In advanced engineering applications, there has been an increasing demand for the service performance of materials under high-strain-rate conditions where a key phenomenon of adiabatic shear instability is inevitably involved. The presence of adiabatic shear instability is typically associated with large shear strains, high strain rates, and elevated temperatures. Significant plastic deformation that concentrates within a adiabatic shear band (ASB) often results in catastrophic failure, and it is necessary to avoid the occurrence of such a phenomenon in most areas. However, in certain areas, such as high-speed machining and self-sharpening projectile penetration, this phenomenon can be exploited. The thermal softening effect and microstructural softening effect are widely recognized as the foundational theories for the formation of ASB. Thus, elucidating various complex deformation mechanisms under thermomechanical coupling along with changes in temperatures in the shear instability process has become a focal point of research. This review highlights these two important aspects and examines the development of relevant theories and experimental results, identifying key challenges faced in this field of study. Furthermore, advancements in modern experimental characterization and computational technologies, which lead to a deeper understanding of the adiabatic shear instability phenomenon, have also been summarized.

The occurrence of adiabatic shear bands, as an instability phenomenon, is viewed as a precursor to failure caused by instability at high strain rates. Metastable β titanium alloys are extensively utilized due to their excellent mechanical properties, which are often subjected to high strain rate loads in service conditions. Understanding and studying their adiabatic shear instability behavior is thus crucial for preventing catastrophic failure and enhancing material performance. In this study via detailed microstructural analyses in the adiabatic shear region of a Ti-10V-2Fe-3Al alloy subjected to high strain rates, it was observed that α″ martensitic transformation and nano-twinning plus β-to-α phase transformation with α″ martensite as an intermediate phase occurred, in addition to substantial fine grains. The grain refinement mechanisms were mainly related to dynamic recovery dominated by dislocation migration alongside severe plastic deformation.

Powder Bed Fusion by Laser Beam Melting (PBF-LB/M) has gained significant interests for fabricating magnesium matrix composites (Mg-MMCs). However, particle agglomeration unavoidably leads to a strength-ductility trade-off. To address this issue, a multi-physics multiscale framework, coupling a microscopic Computational Fluid Dynamics-Discrete Phase Model (CFD-DPM) with a sub-microscopic Sharp Interface Model (SIM), is developed. The CFD-DPM results reveal that Marangoni vortices drive reinforced particles to agglomerate at molten-pool periphery in a single track. Then a layer-wise tuned scheme for hatch spacing is defined as the two-fifths of the molten pool width and confirmed by experiments. The SIM simulations reveal that the particles should not be inert. With the consideration of Brownian motion, insufficient free reorientation of particles ahead of solid-liquid interface can explain why the particle pushing mechanism dominates in the experiments instead of the engulfment mechanism from the perspective of interfacial energy. With the optimized hatch spacing of 60 μm, an increase in tensile strength (~349.2 MPa) with decent ductility (~4.3%) is achieved in the as-fabricated TiCN/AZ91D composites. The solute co-segregation of Al/Zn/Mn elements enhances particle/matrix interfacial bonding and may delay microcrack initiation. The activation of dislocations near particle/matrix interface facilitates the release of stress concentration and enhances deformation uniformity. This particle-dispersion optimization strategy can also be applied for fabricating other high-performance Mg-MMCs by PBF-LB/M.