{10-12} Twinning Mechanism During <i>In Situ</i> Micro-Tensile Loading of Pure Mg: Role of Basal Slip and Twin-Twin Boundary Interaction

Abstract

An SEM in situ uniaxial tensile testing setup allowing HR-EBSD acquisition during deformation was used to study the extension twinning mechanism in magnesium (Mg) at the micron scale. Structures with strain gauge sizes of 20 × 5 × 8 µm were fabricated by Ga-FIB in pure Mg with two different crystal orientations, respectively perfectly aligned with, and at 5° to, the [0001] axis. Limited {10-12} twin formation was identified in the former case, while twinning was found to largely accommodate the plastic deformation in the latter case. These two different mechanisms are explained by the activation of basal slip when loading at 5° to the c-axis, which triggers {10-12} twin nucleation and strongly favors twin growth and propagation. The other orientation shows the activation of pyramidal slip together with only limited {10-12} twin growth. The critical resolved shear stress for {10-12} twinning has been determined to be ten times higher than in bulk material due to size effects. 3D HR-EBSD mapping enabled reconstruction of the three dimensional twin structure after deformation. From this, the interaction between twinning dislocations ahead of a twin tip and a pre-existing twin boundary was studied, where the GND distribution and the local shear stress in the active twin variant coordinate system were determined. The results show plastic accommodation up to ~11% of strain through transmission of twinning dislocations across a twin boundary, activation of several slip systems, and the formation of a third extension twin variant.