Plastic deformation of forsterite: dislocation interactions and lattice friction. A numerical mesoscopic approach

The viscosity profile in the Earth's mantle depends on the rheological properties of high-pressure minerals. The plastic behaviour of minerals is controlled by the motion of crystal defects, including dislocations. Recent progresses in mesoscopic scale simulations enable to make the transition between the individual behaviour of dislocations and plastic deformation of single crystals. The evolution of the dislocation density with stress is important in minerals because it affects the rheological laws, and because it can be directly used as a piezometer, either in experiments, or in naturally deformed rocks. In this perspective, dislocation dynamics simulation, initiated by Kubin and Canova, is used to understand the collective behaviour of dislocations and the strain hardening in forsterite single crystals. Plastic deformation of forsterite is likely to be controlled by lattice friction, at least at temperatures well below the melting point. The simulation enables dislocation segments to move in response to the effective stress with a characteristic mobility. These mobilities have been determined from yield stresses of single crystals deformed in single slip orientations at various temperatures. Strain hardening occurs when, due to multiplication mechanism during plastic deformation, the dislocation density increases. Plastic flow becomes then controlled by the dislocation interactions. We have studied interactions between dislocations from intersecting slip planes based on an elastic calculation and on the simulation. Finally, simulations with several slip system operating simultaneously are in process. This will enable us not only to obtain the strain hardening, but also understand the elementary processes taking place in the plastic deformation of forsterite.