulk nanostructured materials with remarkably high hardness and excellent corrosion performance, potentially useful for a myriad of applications.

High‐Pressure Torsion Processing of Serine and Glutamic Acid: Understanding Mechanochemical Behavior of Amino Acids under Astronomical Impacts

Astronomical impacts by small solar system bodies (meteoroids, asteroids, comets, and transitional objects) are considered a mechanism for delivering amino acids and their polymerization to proteins in early Earth conditions. High‐pressure torsion (HPT) is a new methodology to simulate such impacts and clarify the behavior of biomolecules. Herein, two amino acids, crystalline L‐serine and L‐glutamic acid that are detected in meteorites, are processed by HPT and examined by ex situ X‐ray diffraction, Raman spectroscopy, nuclear magnetic resonance, Fourier‐transform infrared spectroscopy, and in situ mechanical shear testing. No polymerization, chemical reactions, or phase transformations are detected after HPT, indicating that the stability and presence of these two amino acids in meteorites are quite reasonable. However, some microstructural and mechanical changes like crystal size reduction to the nanometer level, crystal defect formation, lattice expansion by vacancy formation, and shear strength enhancement to the steady state are found which are similar to the behaviors reported in metals and ceramics after HPT processing.

Magnetic characterization of HPT-deformed Fe–Si soft magnetic alloys before and after heat treatment and internal stress calculations

Fe, Fe-3wt%Si, and Fe-6.5 wt%Si alloys were subjected to high-pressure torsion (HPT) to achieve a nanostructure (<100 nm grain size). HPT processing was done at 77 and 293 K and heat treatments were performed on the deformed samples to reduce internal stresses that are known to deteriorate soft magnetic properties by impeding the movement of domain walls. X-ray line profile analysis (XLPA) was used to find out the crystallite/subgrain/domain size and the dislocation density in the deformed samples. The coercivity Hc was characterized on ring-shaped samples using a hysteresisgraph. Internal stresses in the deformed and heat-treated samples were determined using magnetic data and the dislocation density data and a comparison is made between the two approaches. HPT deformation increases the coercivity of the samples while heat treatment after deformation reduces the coercivity to a certain extent in Fe samples but has mixed results in Fe–Si alloys. In comparison with Herzer's theory, zero-frequency Hc was plotted against crystallite (or subgrain) size and it was found that coercivity follows a D1.5 relation, where D is crystallite size.

Enhanced strength-ductility combination in the aluminum-gold system by heterogeneous distribution of nanoparticles via ultra-severe plastic deformation and reactive interdiffusion

Ultrafine-grained aluminum alloys are of interest due to their high strength-to-weight ratio, but they usually suffer from poor uniform ductility. In this study, an Al-Au alloy with a good combination of strength and ductility is produced by the heterogeneous distribution of Al2Au nanoparticles in an aluminum matrix. To generate such heterogeneity, the alloy is synthesized by ultra-severe plastic deformation of aluminum and gold powders via the high-pressure torsion (HPT) method. Reactive interdiffusion occurs during the process leading to the heterogeneous formation of intermetallic particles and a good strength-ductility synergy (200 MPa yield stress and 15% uniform elongation). Nanoparticles gradually distribute within the matrix and once a uniform nanoparticle distribution is achieved, the alloy shows no further increase in strength, but it completely loses its ductility. It is concluded that not only the presence of nanoparticles but more importantly the heterogeneity of their distribution can positively influence the strength-ductility combination in ultrafine-grained aluminum alloys. The findings of this study suggest that future studies on heterogeneous precipitation hardening can be a solution to achieve ductile precipitation-hardened alloys.

A Discrete Element Model of High-Pressure Torsion Test to Assess the Effect of Particle Characteristics in the Interface

Abstract Sand particles have been used since the early stages of the railway industry to increase adhesion at the wheel–rail contact. However, there is a limited understanding of how sand particle characteristics affect the tribological performance of the wheel–rail contact. In this work, the high-pressure torsion test used as a small-scale simulation of the interface is numerically modeled using the discrete element method (DEM). The DEM model is then utilized to investigate the effect of different particle characteristics on the frictional performance of wheel–rail contact which can provide more insight into micromechanical observations. The effects of various particle characteristics including their size, their number, the number of fragments the particles break into, and the parameters defining the behavior of the bonds between particle fragments on the coefficient of traction (COT) are systematically investigated. Results show that, in dry contacts, the coefficient of traction decreases when the size or number of sand particles increases. This can be attributed to the formation of weak shear bands between the fragments. Further investigation is needed for wet- and leaf-contaminated contacts. It is also found that the COT is more sensitive to the stiffness of the bond between the fragments of a broken particle compared to the strength of the bond. A limiting value for bond strength was identified, beyond which the sand particles exhibited ductile behavior rather than the expected brittle fracture. The findings from this study can be useful for future research on adhesion management in wheel–rail contact and the modeling approach can be scaled up to the full contact.

An electro-mechanical contact model for particulate systems

This study presents an electro-mechanical contact model to investigate the electrical response within a particulate system under mechanical loading. The model considers the electrical resistance across particle-to-particle and particle-to-wall contacts by formulating bulk and contact resistances, separately. Hertz and Holm's models are utilised for the contact resistance in the overlapping region. The bulk resistance for a particle is estimated considering particle geometry transformations during the compression. The proposed electro-mechanical contact model is verified and validated against published analytical solutions and experimental data. Finally, the contact model is applied to simulate a high-pressure torsion test to investigate the influence of three different materials on the electrical conduction properties of the system after particle fragmentation. The intensity distributions of electric potential, current and resistance of the granular system are analysed at both particle and system scales. An electroactivity index is proposed for evaluating the contribution of each fragment sieve cut on the bulk electrical conduction behaviour.

Oxygen vacancy-rich high-pressure rocksalt phase of zinc oxide for enhanced photocatalytic hydrogen evolution

The generation of hydrogen as a clean energy carrier by photocatalysis, as a zero-emission technology, is of significant scientific and industrial interest. However, the main drawback of photocatalytic hydrogen generation from water splitting is its low efficiency compared to traditional chemical or electrochemical methods. Zinc oxide (ZnO) with the wurtzite phase is one of the most investigated photocatalysts for hydrogen production, but its activity still needs to be improved. In this study, an oxygen-deficient high-pressure ZnO rocksalt phase is stabilized using a high-pressure torsion (HPT) method, and the product is used for photocatalysis under ambient pressure. The simultaneous introduction of oxygen vacancies and the rocksalt phase effectively improved photocatalytic hydrogen production to levels comparable to benchmark P25 TiO2, due to improving light absorbance and providing active sites for photocatalysis without any negative effect on electron-hole recombination. These results confirm the high potential of high-pressure phases for photocatalytic hydrogen generation.