Influence of plastic properties on the grain size effect on twinning in Ti and Mg

Deformation twinning is an inevitable mode of plastic deformation in hexagonal dose packed (HCP) crystals and has been considered a hinderance to achieving formability and ductility. It has long been recognized that reductions in grain size can lower the propensity for twinning. In prior work, EBSD statistical analyses on big data sets of twins in pure Ti and Mg were carried out for the same twin type, loading conditions, initial texture, and range of grain sizes. Here, we revisit these data sets to investigate the role of material plastic properties on grain size effects on deformation twinning. We show that while reductions in grain size dramatically lower the number of twins per grain and the twin thickness in both metals, these effects are several times stronger in Ti than that for Mg. To rationalize this difference, full-field, 3D crystal plasticity calculations are performed. The analysis indicates that the stronger grain size effect in Ti arises due to the larger critical resolved shear stress (CRSS) of its easiest slip system, making Ti less accommodating of the twin shear than Mg. This result points to plastic properties as additional variables that can be tuned towards developing HCP materials for structural applications.