Terence G. Langdon

Achieving an optimal strength-ductility balance is a central challenge in the design of advanced metallic materials. The development of novel metastable alloys that exhibit a deformation-induced martensitic transformation (DIMT) effect presents a promising avenue for addressing this issue. While these alloys exhibit relatively high ductility under standard conditions, a further enhancement is essential to circumvent the strength-ductility trade-off and enable their use in structural engineering applications. This review focuses on the pivotal role of precisely controlling the kinetics of the DIMT as a novel toughening mechanism in order to boost ductility in metastable alloys and particularly high-entropy alloys (HEAs). Initially, the importance of metastability engineering on the strength-ductility synergy is discussed, and then the critical factors affecting the DIMT kinetics are explored. The review also examines important kinetic models relevant to advanced steels and HEAs. Additionally, the key strategies are analyzed for controlling the thermodynamics and kinetics of DIMT: (I) manipulating the chemical composition, (II) grain refinement in highly metastable alloys, (III) a microstructural engineering approach, including implementing partitioning by heat treatment, introducing dislocation substructures, and constructing hierarchical microstructures. Finally, the existing research gaps are examined, and directions are explained for future exploration, including the development of novel chemical compositions, controlled thermomechanical processing routes, and applying machine learning models to predict different features of DIMT.

At temperatures of 290 K and 77 K, the phase composition and mechanical properties ofnonequiatomic medium-entropy (MEA) alloys Fe40Mn40Co10Cr10 and Fe50Mn30Co10Cr10 werecompared in the coarse-grained (CG) and nanostructured (NS) states, in which additionaldeformation mechanisms are activated under load: phase transformations in the MEAFe50Mn30Co10Cr10 (MEA TRIP) and twinning in the MEA Fe40Mn40Co10Cr10 alloy (MEA TWIP). Itis shown that in the NS state in both alloys, in contrast to the CG state, a complete phase transitionfrom the fcc to the hcp phase is observed, the content of which weakly depends on the temperatureand the number of torsion revolutions during high-pressure torsion (HPT). The transition from theCG to the NS state leads to an increase in the microhardness (in the NS MEA TWIP by 3.7 and inthe NS MEA TRIP by 2.25). In the CG state, a thermally activated character of plastic deformationis observed for both alloys in the temperature range of 290 – 77 K. In the NS state, MEA TWIPremains plastic under active compression deformation at 290 K and 77 K, whereas in NS MEATRIP under similar conditions, macroscopic plasticity is absent. Tensile deformation up to 50 % at30 K in the CG state for both alloys leads to a significant decrease in the absolute values of Young'smodulus over the entire temperature range.

The microhardness of CoCrFeNiMnV x (х = 0-2) high-entropy alloys (HEAs) was measured in the temperature range 77-293 K. At x ≤ 0.4, a significant monotonic increase in microhardness occurs with decreasing temperature, which indicates the thermally-activated character of plastic deformation of the material under the indenter. At x = 0.5, as well as at x = 0.75 and 0.85, athermal behavior of microhardness was detected in the ranges of 200-293 K and 150-293 K, respectively. The latter is apparently associated with the appearance in the indicated alloys, along with the FCC phase, of precipitates of the hard intermetallic sigma phase, which are athermal obstacles to the motion of dislocations. For the first time the microhardness of the sigma phase in the range of 77-293 K was measured; at 293 K and 77 K it was about 9.5 GPa and 12.5 GPa, respectively, which is approximately 5 times higher than the microhardness of the FCC alloy with x = 0.25.

This study describes the fabrication of immiscible Cu-V alloys through the application of high-pressure torsion (HPT). For this purpose, stacked Cu/V/Cu disks were subjected to HPT from 0.5 to 250 turns under a pressure of 6.0 GPa at room temperature. The V layers became thinner and fragmented with increasing numbers of HPT turns but finally mixed well with the Cu matrix throughout the disk samples. After 200 turns HPT processing, the nanostructured Cu-V alloy displays a submicron level heterostructure with a mixture of coarser grains (~ 100 nm and high Cu content) and finer grain (~ 20-30 nm and high V content). An ultimate tensile strength (UTS) of 1300 MPa with 3.5% elongation was achieved in a sample subjected to 200 turns HPT processing and post-HPT annealing at 773 K for 1 h. Thus, the HPT-processed immiscible Cu-V alloy achieved not only a significant microstructural refinement but also a remarkable strength enhancement through the solid mixing of Cu and V at room temperature.

