Temperature Dependent Dislocation Mobility in MgSiO3 Perovskite: An Atomic Scale Study

Abstract:

Heat transfer through the mantle is carried by convection, which involves plastic flow of the mantle constituents. Among these constituents, (Mg,Fe,Al)(Si,Al)O₃ perovskite is known to be the most abundant. This material is deformed at very low strain rate (from 10^-12 to 10^-16 s^-1), and under extreme pressure and temperature conditions (from 30 to 140GPa, 1500 to 4000°C). Its plastic behaviour is challenging to reproduce experimentally, but crucial for a better understanding of the Earth's dynamic.
The recent progress in modelling the behaviours of materials, which until now have been mostly used on metals, are applied here on MgSiO₃ perovskite (Mg-Pv). We characterize dislocations at the atomic scale, as the first step of a multi-scale modelling approach on Mg-Pv plastic deformation.

We model dislocations with [100] and [010] Burgers vectors (described within the Pbnm space group), which are the shortest lattice parameters in the orthorhombic structure. Dislocation cores are determined to be described at various pressures. The resistance to glide of the dislocations is quantified indicating that [100](010) and [010](100) are the easiest slip systems in Mg-Pv over the full pressure range of the lower mantle.
The effect of temperature is introduced by assimilating the thermal activation on dislocation lines to vibrations of a string lying into a potential valley. These vibrations allow the dislocation to overcome locally the energy barrier that represents the lattice friction, and then propagates under the effect of stress. With this model, by combining elastic theory of dislocations and calculations at the atomic scale, a first expression of the strain rate produced by dislocation glide is provided.

Left figure : Thermally activated propagation of dislocation over the energy barrier
Right figure : Shape of the crossing dislocation obtained from atomic scale modelling