Grain scale bursts of plasticity in Mg-4Zn via high energy X-rays: Towards twin observation in real-time

Abstract

The elastic-plastic transition in magnesium alloy Mg-4.5Zn is marked by bursts of deformation. A relatively short exposure time (44 ms) polychromatic transmission Laue experiment is employed to quantify the magnitude and frequency of these events. Abrupt displacement events seen in tracked Laue peaks reflect (mostly) sudden changes in grain orientation that accompany bursts of deformation. Deformation bursts are seen that are co-ordinated amongst nearby grains. The burst rate peaks at the onset of full plasticity. We associate the most significant peak displacement events to twinning events and this is supported by the twinned structures observed using electron microscopy. Bursts were often preceded and followed by a stasis in peak movement. This is interpreted via the hypothesis that twins adopt a particular ‘birth size’ upon formation and that subsequent growth occurs at a later point. The twin aspect ratio inferred from peak analysis and those evident in the microstructure are coarser than elastic predictions and we employ numerical analysis to show this is likely to be due to concomitant bursts of accommodating lattice dislocations.