The Effect of Size on the Deformation Twinning Behavior in Hexagonal Close-Packed Ti and Mg

In hexagonal close-packed (HCP) structural materials, the limited activation of different slip mechanisms results in alternative deformation mechanisms, such as twinning, which become relevant to plasticity. As external/internal dimension refinement affects operative mechanisms and is commonly used to tune the mechanical properties of materials, understanding the effect of size on deformation twinning in HCP materials is a critical issue for improving their strength and ductility. Recent in situ and ex situ small-scale testing experiments have generated insights into size effects on twinning by deforming single-crystal systems with different sizes. In this article, we review some of the recent results in this field, including studies of the size-related deformation twinning behavior in Ti, Mg, and their alloys. The effect of size on deformation twinning in these systems is remarkable, resulting in a significant change in the mechanical properties of the materials. Deformation twinning can be restricted by the size effect in certain size regimes and materials but also can be promoted by the presence of surfaces at extremely small scales. The correlation of these two effects in two different HCP materials is discussed.