Plastic deformation of quartz single crystals

Abstract

Dry and wet quartz single crystals with various orientations have been deformed, mostly in uniaxial compression, in a temperature range 400-1300°C and the induced dislocation structure has been studied in TEM. Dry quartz (H/Si < 80 ppm) appears to be quite strong. At T = 1300°C for a deviatoric stress ⋍ 1000 MPa it still exhibits a brittle behaviour. Dislocations with crystallographically controlled directions are induced only at the crack tips. Wet crystals (500 < H/Si < 1000 ppm) show a ductile regime at moderate temperature (450 to 700°C). a and c glides have similar flow stresses and one observes in TEM a progressive change of the dislocation substructure as the deformation proceeds : the beginning of the deformation seems to be controlled by lattice friction (dislocations with marked crystallographic orientations) ; for larger strain, recovery configurations (subgrain boundaries) occur while hardening is observed on the stress-strain curves. Such an evolution of the plastic behaviour of wet quartz is attributed to the slow but continuous precipitation of water during the course of the deformation process. A thermodynamic approach is used to calculate the equilibrium water concentration and the kinetic aspect of water precipitation. The results suggest that, at least for a prismatic glide, the initial dislocation multiplication, at the very beginning of the deformation, is probably driven by water precipitation (which is accompanied by the nucleation of a large density of small dislocation loops). The possible mechanisms for dislocation glide in dry and wet quartz are also discussed.