Deformation Twinning During Impact of a Titanium Cylinder: Numerical Investigation

It is well known that polycrystalline metals (especially those with bcc or hcp structures), when subjected to impact, undergo two inelastic processes — slip and twinning. Since the work of Taylor (1948) the former one has been studied extensively; while more recently, deformation twinning has attracted attention of some researchers, e.g. Bolling & Richman (1965), Armstrong & Worthington (1974) and Zerilli & Armstrong (1988). Zerilli and Armstrong (1988) suggested that the major effect of twinning is a refinement of the grain size. Based on this assumption, they proposed a model for twinning and showed that much better agreement with experiments can be obtained if, in addition to deformation by slip, deformation twinning is also considered. Similar conclusions were reached by Holt, Mock, Zerilli and Clark (1994) who analyzed the Taylor impact of a titanium specimen. In this study, we model the Taylor impact of a titanium cylinder. We assume that the problem is axisymmetric and solve the full dynamic equations by using the Galerkin finite element method. Our results show that the energy absorbed during twinning and the deformation due to twinning are relatively small. We also demonstrate the dependence of the results on the initial grain size of the material. Specifically, by modeling two materials of widely differing grain sizes, we show that the long-grained material twins substantially more than the small-grained material.