Phase Transformations Under High Pressure and Large Plastic Deformations: Multiscale Theory and Interpretation of Experiments

It is known that superposition of large plastic shear at high pressure in a rotational diamond anvil cell (RDAC) or high-pressure torsion leads to numerous new phenomena, including drastic reduction in phase transformation (PT) pressure and appearance of new phases. Here, our four-scale theory and corresponding simulations are reviewed. Molecular dynamic simulations were used to determine lattice instability conditions under six components of the stress tensor, which demonstrate strong reduction of PT pressure under nonhydrostatic loading. At nanoscale, nucleation at various evolving dislocation configurations is studied utilizing a developed phase field approach. The possibility of reduction in PT pressure by an order of magnitude due to stress concentration at the shear-generated dislocation pileup is proven. At microscale, a strain-controlled kinetic equation is derived and utilized in large-strain macroscopic theory for coupled PTs and plasticity. At macroscale, the behavior of the sample in DAC and RDAC is studied using a finite-element approach. A comprehensive computational study of the effects of different material and geometric parameters is performed, and various experimental effects are reproduced. Possible misinterpretation of experimental PT pressure is demonstrated. The obtained results offer new methods for controlling PTs and searching for new high-pressure phases (HPPs), as well as methods for characterization of high-pressure PTs in traditional DAC and RDAC.