Coupled Severe Plastic Deformations, Phase Transformations, and Microstructure Evolution under High Pressure: Four-scale Theory and In-situ Experiments*

During compression in a diamond anvil cell (DAC), materials undergo large plastic deformations, which cause various phase transformations (PTs). We formulated new concept that these PTs should be treated as plastic strain-induced PTs under high pressure rather than pressure-induced PTs. Pressure- and stress-induced PTs occur by nucleation at the pre-existing defects below the yield. Strain-induced PTs occur by nucleation at new defects (e.g., dislocation pileups) permanently generated during plastic flow. Strain-induced PTs require completely different thermodynamic and kinetic treatments and experimental characterization. New in situ experimental results are obtained under compression of materials in a DAC and torsion in rotational DAC (RDAC). Drastic reductions of the PT pressure compared with hydrostatic loading and the appearance of new phases are demonstrated. Thus, graphite in RDAC was transformed to nanocrystalline hexagonal and cubic diamond at 0.4 and 0.7 GPa, respectively, which are 50 and 100 times lower than the PT pressures under hydrostatic loading! This could be a precursor of new technology of plastic strain (defect) induced diamond synthesis. New rules of coupled severe plastic deformations, PTs, and grain and dislocation structure evolution under high pressure are discovered for Zr. Numerous phenomena with potential technological applications are revealed for combined effect of plastic flow and particle size on PTs between 7 Si phases. A four-scale theory was developed. Molecular dynamics and first-principle simulations were used to determine PT criteria under six stress tensor components. At the nanoscale and microscale, nucleation at evolving dislocation pileups was studied with developed nanoscale and scale-free phase-field approaches. A 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/RDAC is studied using the finite element approach. Various experimental effects are reproduced. The obtained results offer new fundamental understanding of strain-induced PTs and methods of searching for new high-pressure phases and phenomena.