Critical engineering applications, such as landing gears and armor protection, require structural materials withstanding high strength and significant plastic deformation. Nanoprecipitate-strengthened high-entropy alloys (HEAs) are considered as promising candidates for structural applications due to their enhanced strength and exceptional work-hardening capability. Herein, we report a FeCoNiAlTi-type HEA that achieves ultrahigh gigapascal yield strength from quasi-static to dynamic loading conditions and superb resistance to adiabatic shear failure. This is accomplished by introducing high-density coherent L12 nanoprecipitates. Multiscale characterization and molecular dynamics simulation demonstrate that the L12 nanoprecipitates exhibit multiple functions during impact, not only as the dislocation barrier and the dislocation transmission medium, but also as energy-absorbing islands that disperse the stress spikes through order-to-disorder transition, which result in extraordinary impact resistance. These findings shed light on the development of novel impact-resistant metallic materials.

It becomes a common practice to adopt high-throughput experiments on superalloys, which can generate a large amount of data. To address this large amount of data, we designed a machine learning (ML) based model to automate the experimental analysis process. More specifically, we adopted the Unet algorithm to segment the precipitated phases from superalloy images and subsequently used a regression algorithm to predict the morphological parameters of the microstructure of the segmented precipitated phases according to their composition. The method proposed in this work may provide guidance for the future design of the superalloy composition. (c) 2021 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Mechanical properties and microstructure of the bovine hoof wall have been studied. The reduced modulus and hardness of the transverse and the longitudinal specimens decrease from the outside to the inside, and reach a peak of 4.69 GPa and 0.15 GPa, respectively. The further work reveals that the enhancement mechanism is the lamellar structure and fiber-refined mode. In addition, a kind of structural model is designed and fabricated by three-dimensional printing technology, whose performance is improved by 55%. We hope this work can provide ideas for a new high-toughness and light-weight armor design. (C) 2020 Elsevier B.V. All rights reserved.

In this study, the high-power solid-state laser was successfully employed to weld the FeCoNiCrMn high entropy alloy plates. The tensile strength of the joint achieves 524 MPa. The FeCoNiCrMn high-entropy alloy joint has improved hardness and excellent cyclic deformation resistance. In the fusion zone, dendrites become finer and elements Mn are distributed at the end of dendrites. Refined grains and ultrafine Mn-C rich precipitates can effectively enhance the mechanical properties of joints. (C) 2019 Elsevier B.V. All rights reserved.

The dynamic mechanical properties and microstructure of the (Al0.5CoCrFeNi)0.95Mo0.025C0.025 high entropy alloy (HEA) prepared by powder extrusion were investigated by a split Hopkinson pressure bar and electron probe microanalyzer and scanning electron microscope. The (Al0.5CoCrFeNi)0.95Mo0.025C0.025 HEA has a uniform face-centered cubic plus body-centered cubic solid solution structure and a fine grain-sized microstructure with a size of about 2 microns. The HEA possesses an excellent strain hardening rate and high strain rate sensitivity at a high strain rate. The Johnson–Cook plastic model was used to describe the dynamic flow behavior. Hat-shaped specimens with different nominal strain levels were used to investigate forced shear localization. After dynamic deformation, a thin and short shear band was generated in the designed shear zone and then the specimen quickly fractured along the shear band.

The equiatomic CoCrFeMnNi high entropy alloy prepared by powder metallurgy has homogenous chemical composition and microstructure. The mechanical properties of the CoCrFeMnNi high entropy alloy at the strain rates (1 x 10(-4) s(-1) to 0.1 s(-1) and 1 x 10(3) s(-1) to 3 x 10(3) s(-1)) and the temperature (298 K, 673 K and 1073 K) were investigated. Results indicate that the yield strength of the CoCrFeMnNi high entropy alloy is in the range of 350-700 MPa, increasing sensitively with increasing the strain rates, especially at a high strain rate (larger than 1 x 10(3) s(-1)). The serration behavior of the high entropy alloy is observed on the flow stress curves of the alloy deformed at a low strain rate of 1 x 10(3) s(-1) and the high strain rates (1 x 10(3) s(-1) to 3 x 10(3) s(-1)). Influences of the strain rate and the temperature on the serration behavior of the CoCrFeMnNi high entropy alloy are discussed.

