Elasto-viscoplastic self consistent modeling of the ambient temperature plastic behavior of periclase deformed up to 5.4 GPa

Abstract

Anisotropy has a crucial effect on the mechanical response of polycrystalline materials. Polycrystal anisotropy is a consequence of single crystal anisotropy and texture (crystallographic preferred orientation) development, which can result from plastic deformation by dislocation glide. The plastic behavior of polycrystals is different under varying hydrostatic pressure conditions, and understanding the effect of hydrostatic pressure on plasticity is of general interest. Moreover, in the case of geological materials, it is useful for understanding material behavior in the deep earth and for the interpretation of seismic data. Periclase is a good material to test because of its simple and stable crystal structure (B1), and it is of interest to geosciences, as (Mg,Fe)O is the second most abundant phase in Earth's lower mantle. In this study, a polycrystalline sintered sample of periclase is deformed at ∼5.4 GPa and ambient temperature, to a total strain of 37% at average strain rates of 2.26 × 10−5/s and 4.30 × 10−5/s. Lattice strains and textures in the polycrystalline sample are recorded using in-situ synchrotron x-ray diffraction and are modeled with Elasto-Viscoplastic Self Consistent (EVPSC) methods. Parameters such as critical resolved shear stress (CRSS) for the various slip systems, strain hardening, initial grain shape, and the strength of the grain–neighborhood interaction are tested in order to optimize the simulation. At the beginning of deformation, a transient maximum occurs in lattice strains, then lattice strains relax to a “steady-state” value, which, we believe, corresponds to the true flow strength of periclase. The “steady state” CRSS of the 11011¯0 slip system is 1.2 GPa, while modeling the transient maximum requires a CRSS of 2.2 GPa. Interpretation of the overall experimental data via modeling indicates dominant 11011¯0 slip with initial strain softening, followed by strain hardening. This approach illustrates the utility of combining EVPSC and experimental data to understand deformation of materials at high pressures.