Anomalous temperature effects on the twinning-dislocation transition in magnesium: in-situ quantitative thermal-mechanical investigations

The effect of temperature on the plastic deformation mechanisms in submicron magnesium (Mg) single crystal is investigated by performing in-situ mechanical tests inside transmission electron microscope over the temperature range of 25 and 300 °C. The deformation in submicron Mg under compression along $\left[01\overline{1}0\right]$ orientation transitions from twinning- to dislocation-dominated one over the temperature range of 210-250 °C. Below this temperature range, deformation is governed by $\left\{10\overline{1}2\right\}$ deformation twins, while it is dominated by non-basal 〈a〉 dislocation slip above it. Under tension along [0001], due to the higher critical resolved shear stress (CRSS) for the pyramidal 〈c+a〉 dislocation slip (compared to that of $\left\{10\overline{1}2\right\}$ deformation twins), no transition involving twins and pyramidal 〈c+a〉 dislocation slip occurs over the tested temperature range. Interestingly, an abnormal increase in the CRSS of $\left\{10\overline{1}2\right\}$ deformation twinning with the temperature is observed, both under compression and tension. Critical experiments show that the observed anomalous temperature dependence is not caused by the stress required for twin boundary migration, which remains invariant with temperature, but may be attributed to the lack of heterogenous nucleation sites for twin nucleation at high temperatures. These findings provide new insights into the transition from twinning- to dislocation-dominated plasticity in Mg and its alloys at elevated temperatures. The methodology can be extended to investigate the high-temperature deformation behavior and mechanisms in other materials.