Shear localization is the main failure mechanism of the Ti-5Al-5Mo-5V-1Cr-1Fe titanium alloy deformed at a high strain rate. Double peaks appear on the curves of the strain and the stress during the shear localization process when the phase transformation occurs. Widths of the shear bands are affected by the nominal strain of the specimens and the treatment temperature. Microstructure of a shear band in the fine grained Ti-5Al-5Mo-5V-1Cr-1Fe titanium alloy was investigated by Scanning Electron Microscopy and High Resolution Transmission Electron Microscopy. Grains in the boundary of the shear band are highly elongated along the shear direction, and the core of the shear band consists of ultrafine equiaxed grains with sizes 0.1-0.5 mu m and with low density of dislocations. Sizes of the grains in the shear band obtained at the cryogenic temperature are smaller than those in the shear band obtained at the room temperature. Evidences of the grain boundaries of the elongated grains and the ultrafine equiaxed grains indicate that the microstructure of the shear band are formed by the way of the rotational dynamic recrystallization.

Dynamic deformation and shear localization of ultrafine-grained (similar to 120 nm) pure titanium are examined. The strain hardening can be considered as having two regimes: below and above a strain similar to 0.04; at this point there is a drastic decrease in the slope. The strain-rate sensitivity of ultrafine-grained titanium is found to be approximately the same as its coarse grained counterpart. Based on experimentally determined parameters, the Zerilli-Armstrong equation is modified to describe the mechanical response of the ultrafine-grained titanium over the strain rate range 10(-5) to 10(3) s(-1). Adiabatic shear banding is examined in a forced shear configuration where large strain is imposed in a narrow region. The microstructure inside the adiabatic shear band consists of a mixture of elongated grains and equiaxed nanograins (similar to 40 nm) that are significantly smaller than the initial grains (similar to 120 nm). The formation of equiaxed nanograins is modeled through a mechanism of rotational dynamic recrystallization. This further reduction in grain size from the one generated by ECAP is interpreted in terms of the Zener-Hollomon parameter for quasistatic and dynamic deformation. The adiabatic shear band eventually fractures by a combination of brittle and ductile failure. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Localized shear is an important mode of deformation; it leads to catastrophic failure with low ductility, and occurs frequently during high strain-rate deformation. The hat-shaped specimen has been successfully used to generate shear bands under controlled shock-loading tests. The microstructure in the forced shear band was characterized by optical microscopy, microhardness, and transmission electron microscopy. The true flow stress in the shear region can reach 800MPa where the strain is about 2.2. The whole shear localization process lasts for about 100 μs. The shear band is a long and straight band distinguished from the matrix by boundaries. It can be seen that the grains in the boundary of the shear band are highly elongated along the shear direction and form the elongated cell structures (0.2µm in width), and the core of the shear band consists of a number of recrystallized equiaxed grains with 0.2−0.3µm in diameters, and the second phase particles distribute in the boundary of the ultrafine equiaxed new grains. The calculated temperature in the shear band can reach about 667K. Finally, the formation of the shear band in the ultrafine grained 6061 aluminum alloy and its microstructural evolution are proposed.

Adiabatic shear localization plays an important role in the deformation and failure of near beta Ti–5Al–5Mo–5 V–1Cr–1Fe alloy used in aircraft's gear at high rate deformation. Hat shaped specimens with different nominal shear strains are used to induce the formation of an adiabatic shear band under controlled shock-loading experiments. When the nominal shear strain is about 0.68, unstable shear deformation of the alloy emerges after the true flow stress reaches 1100MPa, the first vibration peak during the split Hopkinson pressure bar testing, and the whole process lasts about 62μs. The microstructures within the shear band in the Ti–5Al–5Mo–5V–1Cr–1Fe alloy are investigated by means of optical microscopy, scanning electron microscopy and transmission electron microscopy. Phase transformation occurs in the shear band when the nominal shear strain increases to 0.68. A number of equiaxed grains with sizes 50–200nm and alpha″-phase are in the center of the shear band. Kinetic calculations indicate that during the deformation process, the recrystallized nanosized grains can be formed in the shear band by way of the subgrain boundaries rotation, and the alpha″ phase transformation start after the subgrain boundaries rotated to 30°.

Adiabatic shear localization plays an important role in the deformation and failure of the coarse grained beta titanium alloy Ti-5 Al-5 Mo-5V-1 Cr-1 Fe with grain size about 1mm at high strain rate deformation. Hat shaped specimens with different nominal shear strains are used to induce the formation of shear bands under the controlled shock-loading experiments. The true stress in the specimens can reach about 1040MPa where the strain is about 1.83. The whole shear localization process lasts about 35μs. The microstructures within the shear band are investigated by optical microscopy, scanning electron microscopy / electron backscatter diffraction, and transmission electron microscopy. The results show that the width of the shear bands decreases with increasing nominal shear strain, and the grains in the transition region near the shear band are elongated along the shear band, and the core of the shear band consists of the ultrafine deformed grains with width of 0.1μm and heavy dislocations. With the aims of accommodating the imposed shear strain and maintaining neighboring grain compatibility, the grain subdivision continues to take place within the band. A fiber texture is formed in the core of the shear band. The calculated temperature rise in the shear band can reach about 722K. Dynamic recovery is responsible for the formation of the microstructure in coarse grained beta titanium alloy.

