Severe plastic deformations, mechanochemistry, and microstructure evolution under high pressure: In Situ Experiments, Four-Scale Theory, New Phenomena, and Rules

Processes involving severe plastic deformations (SPD) and phase transformations and chemical reactions (PTs/CRs) under high pressures are widespread for obtaining new nanostructured phases and their processing, mechanochemical synthesis, military applications, and nature. SPD strongly reduce the pressure required for PTs/CRs (by one-two orders of magnitude) and PT hysteresis; lead to hidden metastable phases, which cannot be obtained otherwise, and substitute reversible PTs/CRs with irreversible ones. This review is devoted to breakthroughs in understanding multifaceted interactions between high-pressure PTs/CRs, SPD, and microstructure evolution from the viewpoint of advanced mechanics and thermodynamics of materials under stress and plastic strain tensors. A novel concept of plastic strain-induced PTs/CRs under high pressure is explored using four-scale theory and simulations (from atomistic to nano- and scale-free phase-field approaches to macroscale) coupled to in situ experiments in traditional and rotational diamond anvil cells, and their integration. Its development revealed various phenomena and misinterpretations, resolved numerous puzzles, found the first general rules in these fields, and suggested ways for economic defect-induced synthesis of high-pressure phases and nanostructures. Coupled analytical/computational/experimental approaches are developed for complete characterization of occurring processes and finding all heterogeneous scalar and tensorial fields. Applications include high-pressure torsion, surface treatment, high-pressure tribology, PTs/CRs in shear bands leading to severe transformation/reaction-induced plasticity and self-blown-up processes, mechanisms of deep-focus earthquakes, the appearance of microdiamonds in low-pressure-temperature Earth crust, and the mechanochemical origin of life beyond Earth. Unresolved problems and future directions are outlined.