A decade of research combining multi-principal element alloys (MPEAs) processed by high-pressure torsion (HPT) and possessing unique effects has generated considerable anticipated and unexpected insights related to the deformation behavior and properties of these alloys. Processing by HPT offers a simple route for obtaining nanostructured grains, thereby overcoming the long-standing issue of the low yield strength in face-centered cubic (FCC) MPEAs. This review provides the first comprehensive report on the processing‒structure‒property relationship in the realm of FCC MPEAs. It casts light on the breakdown of the conventional stacking fault energy‒deformation mechanism correlation for HPT-processed FCC MPEAs, the unexpected occurrence of deformation-induced phase transformations and it clarifies the role of different material-specific as well as processing-dependent factors dictating the grain refinement down to the nanoscale regime. Additionally, a detailed discussion is presented on the potential of HPT processing to achieve outstanding mechanical properties for FCC MPEAs. The multifunctional aspects of the nanostructured FCC MPEAs are critically examined from the viewpoint of their high temperature stability, corrosion resistance and susceptibility to hydrogen embrittlement. Accordingly, this review provides a pathway for future research by highlighting the key research gaps and the opportunities for niche industrial applications of FCC MPEAs processed using HPT.

Understanding the mechanistic interplay between phase transformation and grain fragmentation is critical for microstructural control in advanced structural steels subjected to severe shear. Here, we investigate the activation sequence of retained-austenite transformation and grain fragmentation along the radial strain gradient of a single QP1180 steel disk processed by high-pressure torsion. Synchrotron-based high-energy X-ray diffraction and microscopy reveal a pronounced austenite (γ) → martensite (α′/α) transformation that saturates at a critical equivalent von Mises strain ε ‾ T ∼ 8.5. Concomitantly, γ grain size decreases sharply up to ε ‾ T , while γ peak broadening and microstructural analysis suggest limited grain fragmentation of austenite during transformation. These findings demonstrate that γ-phase reduction is primarily driven by phase transformation prior to the onset of defect-induced fragmentation. This mechanistic activation order and the critical strain ε ‾ T provide key inputs for calibrating physics-based constitutive models and defining robust process windows for industrial forming operations and component design.

Creep refers to the nonrecoverable plastic strain that accumulates in a material when it is subjected to an applied stress for a prolonged period of time. For polycrystalline materials, creep may occur through intragranular mechanisms such as dislocation climb and glide or through intergranular mechanisms such as diffusional creep and grain boundary sliding (GBS). For GBS, the grains become displaced relative to each other and analysis shows that the flow mechanism then depends on the grain size. Subgrains form within the grains at reasonably large grain sizes but no subgrains are formed if the grain size is equal to or smaller than the subgrain size. The latter process corresponds to superplastic flow where it is possible to achieve tensile elongations of several hundreds or even thousands of per cent. This review summarises the principles of GBS in polycrystalline materials and then examines the potential for achieving GBS at low temperatures.

Ceramics are typically brittle at ambient conditions due to their covalent or ionic bonding and limited dislocation activities. While plasticity, and occasionally superplasticity, can be achieved in ceramics at high temperatures through thermally activated phenomena, creep, and grain boundary sliding, their deformation at ambient temperature and pressure remains challenging. Processing under high pressure via the high-pressure torsion (HPT) method offers new pathways for severe plastic deformation (SPD) of ceramics. This article reviews recent advances in HPT processing of ceramics, focusing primarily on traditional ceramics (e.g., oxides, carbides, nitrides, oxynitrides) and to a lesser extent advanced ceramics (e.g., silicon, carbon, perovskites, clathrates). Key structural and microstructural features of SPD-processed ceramics are discussed, including phase transformations and the generation of nanograins and defects such as vacancies and dislocations. The properties and applications of these deformed ceramics are summarized, including powder consolidation, photoluminescence, bandgap narrowing, photovoltaics, photocatalysis (dye degradation, plastic waste degradation, antibiotic degradation, hydrogen production, CO 2 conversion), electrocatalysis, thermoelectric performance, dielectric performance, and ion conductivity for Li-ion batteries. Additionally, the article highlights the role of HPT in synthesizing novel materials, such as high-entropy ceramics (particularly high-entropy oxides), black oxides, and high-pressure polymorphs, which hold promise for energy and environmental applications.

