Thermally activated flow parameters in the deformation of MgO single crystals

Abstract

Strain-rate cycling methods have been used to determine the thermally activated flow parameters in the compressive deformation of MgO single crystals. Activation volumes of the order of 100b ³ have been measured for deformation below room temperature, indicating dislocation-point defect interactions as the rate controlling mechanism for hardening at these temperatures, whileV ^* values of the order of 1000b ³ have been measured at and above room temperature. This, together with the fact that the effective stress remains constant with strain at these temperatures identifies the interaction of dislocations with dislocation dipoles as the rate controlling mechanism in this temperature range. The Gibbs free energy for deformation,ΔG, equals the activation enthalpy,ΔH, at low temperatures and the deviation ofΔG fromΔH above room temperature may be attributed to entropy effects.