Twinning and phase transformations in Zr crack tips

We present atomistic studies of the deformation mechanisms occurring in the crack-tip region during low temperature fracture of hcp Zr. Static simulations of mode I fracture in Zr single crystals were performed using empiric embedded atom (EAM) potentials. Cracks were introduced in six different crystallographic orientations. For each crack orientation two different crack-propagation directions were chosen. Plasticity on the crack tip in single crystal hcp Zr depends strongly on the orientation of the crack. Several competing processes are involved, including the phase transformation to the bcc structure. Twinning and dislocation emission are the other major mechanisms responsible for ductility. For the cases where the transformation to bcc was observed, a significant improvement in ductility results from the transformation.