Metal matrix composites with improved mechanical properties and thermal stability were produced using mechanical milling, spark plasma sintering (SPS) and high-pressure torsion (HPT). Three types of reinforcing particles were used, i.e., GO, Al 2 O 3 and rGO/Al 2 O 3 . All of the produced composites exhibit higher hardness and tensile strength than pure coper, reaching values of 250 Hv for Cu-GO, 240 Hv for Cu- Al 2 O 3 , 210 Hv for Cu- rGO/Al 2 O 3 and 185 Hv for Cu after HPT. STEM analyses reveal that the HPT significantly refines the grain size of pure copper to ~210 nm, and even more in the Cu-based composites achieving grain sizes as small as ~55-75 nm. Pure Cu after HPT recrystalizes after annealing at 573 K. The Cu- Al₂O₃ composite demonstrated the best thermal stability with a hardness after annealing at 773 K of 220 Hv and a grain size of ~100 nm. The composite of Cu-GO after annealing at 773 K showed slight grain growth up to ~150 nm. The composite Cu-GO/Al 2 O 3 exhibits improved microhardness and tensile strength up to 673 K and annealing of this composite at 773 K which led to a bimodal microstructure. All of the composites annealed at 773 K showed hardness above 180 Hv.

Studies of creep may be traced back for more than 100 years and this extensive experimentation has produced a comprehensive understanding of the various flow mechanisms occurring in the steady-state or secondary stage of creep. These mechanisms range from diffusional creep and Harper-Dorn creep at low stresses to dislocation processes such as glide and climb at higher stresses and to grain boundary sliding where the rate is dependent primarily upon the grain size. This review examines the nature and characteristics of these flow processes and then demonstrates that the theoretical predictions are generally in good agreement with the experimental data. Finally, two examples are presented, in the fields of structural engineering and glaciology, to illustrate the potential for making significant new contributions to the understanding of the creep processes.

High-pressure torsion (HPT) processing disrupts the thermodynamic equilibrium in immiscible systems and often produces nonequilibrium microstructures with unique properties. This study investigates the microstructural evolution and mechanical behaviour of a Cu-Nb immiscible alloy subjected to HPT under 6 GPa compressive stress. The HPT processing was performed on stacked Cu-Nb-Cu layers by up to 200 turns and this produced mechanically alloyed, homogenized disks free of porosity or cavities. Microstructural characterization using X-ray diffraction and scanning electron microscopy, coupled with energy-dispersive X-ray spectroscopy, revealed a stepwise evolution, including the reduction of segregation layers, the formation of nonequilibrium Cu-17 at. %Nb solid solution in the disc processed at 200 HPT turns and an increased Nb insertion into the Cu lattice. Additionally, grain refinement and residual strain increments were observed with increasing torsional turns. Thereafter, the mechanical properties were evaluated using hardness mapping and tensile testing. The material exhibited strain hardening behaviour and achieved an ultimate tensile strength (UTS) exceeding 1.25 GPa. Following post-deformation annealing, the UTS decreased to ~700 MPa due to recrystallization and recovery. These results provide a preliminary understanding of microstructural transformations and their impact on the mechanical properties of immiscible systems subjected to extreme deformation. • HPT process is able to produce bulk disk of immiscible Cu-Nb defect-free mechanically alloyed sample after 200 turns. • Homogeneity of the composition increases by increasing number of turns. Edge is easily homogenized while centre needs more shear strain provided by applying further turns. • Residual strain and crystallite size are increased and reduced respectively by increasing number of turns. • Negative work-hardening behaviour was observed after tensile properties of 200 turns with UTS of 1250 MPa.

