Terence G. Langdon’s research while affiliated with University of Southampton and other places

The corrosion behavior of a commercial Al-6061 alloy was explored in a 3.5% (wt%) NaCl solution after high-pressure torsion (HPT) processing at room temperature for numbers of revolutions of N = 0, 1/2, 2 and 10 turns. The microstructures revealed by electron backscatter diffraction (EBSD) showed excellent grain refinement from 121±5 to 0.44±0.1 µm after N = 10 turns with a high fraction of high-angle grain boundaries (~65%). The results from electrochemical tests demonstrate that HPT processing significantly improves the corrosion resistance and reduces the corrosion rate due to a combination of grain refinement, an increased dislocation density and texture weakening. The corrosion mechanism was not affected by the HPT processing and found to be controlled by charge transfer. The corrosion morphology of the HPT-processed sample taken through N = 10 turns and observed after 14 days of immersion showed a smooth surface except for the presence of some corrosion microcracks around large particles enriched with Zn and Fe elements.

Electrically assisted forming (EAF) is a reliable method of reducing the deformation resistance of metallic materials and enhancing their formability. In this study, the mechanical properties and microstructure of Al0.5CoCrFeNi high-entropy alloy (HEA) under electrically assisted compression (EAC) were investigated. The results showed that the flow stress decreased with increasing current density in the EAC. Specifically, the flow curves exhibited S-shaped softening at a higher current density, which was dominated by the non-uniform distribution of the Joule heating temperature during EAC. When the flow stress was fixed at 500 MPa and 80 A·mm−2, compressible deformation amounts of 63.7% were observed at a strain rate of 1 s−1, indicating full compression of Al0.5CoCrFeNi HEA at low-stress levels. Based on the microstructure, the flowability of Al0.5CoCrFeNi HEA was improved during EAC, and the flow direction shifted from 45°to the horizontal direction. The current density, which influences the Joule heating temperature and strain rate, synergistically affects the stacking fault energy (SFE) and critical resolved shear stress (CRSS), which affect the tendency for twinning behavior. Thererfore, deformation nanoscale twins (DTs) were observed, indicating a shift in the deformation mechanisms from dislocation slip domination to a mixed pattern of dislocation slip and twinning. This study confirmed the deformability of Al0.5CoCrFeNi HEA during EAC and provided an experimental foundation and theoretical support for the formation of HEAs.

High-entropy alloys (HEAs) are a new class of material producing superior properties that have a potential for replacing many structural materials in industry. Single-phase solid solution HEAs with face-centered cubic crystal structure show significant ductility and toughness over a wide temperature range including at cryogenic temperatures. Nevertheless, the occurrence of decomposition at elevated temperatures is challenging for many applications. These materials reveal sluggish diffusion and therefore high thermal stability so that processing by severe plastic deformation gives increased kinetics of decomposition and leads to fine-multiphase microstructures which provide a potential for achieving superior superplastic elongations. The present review is designed to examine the available superplastic data for HEAs and thereby to compare the behavior of HEAs with conventional superplastic alloys. Mater. Trans. 64 （2023） 1526-1536に掲載. Figs. 6, 12を修正． Fig. 12. Temperature and grain size compensated strain rate versus normalized stress showing excellent agreement with the theoretical prediction for conventional superplasticity [64] Fullsize Image

The dissolution reaction kinetics were determined by differential scanning calorimetry (DSC) analysis for Mg-0.6Gd (wt.%) in an as-cast condition and after processing by high-pressure torsion (HPT) at room temperature for 20 turns. For the as-cast condition, the dissolution temperature was in the range of 634–658 K with an activation energy of 135±9 kJ/mol. After HPT, the range of dissolution temperature decreased to 601–633 K, with a lower activation energy of 118±10 kJ/mol. In both conditions, the Avrami and growth exponents are around 1.5, which suggests that the dissolved precipitates exhibit polyhedron-like morphology and that the dissolution process is dominated by bulk dissolution with a constant rate controlled by diffusion.

The growing demand for lightweight and high-strength materials in the aerospace and automotive industries, as well as the need for highly conductive materials such as heat sinks, electrodes and integrated circuits, has fueled the exploration of innovative composites. Metal matrix composites (MMCs) reinforced with graphene offer a promising solution, combining the inherent properties of metals with the unique characteristics of graphene. However, the fabrication of MMCs reinforced with graphene poses several challenges such as poor wettability of graphene within the metal matrix, a non-uniform distribution of graphene and graphene clustering. Various fabrication methods have been used to address these challenges; among them, high-pressure torsion (HPT) is a promising solution due to the introduction of a fine- or even nanograined structure with well-distributed graphene within the matrix through severe shear deformation. Grain boundary strengthening, Orowan bypassing due to the presence of non-shearable graphene particles, stress transfer to the reinforcements and the inherent properties of graphene can also enhance the mechanical properties of the graphene-containing MMCs produced by HPT. On the other hand, HPT negatively affects the electrical conductivity of the metal matrices by increasing the dislocation density and the number of grain boundaries. Nevertheless, graphene can also enhance the electrical conductivity of the composite by endowing the metal matrix with its π electrons. A current comprehensive examination of the literature provides valuable insights into the development of graphene-reinforced metal matrix composites fabricated by HPT and gives additional information on their potential applications.

