Cascade morphology transition in bcc metals

Energetic atom collisions in solids induce shockwaves with complex morphologies. In this paper, we establish the existence of a morphological transition in such cascades. The order parameter of the morphology is defined as the exponent, $b$, in the defect production curve as a function of cascade energy ($N_F \sim E_{MD}^b$). Response of different bcc metals can be compared in a consistent energy domain when the energy is normalized by the transition energy, $\mu$, between the high- and the low-energy regime. Using Cr, Fe, Mo and W data, an empirical formula of $\mu$ as a function of displacement threshold energy, $E_d$, is presented for bcc metals.