Abstract 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 Al 0.5 CoCrFeNi 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 Al 0.5 CoCrFeNi HEA at low‐stress levels. Based on the microstructure, the flowability of Al 0.5 CoCrFeNi 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 Al 0.5 CoCrFeNi HEA during EAC and provided an experimental foundation and theoretical support for the formation of HEAs.

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.

The grain size is an important structural parameter in polycrystalline materials contributing to the strength of the material and the ability to achieve a superplastic forming capability. Grain refinement is especially important because small grains lead to stronger materials and they provide more opportunities for attaining superplastic flow. Traditionally, the grain size was modified through the use of various thermo-mechanical treatments but this had a significant limitation because it was not possible to produce materials with grain sizes smaller than a few micrometers. The situation has changed over the last forty years with the demonstration that much smaller grain sizes may be produced by processing through the application of severe plastic deformation (SPD) where a high strain is imposed without causing any significant change in the overall dimensions of the sample. This report summarizes the principles of the main SPD processing techniques and then demonstrates the significance of producing submicrometer grain sizes.

Correction for ‘Recent advances in using severe plastic deformation for the processing of nanomaterials’ by Terence G. Langdon, Nanoscale , 2025, 17 , 17417–17427, https://doi.org/10.1039/D5NR01886B.

The well-known indentation size effect (ISE) is reviewed with special emphasis on the effect of grain size in the polycrystalline matrix. It is demonstrated that there is a close connection between the Hall-Petch relationship and the characteristics of the ISE phenomenon such that the ISE phenomenon may disappear in an ultrafine-grained (UFG) matrix. This finding is significant in any attempts to interpret nanoindentation measurements performed on UFG materials.

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.

Experiments were conducted to evaluate the creep characteristics of a high purity 2.5 mol % yttria-stabilized zirconia (termed 2.5Y-TZP). Tensile creep tests were performed at elevated temperatures and measurements were taken to record the creep strain as a function of time. The results are compared with published creep data for 3Y-TZP under both tensile and compressive testing conditions. A comparison is made also with tensile creep data for low purity 3Y-TZP. It is shown there is a tendency for the creep rate to decrease continuously in tests performed at the lower stresses. Observations using scanning electron microscopy suggest this decrease is not related to the occurrence of grain growth or the development of internal cavitation. Detailed analysis shows also that the decreasing creep rate at low stresses has no influence on some of the basic parameters associated with high temperature creep, including the exponent of the inverse grain size where p ~ 1 and the apparent activation energy for creep where Qapp ~ 570 kJ mol'1. However, there is a change in the value of the stress exponent from n ~ 2 in the early stages of the tests to n ≥ 3 in the later stages.

A comparison between the creep characteristics of squeeze-cast magnesium alloys AZ 91 and QE 22 reinforced with 20 vol. %A12O3 short fibres and unreinforced AZ 91 and QE 22 matrix alloys shows that the creep resistance of the reinforced materials is considerably improved compared to the matrix alloys. It is suggested that the creep strengthening in the short fibre composites arizes mainly from the existence of a load transfer effect and a threshold stress. By contrast, investigations of the creep behaviour of a particulate QE 22-15 vol. %SiC composite and its unreinforced QE 22 matrix alloy prepared by power metallurgy revealed no substantial increase in the creep strength of the composite. This unexpected result has been explained by the microstructural changes induced by the presence of particle reinforcement and by creep loading in the composite matrix.

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.

An analysis of the processes of plastic deformation and acoustic relaxation in a high-entropy alloy, Al0.5CoCrCuFeNi, was carried out. The following were established: dominant dislocation defects; types of barriers that prevent the movement of dislocations; mechanisms of thermally activated movement of various elements of dislocation lines through barriers at room and low temperatures. Based on modern dislocation theory, quantitative estimates were obtained for the most important characteristics of dislocations and their interaction with barriers.