Strain-induced phase transformations under high pressure and large shear in a rotational diamond anvil cell: Simulation of loading, unloading, and reloading

Coupled plastic flow and strain-induced phase transformations (PTs) under high pressure and large plastic shear in a micron scale sample under loading, unloading, and reloading in a rotational diamond anvil cell (RDAC) are studied in detail, utilizing finite element approach. A plastic strain-controlled, pressure-dependent kinetic equation, which describes strain-induced PTs, is used. The effects of four main material parameters in this equation on PTs and plastic flow in RDAC in three-dimensional formulation are systematically analyzed. Multiple experimental phenomena are reproduced and interpreted, including pressure self-multiplication/demultiplication effects, small 'steps' on pressure distribution in the two-phase region, simultaneous occurrences of direct and reverse PTs, oscillatory distribution of pressure for weaker high-pressure phase, and a thin layer of high-pressure phase on a contact surface. During unloading, unexpected intensive plastic flow and reverse PT are revealed, which change the interpretation of experimental results. The effect of unloading and reloading paths on PTs is examined. Two types of pressure variations are revealed, which are qualitatively consistent within experimental observations for ZnSe and KCl. Obtained results lead to ways of controlling PTs by varying compression-torsion paths and can be utilized for the search of new high pressure phases, ways to reduce pressure for the synthesis of high pressure phases, and to retain them at ambient pressure.