In-situ, ex-situ EBSD and (HR-)TEM analyses of primary, secondary and tertiary twin development in an Mg–4 wt%Li alloy

This study presents experimental evidence for the sequence of activation and the evolution of primary tension (TTW), secondary compression (CTW) and tertiary double twinning (DTW) in an extruded Mg–4 wt%Li alloy during uniaxial compression by means of in-situ and ex-situ EBSD and (HR-)TEM. In-situ EBSD analysis reveals that the secondary CTWs propagate very slowly, while the tertiary DTWs propagate faster once nucleated. Thereby, when DTW-ing overtakes the CTW, CTW boundary migration stops, indicating that the thickness of the CTW is limited by the onset of internal DTW-ing. This finding confirms some prior proposals based on post-mortem analyses. High-resolution (HR-)TEM analysis of a −18% compressed sample reveals significant basal slip activity and a high amount of basal stacking faults within the primary TTW as well as within the secondary CTW. Additionally, the HR-TEM analysis showed a large amount of basal dislocations within the area of the CTW-DTW transition. These findings give strong experimental evidence that the observed predominance of type 1 DTW-ing is related to the dissociation of extended basal dislocations.