A computer simulation study of displacement cascades in α-titanium

The damage produced by displacement cascades in the h.c.p. metal α-titanium at 100K has been investigated by molecular dynamics using a many-body interatomic potential. This has been the first simulation of cascades in the h.c.p. structure. 32 cascades ranging in energy from 0.3 to 5 keV were generated in order to study the effects of the crystal structure and cascade energy. Computer-generated plots have been used to visualize the nature and arrangement of the point defects produced in the cascade events. There is no discernible effect of the primary knock-on atom direction, and individual replacement sequences make only a minor contribution to the final damage state. Views of the displaced atoms looking along the basal planes show very clearly regions with a liquid-like disorder, but they are small, even at 5 ke V, and do not lead to vacancy clustering when crystalline order is restored. The efficiency of production of Frenkel pairs declines with increasing cascade energy in a similar fashion to that found recently for cubic metals. Some interstitial clusters are formed in the cascade process in titanium and self-interstitial atoms were observed to migrate preferentially along basal planes during the thermal spike. Although the h.c.p. structure may be expected to exhibit anisotropy in the atomic mixing in cascade-inducing radiation, analysis of the mean square displacement of atoms in the final state of two 2 and 5 keV cascades shows that the mixing in our model of α-titanium is essentially isotropic.