Twinning behavior and its effect on mechanical behavior of commercial titanium at cryogenic temperature

Twinning behavior in tensile deformation of commercial pure titanium as well as its grain size dependence were investigated. The microstructure after deformation was observed by use of optical microscopy and transmission electron microscopy, and the twin fraction was obtained as well. The results show that twins appear mainly at the stage of strain hardening and twin fraction increases with increase of the plastic deformation. The size of twins at early stage of plastic deformation is larger than that at latter stage, however, larger twins will break under large deformation, which forms some zones with high density of twins together with small twins. The microstructures also show that twinning cooperates with slip in the deformation of titanium at cryogenic temperature. Commercial pure titanium with grain size of 18 μm and 55 μm were obtained after annealed at 650°C and 800°C, respectively. The tensile properties show that ductility of titanium increases with increase of grain size from 18 μm to 55 μm at room temperature and -196°C, and twinning seems to become more common in coarsening grains. All the results imply that higher ductility in coarsening grain is related to the mechanisms of twinning induced plasticity.