Erratic grain rotation during annealing of a Mg alloy interpreted via shear-coupling

We present an observation of erratic grain rotation in Mg alloy AZ91 during annealing at 500 °C. This confirms a finding reported recently by Liss et al. [Adv. Eng. Mater. 2024, 26(4), 2300470]. However, we show that the erratic rotation is actually not completely erratic. For the cases examined, rotation follows distinct segments in time characterised by a common rotation axis and a common shear coupling factor (inferred by comparing grain rotation with X-ray Laue spot intensity change). While the phenomenon is accentuated by the presence of a small fraction of molten phase at certain triple junctions and grain boundaries, which reduces constraint against grain rotation, its key features can be adequately explained in terms of disconnection-mediated shear coupling.