High-pressure Torsion Research Articles

Structure and Electrical Properties of AlFe Matrix Composites with Graphene

Composites based on Al-Fe alloys and reinforced with three-layer graphene sheets were synthesized “in situ” under a layer of molten salts. Composites containing 0.12 wt.% graphene were subjected to severe plastic deformation via high-pressure torsion. SEM and TEM were used to study the morphological and size characteristics of the composite structure in the as-cast and deformed states. Herein, it is shown that severe plastic deformation leads to the deformation-induced dissolution of iron-containing aluminides with the formation of a supersaturated Al solid solution with a submicrocrystalline structure. The hardness and electrical properties of the graphene composites were measured in different structural states and compared to the characteristics of the matrix alloy.

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.

Assessment of different processing strategies to fabricate bulk Mg-Fe nanocomposites

Recent observations indicate that very high deformation temperatures (673 K corresponding to 0.7 of Mg's melting point) by high pressure torsion (HPT) are mandatory to successfully synthesize bulk Mg-Fe nanocomposites with homogenous microstructure and exceptionally high strength (2.2 GPa Hardness). This spurs the development of new fabrication strategies to further strengthen the material while maintaining microstructural homogeneity for lower processing temperatures (T < 673 K). In this regard the present work deals with various strategies of severe plastic deformation at temperatures below 673 K including ball milling the Mg50Fe50 powder followed by HPT, HPT at 673 K followed by HPT at room temperature, inter-stage annealing during deformation, and changing the geometry of HPT anvils. Microstructural analysis using a scanning electron microscope and results of microhardness tests revealed that although abovementioned strategies accelerated microstructural refinement in Mg-Fe composites, strain localization cannot be suppressed. The strain localization and consequently inhomogeneous microstructure can be attributed to the immediate strengthening of basal texture intensity in Mg phase at low temperature driven by shear strain. Hence, the lack of structural reproducibility and hardness values less than 2.2 GPa show that all strategies are not sufficiently effective to synthesize bulk Mg50Fe50 nanocomposites. Only a high deformation temperature is the main key to fully transform a coarse structure into a homogeneous nano-structure with exceptional strength and good reproducibility.

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.

Investigating the effect of different adhesion materials on electrical resistance using a high pressure torsion rig

This paper presents an assessment of newly-developed conductive adhesion materials (Products A-E) in comparison to standard rail sand used in Britain. Current rail sand is an insulating material which can affect track circuits; newly-developed conductive materials could reduce the risk of this and allow for more material to be applied to further mitigate against low adhesion. The particles were characterised to determine their densities, and size and shape distributions. Bulk behaviour was assessed through three characteristics: angle of repose, bulk shear strength, and particle breakage index. Materials were then assessed using a high pressure torsion approach to measure their effects on adhesion and electrical resistance in dry, wet, and leaf contaminated conditions. It was found that all products produced better or equivalent conductivity compared to the currently used GB rail sand and that Product D and Product E should be considered for future field testing.

Effect of Deformation-Induced Plasticity in Low-Alloyed Al-Mg-Zr Alloy Processed by High-Pressure Torsion

The influence of additional deformation heat treatments (DHTs), implemented by two regimes: (1) annealing and small additional deformation by high-pressure torsion (HPT) at room temperature (RT) and (2) HPT at elevated temperature to 10 turns and small additional HPT at RT, has been studied on the microstructure, mechanical properties and electrical conductivity of ultrafine-grained (UFG) Al-0.53Mg-0.27Zr (wt.%) alloy structured by HPT to 10 turns at RT. As is shown, both types of additional DHT lead to a substantial increase in plasticity (2–5 times) while maintaining high electrical conductivity (~53% IACS) and strength comprising 75–85% of the value in the pre-DHT state of the UFG alloy. The possible physical reasons for the revealed changes in the physical and mechanical properties are analyzed. Comparison of the strength and plasticity changes with the microstructure evolution after DHT of both types indicates that the increase in the density of introduced grain boundary dislocations is the most probable factor providing a tremendous increase in plasticity while maintaining a high level of strength in the UFG alloy under study. An outstanding combination of high strength (370 MPa), high elongation to failure (~15%) and significant electrical conductivity (~53% IACS) was achieved for the Al-Mg-Zr alloy. This combination of properties exceeds those obtained to date for this system, as well as for a number of other commercial conductor alloys based on the Al-Zr system.

