Proliferation of Twinning in HCP Metals: Application to Magnesium

Abstract: Plastic deformation of metallic alloys usually takes place through slip, but occasionally involves twinning. In particular, twinning is important in hexagonal close packed materials where the easy slip systems are insufficient to accommodate arbitrary deformations. While deformation by slip mechanisms is reasonably well understood, less remains known about deformation by twinning. Indeed, the identification of relevant twinning modes remains an art. In this paper, we develop an universal framework combining fundamental kinematic definition of twins with large scale atomistic calculations to predict twinning modes of crystalline materials. We apply this framework to magnesium where there are two accepted twin modes -- tension and compression, but a number of anomalous observations. Surprisingly, our framework shows that there are a very large number of twinning modes that are important in the deformation process of magnesium consistent with the anomalous observations. Thus, in contrast to the traditional view where plastic deformation is kinematically partitioned between a few modes, our result argues that the physics of deformation in HCP materials is governed by an energetic and kinetic competition between numerous possibilities. Consequently, our findings suggest that the commonly used models of deformation physics need to be revisited in order to take into account a broader and richer variety of twin modes, and potentially points to new avenues of improving the mechanical properties.