Micromechanical response of pure magnesium at different strain rate and temperature conditions: twin to slip and slip to twin transitions

Abstract

The strain rate (ε˙) and temperature (T) dependent mechanical response of single crystal magnesium (Mg) micropillars compressed along the [2¯110] direction (a-axis) is investigated from room temperature to 573 K and from 10-3 to 100 s-1. The loading direction was chosen to disfavour basal slip activation by a low Schmid factor, allowing the investigation of the rate-sensitivities of extension twinning and prismatic slip. For T ≤ 423 K, the plasticity was governed by extension twinning for the entire range of the applied strain rates. At T > 423 K and for ε˙ ≤ 10 s-1, extension twinning did not occur and a continuous plastic flow induced by prismatic dislocation mediated plasticity was observed: the twin to slip transition takes place due to the decrease of the critical resolved shear stress of non-basal slip. For ε˙ > 10 s-1, however, the accommodation of the plastic deformation by activation of prismatic slip is not enough to match the applied deformation rate, favouring again deformation twinning. The first part of this work provides a complete overview of the mutual effects of T and ε˙ on the transition points of deformation modes in Mg at the microscale. In a second stage, the influence of thermal and kinetic contributions on the evolution of the flow stress leading to the slip to twin transition at 573 K has been assessed in more detail. Within the slip-dominated plasticity regime, this work provides a quantitative assessment of the increases in the saturation stress (stage III) with ε˙ at high temperature, showing how the strain rate dependency of the dislocation generation rate in the pillar and escape rate at the free surfaces of the structure controls the stress evolution in Mg microcrystals.