TEMPERATURE AND STRESS DEPENDENCE OF RHOMBOHEDRAL TWINNING IN SAPPHIRE

Temperature and stress dependence of rhombohedral twinning behavior of sapphire single crystals was investigated. Specimens were deformed in compression along the c-axis with an Instron universal testing machine below 1100 degrees C in order to avoid basal twinning and dislocation slip. Rhombohedral twinning formation can be explained by nucleation of a double kink in the twinning dislocation, while twin growth can be treated as a twinning dislocation movement which is driven by atomic diffusion at elevated temperatures and by mechanical slip at relatively low temperatures. The activation enthalpy and activation volume of rhombohedral twinning dislocation and also the activation energy of twinning dislocation movement were measured using Smidt's empirical equation and Arrhenius' equation, respectively.