The effect of high-pressure torsion on irradiation hardening of Eurofer-97

The effect of high-pressure torsion on irradiation hardening of Eurofer-97

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.

Low-temperature precipitation of Al3(Zr,Er) dispersoids in an Al-0.4Fe-0.15Zr-0.25Er alloy processed by high pressure torsion

The Al3(Zr,Er) precipitation behavior and property evolution in an ultra-fine grained (UFG) Al-0.4Fe-0.15Zr-0.25Er alloy processed by high pressure torsion (HPT) were investigated. It was found that the Al3(Zr,Er) dispersoids with core–shell structure were formed at much lower temperature of 150 °C as compared with that in coarse-grained alloys. This were explained as the enhanced diffusivity and nucleation of Zr and Er atoms due to the high density of defects introduced by HPT. The microhardness and conductivity of Al-0.4Fe-0.15Zr-0.25Er alloy were improved simultaneously because of the formation of Al3(Zr,Er) dispersoids. This work may provide a new insight in development of Al alloys with Al3(Zr,Er).

Microstructures, Mechanical Properties and Transformation Behavior in Ni49.6Ti35.4Hf15 Alloy Produced with High-Pressure Torsion

High-pressure torsion (HPT) was applied for the Ni49.6Ti35.4Hf15 (at.%) alloy up to 1/4, 2 and 16 turns under a pressure of 4.0 GPa. The samples were examined using X-ray diffraction (XRD), transmission electron microscope (TEM) and microhardness measurements. The results indicate that the mixture of an amorphous and nanocrystalline microstructure developed in the investigated NiTiHf alloy as the number of HPT turns was increased to two. The average hardness of the samples increased from 330 Hv to 500 Hv after 16 turns of HPT. Very fine martensite developed when the HPT-processed samples were annealed at 550 °C and the finer microstructures were attained with higher HPT turns. Differential scanning calorimetry (DSC) tests were performed in the post-HPT annealing samples to clarify the transformation behavior after severe plastic deformation in HPT and subsequent annealing, so as to provide an experimental basis for the application of the shape memory alloy. The transformation temperature of the alloy decreased remarkably when the number of turns of HPT reached 16. It is suggested that the deformation extent and annealing temperatures should be considered in order to maintain a high transformation temperature while utilizing the strengthening effect of HPT in the NiTiHf alloy.

A Comparative Study of a Machine Learning Approach and Response Surface Methodology for Optimizing the HPT Processing Parameters of AA6061/SiCp Composites

This work investigates the efficacy of high-pressure torsion (HPT), as a severe plastic deformation mechanism for processing plain and silicon-carbide-reinforced AA6061, with the broader objective of using the technique for improving the properties of lightweight materials for a range of objectives. The interactions between input variables, such as the pressure and equivalent strain (εeq) applied during HPT processing, and the presence of SiCp and response variables, like the relative density, grain refinement, homogeneity of the structure, and the mechanical properties of the AA6061 aluminum matrix, were investigated. Hot compaction (HC) of the mixed powders followed by HPT were employed to produce AA6061 discs with and without 15% SiCp. The experimental findings were then analyzed statistically using the response surface methodology (RSM) and a machine learning (ML) approach to predict the output variables and to optimize the input parameters. The optimum combination of HPT process parameters was confirmed by the genetic algorithm (GA) and ML approaches. Furthermore, the constructed ML and RSM models were validated experimentally by HPT processing the same material under new conditions not fed into the models and comparing the experimental results to those predicted by the model. From the ML and RSM models, it was found that processing the AA6061/SiCp composite HPT via four revolutions at 3 GPa produced the highest mechanical properties coupled with significant grain refinement compared to the HC condition. ML analysis revealed that the equivalent strain induced by the number of revolutions was the most effective parameter for grain refinement, whereas the presence of SiCp played the highest role in improving both the hardness values and the compressive strength of the AA6061 matrices.

Magnetic Properties of a High-Pressure Torsion Deformed Co-Zr Alloy.

Co-Zr amorphous alloys exhibit soft magnetic properties, whereas the Co-rich crystalline magnetic phases in this alloy system displayed a hard magnetic behavior. In this study, an initial two-phase Co-Zr composite with an overall composition of 75 at.% Co and 25 at.% Zr was processed by high-pressure torsion (HPT), and the effects of severe plastic deformation and subsequent thermal treatment on the composite's structural evolution and its magnetic properties were investigated. HPT processing allowed us to achieve an amorphous microstructure with low coercivity in its as-deformed state. To further tune the alloy's magnetic properties and study its crystallization behavior, various annealed states were investigated. The microstructural properties were correlated with the magnetic properties, and a decreasing coercivity with increasing annealing temperatures was observed despite the onset of crystallization in the amorphous alloy. At higher annealing temperatures, coercivity increased again. The results appear promising for obtaining tuneable rare-earth free magnetic materials by severe plastic deformation.

