Stochastic twinning in confined volumes of Mg: Insights from in-situ micromechanical testing and atomistic simulations
Tensile twinning plays a central role in accommodating -axis plasticity in Mg. In bulk Mg, twinning typically shows a relatively deterministic response with a low critical stress, whereas in confined volumes it exhibits pronounced scatter, complicating the prediction of small-scale mechanical behavior. In this study, we investigate the origin of this stochasticity by combining site-specific micropillar compression with atomistic simulations. Experiments show that under -axis compression, plastic deformation is dominated by {10-12} twinning, with each discrete stress drop in the stress-strain response marking the activation and rapid advance of a twin. Atomistic simulations further separate twinning into two mechanistic regimes: nucleation and longitudinal propagation occur in a high-stress, shuffle-assisted regime, whereas lateral thickening proceeds in a low-stress regime controlled by disconnection glide. Linking these mechanistic insights with post-mortem characterization of deformed pillars demonstrates that the scatter in measured yield stresses arises from stochastic selection among competing twinning pathways, governed by the local defect landscape (presence, distribution, and morphology of pre-existing defects). Overall, this work identifies an atomistic basis for size-dependent stochastic twinning in Mg and provides a general framework for materials whose plasticity is controlled by discrete activation events.