Dynamic dislocation behavior in iron-doped magnesium oxide crystals containing dislocation dipoles

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Abstract
Edge- and screw-dislocation velocities in iron-doped magnesium oxide single crystals containing dislocation dipoles and 90 ppm iron have been measured as a function of the stress (at stresses below the macroscopic yield 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 Fe2+ and Fe3+ valence states over the stress and temperature regimes investigated. The edge and screw dislocations move faster in reduced samples than in oxidized samples. From the analysis of the edge- and screw-dislocation velocity data in terms of the activation parameters (volume, enthalpy, total enthalpy, and the stress exponent of dislocation velocity), it is proposed that the screw-dislocation mobility in MgO single crystals containing dislocation dipoles and 90 ppm iron in the oxidized state is controlled by the interaction of dislocations with noncentrosymmetric defects (FeMg ·−V″Mg). However, the edge-dislocation mobility in the oxidized crystals is governed by the interaction of the edge dislocations with the dislocation dipoles. Moreover, the edge- and screw-dislocation mobilities in the reduced state are also controlled by the interaction of the edge dislocations with the dislocation dipoles.