Drag Mechanisms Controlling Dynamic Dislocation Behavior in MgO Single Crystals

Abstract

Edge and screw dislocation velocities in “pure” and iron-doped magnesium oxide single crystals, with and without dislocation dipoles, have been measured as a function of stress, temperature, and valence state of the iron impurities to identify the rate-controlling drag mechanisms for dislocation mobility. Edge dislocations have been observed to move faster than screw dislocations in the valence states of iron impurities over the stress and temperature regimes investigated. The edge and screw dislocations move faster in reduced (Fe+2) samples than in oxidized (Fe+3) samples. From the analysis of the data for the edge and screw dislocation velocities in terms of the activation parameters (activation volume, activation enthalpy, total activation enthalpy, and the stress exponent of dislocation velocity), it is suggested that the edge and screw dislocation mobilities in “pure” MgO single crystals in the reduced state are controlled by Peierls mechanism, with thermally activated double-kink nucleation as the rate-limiting step. The calculated values of the Peierls stress for edge and screw dislocation mobilities in “pure” MgO crystals in the reduced state are 0.6 x 108 and 1.7 x 108 Nm−2, respectively.