New metal matrix nanocomposites with enhanced thermal stability were produced in a three step process consisting of mechanical milling, spark plasma sintering and High-Pressure Torsion (HPT). The nanocomposites consisted of a copper matrix and the addition of 1 wt.% Graphene Oxide (GO) as a reinforcement. A nanocrystalline microstructure, enhanced hardness and improved thermal stability were achieved. The grain size of the nanocomposites was ~55 nm which is almost four time smaller than for Cu HPT at 210 nm. Hardnes and ultimate tensile strength of the nanocomposites reach 250 Hv and 700 MPa, respectively, which was more than three times higher than for the initial material. The most important result is that the nanocomposites remained ultrafine-grained up to 500 ⁰C whereas the Cu HPT fully recrystalized after annealing at 300 ⁰C The report also includes an investigation of the electrical conductivity of the copper-based composite which was slightly better than for copper after HPT together with the wear behaviour of this material. This is one of the first reports on copper reinforced with graphene oxide composites produced by HPT and it gives information on its thermal stability, electrical conductivity and wear behaviour together with the microstructural characteristics and mechanical properties.

Experiments were conducted to systematically assess the superplastic properties and the microstructural changes of an Al-3Mg-0.2Sc alloy processed by 10 HPT revolutions at room temperature (RT ≈ 300 K) or at 450 K after subsequent tensile testing at temperatures from 473 to 723 K over a wide range of strain rates. The HPT processing at RT led to the development of elongated grains with an average size of ~140 nm whereas the grain structures were equiaxed and slightly larger (~150 nm) after HPT at 450 K. After HPT processing at RT, the Al-Mg-Sc alloy exhibited true superplastic flow at low homologous temperatures and attained a maximum elongation of ~850 % at 523 K. Nevertheless, the elongations decreased at temperatures T ≥ 623 K and an elongation higher than 400 % was only achieved at 673 K for a strain rate of 𝜀̇ = 4.5 x 10-3 s-1. The material processed by HPT at 450 K displayed superior microstructural stability and substantially higher superplastic ductilities. Elongations of > 1100 % were attained at 673 K for strain rates from 3.3 x 10-4 to 1.0 x 10-1 s-1 and a record elongation of ~1880 % for an HPT-processed metal was achieved at 1.5 x 10-2 s-1 at 673 K. High strain rate superplasticity was also reached for an extended range of temperatures and strain rates. Analysis of the data confirms a stress exponent of ~2 which is consistent with superplastic flow by grain boundary sliding accommodated by dislocation glide and climb.

Ultrafine-grained and heterostructured materials are currently of high interest due to their superior mechanical and functional properties. Severe plastic deformation (SPD) is one of the most effective methods to produce such materials with unique microstructure-property relationships. In this review paper, after summarizing the recent progress in developing various SPD methods for processing bulk, surface and powder of materials, the main structural and microstructural features of SPD-processed materials are explained including lattice defects, grain boundaries and phase transformations. The properties and potential applications of SPD-processed materials are then reviewed in detail including tensile properties, creep, superplasticity, hydrogen embrittlement resistance, electrical conductivity, magnetic properties, optical properties, solar energy harvesting, photocatalysis, electrocatalysis, hydrolysis, hydrogen storage, hydrogen production, CO2 conversion, corrosion resistance and biocompatibility. It is shown that achieving such properties is not limited to pure metals and conventional metallic alloys, and a wide range of materials are currently processed by SPD, including high-entropy alloys, glasses, semiconductors, ceramics and polymers. It is particularly emphasized that SPD has moved from a simple metal processing tool to a powerful means for the discovery and synthesis of new superfunctional metallic and nonmetallic materials. The article ends by declaring that the borders of SPD have been extended from materials science and it has become an interdisciplinary tool to address scientific questions such as the mechanisms of geological and astronomical phenomena and the origin of life.

The Ti–29Nb–13Ta–4.6Zr alloy (TNTZ) is a β-Ti alloy that has a potential for use in biomedical applications as an alternative to the less-compatible Ti64 alloys. Enhancing the strength and the surface finish of TNTZ is essential for biomedical applications. In this research, a combination of high-pressure torsion (HPT) and laser treatment was used to improve the TNTZ properties. The HPT-treated samples showed significantly enhanced mechanical properties when compared with the traditional solution-treated TNTZ. A laser surface treatment immediately forms a hydrophilic surface that transforms into a steady hydrophobic state after 14 days in air and the surface roughness increases with an increase in laser power and a slower scanning rate. The corrosion resistance of TNTZ improves significantly after laser treatment, with the corrosion current dropping from 1 × 10⁻⁸ to 1.2 × 10⁻⁹ A and the corrosion potential peak shifting to a more positive value from − 0.349 to − 0.158 V. The friction coefficient after laser treatment decreased from 0.134 to 0.093 and then further reduced to 0.082 after 14 days in air thereby suggesting an enhancement in the tribological properties. Overall, the results show that HPT processing combined with a post-HPT laser treatment is beneficial for enhancing the mechanical properties and the corrosion and wear performance of the TNTZ alloy.