Influence of temperature on twinning dominated pop-ins during nanoindentation of a magnesium single crystal

Abstract

The present paper examines the temperature sensitivity of tensile twinning in a magnesium single crystal during nanoindentation of the prismatic plane. High temperature indentations from 25 °C to 250 °C were employed on a well polished magnesium single crystal {10-10} plane. For a indentation curve displaying a pop-in, a single twin was seen on the sample surface using Atomic Force Microscopy (AFM) imaging. For indentations that produced no pop-in, no twinning was observed. We thus conclude the pop-in arises from a twinning event in the present case. With increasing temperature, the mean pop-in load (measured from 200 repeat indentations of each testing temperature) drops markedly. This is interpreted by the thermal activation of nucleation of lattice dislocations, which immediately trigger a twinning event. Thermal activation analysis yields activation energies that are consistent with this idea. With increasing temperature the pop-ins became deeper and the twins, after further indentation, showed more growth. It is likely that non-basal slip is activated in the stress concentrations that arise during twinning and the thermal activation of this slip accounts for the observed temperature effects. It is concluded that in interpreting the temperature sensitivity of twinning stresses, any associated lattice dislocation activity must be considered.