Low-Temperature Superplasticity of the 1565ch Al–Mg Alloy in Ultrafine-Grained and Nanostructured States

—Homogeneous nanostructured and ultrafine grained (NS, UFG) states with mean grain sizes of 95 and 200 nm, respectively, have been formed in a 1565ch Al–Mg alloy (Al–5.66Mg–0.81Mn–0.67Zn–0.09Zr–0.07Cr–0.04Ti–0.001Be–0.3(Fe + Si) wt %). Microstructure of both states is represented by grain boundaries with predominantly high-angle misorientations. The alloy, produced both by high pressure torsion at room temperature and equal channel angular pressing at 200°C using the Conform approach, exhibits superplasticity at low temperatures in the range 250–300°C and strain rates in the range of 5 × 10–4–10–2 s–1. Elongation values range 170–560%, while the rate sensitivity coefficient (m) varies from 0.3 to 0.73 at low flow stress for both NS and UFG structures. The temperature range for the stability of strength properties of the 1565ch alloy in NS and UFG states after thermal and thermal mechanical treatments has been determined. The material in both structural states maintains a high level of strength after undergoing deformation under SP conditions. The deformation relief formed on the gage surface of the NS and UFG specimens of the 1565ch alloy during the established SP yield stage has been analyzed.

Review - Phase Transitions in Ti Alloys Driven by the High Pressure Torsion

Review - Phase Transitions in Ti Alloys Driven by the High Pressure Torsion

Some Unresolved Problems of High-Pressure Torsion

This overview highlights some salient features of one of the most popular severe plastic deformation techniques: high-pressure torsion (HPT). It focuses on the unresolved challenging problems of HPT. The problems selected touch upon some fundamental questions of mechanics of plasticity, fracture, and friction that are at the core of the HPT process. The scientific significance of these problems and the proposed pathways to resolving them are discussed. The article is meant to promote the use of HPT as a potent tool for studying plasticity at large strains theoretically and also as a practical method enabling novel micromanufacturing routes.

Recent &lt;i&gt;In Situ&lt;/i&gt; Experimental and Theoretical Advances in Severe Plastic Deformations, Strain-Induced Phase Transformations, and Microstructure Evolution under High Pressure

Severe plastic deformations (SPD) under high pressure, mostly by high-pressure torsion, are employed for producing nanostructured materials and stable or metastable high-pressure phases. However, they were studied postmortem after pressure release. Here, we review recent in situ experimental and theoretical studies of coupled SPD, strain-induced phase transformations (PTs), and microstructure evolution under high pressure obtained under compression in diamond anvil cell or compression and torsion in rotational diamond anvil cell. The utilization of x-ray diffraction with synchrotron radiation allows one to determine the radial distribution of volume fraction of phases, pressure, dislocation density, and crystallite size in each phase and find the main laws of their evolution and interaction. Coupling with the finite element simulations of the sample behavior allows the determination of fields of all components of the stress and plastic strain tensors and volume fraction of high-pressure phase and provides a better understanding of ways to control occurring processes. Atomistic, nanoscale and scale-free phase-field simulations allow elucidation of the main physical mechanisms of the plastic strain-induced drastic reduction in phase transformation pressure (by one to two orders of magnitude), the appearance of new phases, and strain-controlled PT kinetics in comparison with hydrostatic loading. Combining in situ experiments with multiscale theory potentially leads to the formulation of methods to control strain-induced PT and microstructure evolution and designing economic synthetic paths for the defect-induced synthesis of desired high-pressure phases, nanostructures, and nanocomposites.

Magnesium Alloys Processed by Severe Plastic Deformation (SPD) for Biomedical Applications: An Overview

Ultra-fine grained and even nanostructured magnesium alloys obtained by processing with methods of severe plastic deformation (SPD) are promising biomaterials for absorbable orthopaedic implants due to their enhanced mechanical properties, adequate corrosion resistance and biocompatibility. This paper presents an overview of the impact of the most important SPD methods – equal-channel angular pressing (ECAP) and high-pressure torsion (HPT) – on microstructure refinement and improvement of the mechanical properties of magnesium alloys intended for medical implants. Several selected groups of magnesium alloys which have the potential for use as bioabsorbable implants are discussed. The presented results of many years of research indicate the beneficial effect of SPD methods on obtaining ultra-fine and even nanostructures of magnesium alloys with improved mechanical and better functional properties, which are necessary for bioabsorbable implants.

