Neutron diffraction and acoustic emission were used in a single in situ experiment in order to study the deformation twinning of two ZM20 Mg alloys with significantly different grain sizes at room temperature. The combination of these two techniques facilitates the distinction between twin nucleation and twin growth. It was shown that yielding and immediate post-yielding plasticity in compression along the extrusion direction was governed primarily by twin nucleation, whereas plasticity at higher strains was presumably governed by twin growth and dislocation slip. It was further shown that, in the fine-grained alloy, collaborative twin nucleation in many grains dominates yielding, whereas twin nucleation in the coarse-grained alloy was progressive and occurs over a larger strain range. In addition, it was shown that, despite twin nucleation stresses increasing with decreasing grain size, roughly the same overall volume fraction of twins was formed in both fine and coarse parent grains. This confirms the difficulty of the alternative deformation modes and suggests a negligible suppressive effect of grain size on twinning in the case of the strongly textured fine-grained alloy. The current results also show that twins in the coarse-grained alloy were born less relaxed with respect to surrounding polycrystalline aggregate than those in the fine-grained alloy. This was believed to lead to lower reversal stresses acting on twin grains in the coarse-grained alloy upon unloading and thus to less untwinning and thus to a smaller pseudoelastic-like hysteresis.

Investigation of Deformation Twinning in a Fine-Grained and Coarse-Grained ZM20 Mg Alloy - Combined in situ Neutron Diffraction and Acoustic Emission. O.Muránsky, M.R.Barnett, D.G.Carr, S.C.Vogel, E.C.Oliver: Acta Materialia, 2010, 58[5], 1503-17