During compression in diamond anvil cell (DAC), materials undergo large plastic deformations, which cause growth of pressure and various phase transformations (PTs). The key point is that these PTs should be treated as strain-induced PTs under high pressure rather than pressure-induced PTs. Pressure- and stress-induced PTs occur by nucleation at the pre-existing defects (e.g., dislocations) below the yield strength of a material. Strain-induced PTs occur by nucleation at new defects generated during plastic flow. Strain-induced PTs require completely different thermodynamic and kinetic treatment and experimental characterization.1 Also, superposition of plastic shear on high pressure in rotational DAC (RDAC) leads to numerous phenomena, including reduction in PT pressure up to one to two orders of magnitude and appearance of new phases.1-4 For example, plastic shear reduced PT pressure from graphite to diamond from 70 GPa under quasi-hydrostatic conditions to 0.7 GPa.4 Here, four-scale theory was developed and corresponding simulations were performed. Molecular dynamic5 and first-principle6 simulations were used to determine lattice instability conditions under all six components of the stress tensor, which demonstrate strong reduction of PT pressure under nonhydrostatic loading. At the nanoscale and microscale, nucleation at various evolving dislocation configurations was studied utilizing developed nanoscale7,8 and scale-free9 phase field approaches. Possibility of reduction of PT pressure by more than an order of magnitude due to stress concentration at the shear-generated dislocation pile up is proven. At the microscale, strain-controlled kinetic equation was derived and utilized in the large-strain macroscopic theory for coupled PTs and plasticity. At the macroscale, the behavior of the sample in DAC and RDAC is studied using finite element approach.10-12 Various experimental effects are reproduced. Possible misinterpretation of experimental PT pressure is demonstrated. The obtained results offer new fundamental understanding of strain-induced PTs under high pressure in DAC and RDAC and methods of controlling PTs and searching for new high pressure phases. They also propose the first steps in new characterization of high pressure PTs both in traditional DAC and RDAC.
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