Strain-induced phase transformations under compression, unloading, and reloading in a diamond anvil cell

Abstract

Strain-induced phase transformations (PTs) in a sample under compression, unloading, and reloading in a diamond anvil cell are investigated in detail, by applying finite element method. In contrast to previous studies, the kinetic equation includes the pressure range in which both direct and reverse PTs occur simultaneously. Results are compared to the case when "no transformation" region in the pressure range exists instead, for various values of the kinetic parameters and ratios of the yield strengths of low and high pressure phases. Under unloading (which has never been studied before), surprising plastic flow and reverse PT are found, which were neglected in experiments and change interpretation of experimental results. They are caused both by heterogeneous stress redistribution and transformation-induced plasticity. After reloading, the reverse PT continues followed by intense direct PT. However, PT is less pronounced than after initial compression and geometry of transformed zone changes. In particular, a localized transformed band of a weaker high pressure phase does not reappear in comparison with the initial compression. A number of experimental phenomena are reproduced and interpreted. Strain-induced phase transformations (PTs) in a sample under compression, unloading, and reloading in a diamond anvil cell are investigated in detail, by applying finite element method. In contrast to previous studies, the kinetic equation includes the pressure range in which both direct and reverse PTs occur simultaneously. Results are compared to the case when "no transformation" region in the pressure range exists instead, for various values of the kinetic parameters and ratios of the yield strengths of low and high pressure phases. Under unloading (which has never been studied before), surprising plastic flow and reverse PT are found, which were neglected in experiments and change interpretation of experimental results. They are caused both by heterogeneous stress redistribution and transformation-induced plasticity. After reloading, the reverse PT continues followed by intense direct PT. However, PT is less pronounced than after initial compression and geometry of transformed zone changes. In particular, a localized transformed band of a weaker high pressure phase does not reappear in comparison with the initial compression. A number of experimental phenomena are reproduced and interpreted.

Plastic flows and phase transformations in materials under compression in diamond anvil cell: Effect of contact sliding

“…11 As a result, theoretical and finite-element methods have been developed and applied for investigation of the evolution of stresses, strains, and concentration of phases in the entire sample during plastic flow and PTs with the growth of external force. 10,14,15,17,18 It is a coupled problem of mechanics and PT with a large deformation, which thus leads to high complexity in simulations using FEM software.…”

“…Thus, the entire field of high pressure physics and material science is based on the ability to create frictional resistance to the radial plastic flow in the thin sample during its compression. At the same time, though the first numerical results [15][16][17][18][19] were successful in interpreting multiple experimental phenomena, the simplest model was considered based on a strong assumption: there is no slipping on the contact surface between the sample and the diamond anvil. As a result, a real large-sliding contact problem was degraded into a problem under zero displacements along the boundary.…”

Coupled phase transformations and plastic flows under torsion at high pressure in rotational diamond anvil cell: Effect of contact sliding

“…12,13,15,16,19 The applicability of the perfectly plastic and isotropic model with the yield strength independent of the deformation history is justified in Ref. 20 under monotonous loading for various classes of materials (metals, rocks, powders, etc.)…”

“…While first numerical results for modeling strain induced PTs in DAC 12,16,19 and in RDAC 13 have been very successful in interpreting multiple experimental phenomena, they involve a strong assumption: there is full cohesion on the contact surface between sample and diamond anvils. In this case, radial displacements at the contact surface are zero and circumferential displacements of material are equal to circumferential displacements of an anvil.…”

“…9 That is why finite-element method (FEM) simulations have been developed to study the evolution of all fields, including concentration of high pressure phases, and effects of material parameters on them, as well as to interpret experimental phenomena. 12,13,15,16 It is a nonlinear problem of coupled PTs and mechanics with large plastic deformations and displacements, which leads to nontrivial simulation challenges using FEM, including convergence.…”