The hot compression testing of Ti–5Al–5Mo–5V–1Fe–1Cr alloy was performed by a thermal simulation machine Gleeble 1500 in the temperature range of 700–950°C with the strain rate range of 0.001–10s−1. The stress–strain behavior showed flow softening in α+β phase region, whereas in β phase region temperature range and strain rate higher than 0.1s−1, discontinuous yield behavior was also observed. The average activation energy is about 137kJ/mol and 288kJ/mol in β phase and α+β phase region, respectively. The alloy exhibits flow instability under the deformation conditions of strain rates about 0.01s−1 and temperature range 700–810°C, which should be avoided during hot working. In addition, the instability area enlarged in processing map with the increasing of true strain. In the α+β phase region the morphology of α phase was bent and intended to globularization, and dynamic recrystallization occurred in β phase region. Based on the stress exponent and microstructure features, dislocation slip controls the creep process of alloy under the conditions of 400°C and all applied stresses. The isothermal ω phase precipitated during creep at 400°C but was not observed at 500°C. The creep process was controlled by dislocation climb under the conditions of 500°C and higher stress.

Ultrafine grained titanium has unique mechanical properties and attracts tremendous interest due to its scientific and technological application. Shear localization is frequently denoted as adiabatic shear band, and is one of the most important deformation and failure mechanisms for it used at high rate deformation. Hat shaped specimens are used to induce the formation of an adiabatic shear band under controlled dynamic conditions. Unstable shear deformation of the alloy emerges after the true flow stress reaches about 750MPa, the first vibration peak during the split Hopkinson pressure bar testing, and the whole deformation process lasts about 50μs. The microstructure and microtextures in the shear band with width about 16.7μm in ultrafine grained titanium processed by multi-axial compression are investigated by means of optical microscopy, scanning electron microscopy/electron back-scattered diffraction, and transmission electron microscopy. The results show that the grains in the boundary of the shear band are highly elongated along the shear direction, and the core of the shear band consists of a number of scattered recrystallized equiaxed grains with 50–80nm in diameters and coalesced grains with 100–150nm in diameters. Some new microtextures (20°, 4°, 0°), (81°, 0°, 0°), and (55°, 0°, 30°) with recrystallized characteristics are generated in the shear band. The grain boundaries in the core of an adiabatic shear band are high-angle boundaries and geometrical necessary boundaries created with aims of accommodating the imposed shear strain. Calculations of temperature rise suggest that the maximum temperature in the shear band is about 870K being sufficient for the recrystallization. Finally, the mechanisms of formation of an adiabatic shear band in the ultrafine grained titanium processed by multi-axial compression and its microstructural evolution are proposed.

Adiabatic shear band is an important materials phenomenon often observed in metals when processed at high strain rates. The mechanical responses and microstructure evolution in it attract strong interests from the scientists of materials science and engineering. We report the results of the microstructure characteristic of a 200 series Fe–Cr–Ni austenitic stainless steel with low nickel contents deformed at high strain rates (about 5.8×105s−1) by a split Hopkinson pressure bar. The hat shaped specimens are used to induce the formation of the adiabatic shear bands under shock-loading tests. The microstructure and microtexture of the shear band in a 200 series Fe–Cr–Ni austenitic stainless steel are investigated by means of optical micrograph electron backscatter diffraction. The shear bands can be generated at about 78μs after the true flow stress reaches the value about 923MPa. The grains in the boundary of the shear band are elongated along the shear direction, and the core of the shear band consists of ultrafine equiaxed grains with diameter 0.1–0.3μm and low dislocation density. According to the orientation distribution, the microtexture peaked at (45°, 65°, 0°) in the matrix slightly shifts towards the recrystallization microtexture (60°, 60°, 0°) in the shear band center, and the grain boundaries in the shear band are geometrical necessary boundaries with high-angles. Calculations of temperature rise about 943K suggest that the temperature in the shear band is above the recrystallization point. Finally, the grain refinement in an adiabatic shear band in the 200 series Fe–Cr–Ni austenitic stainless steel is described as a consequence of the rotational dynamic recrystallization.