Stress and plastic strain-induced phase transformations under high pressure: toward defect-induced material synthesis
|Date/Time:||Thursday, 14 Sep 2017 from 4:10 pm to 5:00 pm|
|Channel:||Condensed Matter Physics|
During compression in diamond anvil cell (DAC) without hydrostatic medium, 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 mostly by nucleation at the pre-existing defects (e.g., dislocations) below the yield 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 large plastic shear on high pressure in rotational DAC (RDAC) leads to numerous new phenomena, including drastic reduction in PT pressure and appearance of new phases that were not obtained without shear.1-3 A four-scale theory was developed and corresponding simulations were performed. Molecular dynamic simulations were used4 to determine lattice instability conditions under six components of the stress tensor, which demonstrate strong reduction of PT pressure under nonhydrostatic loading. At the nanoscale, nucleation at various evolving dislocation configurations is studied utilizing developed phase field approach.5,6 Possibility of reduction of PT pressure by an order of magnitude due to stress concentration at the shear-generated dislocation pile up is proven. At the microscale, strain-controlled kinetic equation is derived and is 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.7-9 A comprehensive computational study of the effects of the kinetic parameter, ratio of the yield strengths of high and low-pressure phases and the gasket, sample radius, and initial thickness on the PTs and plastic flow is performed. Various experimental effects are reproduced. Possible misinterpretation of experimental PT pressure is demonstrated, including experiments for pressure-induced PTs. The obtained results offer new methods of controlling PTs and searching for new high pressure phases, methods of retaining of high pressure phases at normal pressure, as well as methods of characterization of high pressure PTs in traditional DAC and RDAC. The results are summarized as possible routes for plastic strain and defect-induced material synthesis under high pressure.
Bio: Valery Levitas is currently Vance Coffman Faculty Chair Professor at Departments of Aerospace Engineering and Mechanical Engineering of ISU and Faculty Scientist at Ames Laboratory. He works for almost 40 years on different aspects of phase transformations, plastic deformations, and their interactions in various materials. He brought to the USA a unique device, rotational diamonad anvil cells, and initiated experimental and theoretical research on interaction between phase transformations and plasticity under high pressure.
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