Unique Microstructure Evolution of HPT-Processed (α + γ) Two-Phase Stainless Steel

A high-pressure torsion (HPT) processed Fe-21Cr-5Ni-2Mo (mass%) two-phase stainless steel was used to study the morphology and crystallographic features of austenite (γ) precipitated from ferrite (α) during aging in the (α + γ) two-phase region. The starting material was a gas-atomized powder with a completely ferritic structure. The HPT process was carried out to produce a fully dense compact under 6 GPa for 5 revolutions. The compact was given an equivalent strain of about 130. After the HPT process, the matrix ferrite formed a pancake–like nanograined structure with a strong texture, i.e. ND (Normal Direction) // {110} α. By annealing at 1173 K for 3.6 k, an ultrafine (α + γ) microduplex structure with high-angle grain boundaries was formed. In addition, the strong texture formation of {110} α / {111} γ / ND plane was formed in the α and the γ grain duplex structure. The α and γ phases had average grain sizes of 2.1 μm and 1.6 μm, respectively. The area fraction of the γ phase was 37.2%, which exceeded that of a cold-pressed compact, 6.7%. Both ultrafine grain refinement and γ precipitation were accelerated by the HPT process. In other words, the application of the HPT process to the two-phase alloys enables the formation of the ultrafine microduplex structure. The Kurdjumov-Sachs (K-S) orientation relationship between α and γ phases is usually observed in the alloy, however, the K-S orientation relationship was not dominant except for the close packing plane parallel orientation relationship, {110} α / {111} γ, in the HPT-processed material.

MANIFESTATION OF SUPERPLASTICITY AT LOWER TEMPERATURES OF 1565CH ALLOY OF THE Al−Mg SYSTEM IN ULTRAFINE-GRAIN AND NANOSTRUCTURAL STATES

In this work, homogeneous nanostructured and ultrafine-grained (NS and UFG) states with an average grain size of 95 nm and 200 nm, respectively, are formed in the 1565ch alloy of the Al-Mg system. In both states, the microstructure is formed by a network of predominantly high-angle grain boundaries. It has been shown that an alloy with an NS and UFG structure formed at room temperature by high-pressure torsion (HPT) and at 200°C by equal-channel angular pressing according to the Conform scheme (ECAP-С) exhibits similar signs of superplastic (SP) behavior at low temperatures 250 …300 °С in the range of strain rates 5×10-4 s-1…10-2 s-1: elongation values were 170…560%, the rate sensitivity coefficient (m) was 0.3-0.73 at low flow stresses . The temperature range of stability of the strength characteristics of the 1565ch alloy in the NS and UFG states was established, both after thermal and deformation-thermal treatment. It is shown that the material in both studied structural states retains a high level of strength after deformation under SP conditions. The deformation relief formed on the working part of the NS and UFG samples of the 1565ch alloy at the stage of a steady-state SP flow is analyzed.

Synchrotron High-Energy X-ray &amp; Neutron Diffraction, and Laser-Scanning Confocal Microscopy: &lt;i&gt;In-Situ&lt;/i&gt; Characterization Techniques for Bulk Nanocrystalline Metals

This report is aimed at giving an overview of the significance of the novel and innovative microstructural and microscopic characterization techniques for bulk nanostructured metals processed by severe plastic deformation, specifically high-pressure torsion (HPT). In practice, the microstructural relaxation behavior upon heating of nanostructured 316L stainless steel and CoCrFeNi high-entropy alloy was characterized by in-situ heating neutron diffraction measurements; the heterogeneous phase distribution of an HPT-bonded hetero-nanostructured Al-Mg alloy was examined using synchrotron high-energy X-ray diffraction; and the microstructural evolution upon heating of a nanostructured CoCrFeNiMn high-entropy alloy was examined by laser-scanning confocal microscopy. These novel techniques are complementary to each other and any other in- or ex-situ testing methods, especially when nanocrystalline metals are transforming microstructurally and compositionally with temperature and time in a hierarchical manner. The outcomes of the studies emphasize the importance of the methodologies and the development of characterization techniques for further in-depth exploration in the research field of severe plastic deformation.