A COMPUTER-SIMULATION STUDY OF DISPLACEMENT CASCADES IN ALPHA-TITANIUM

The damage produced by displacement cascades in the h.c.p. metal alpha -titanium at 100 K has been investigated by molecular dynamics using a many-body interatomic potential. This has been the first simulation o f 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 crys tal structure and cascade energy. Computer-generated plots have been u sed to visualize the nature and arrangement of the point defects produ ced in the cascade events. There is no discernible effect of the prima ry knock-on atom direction, and individual replacement sequences make only a minor contribution to the final damage state. Views of the disp laced atoms looking along the basal planes show very clearly regions w ith a liquid-like disorder, but they are small, even at 5 keV, and do not lead to vacancy clustering when crystalline order is restored. The efficiency of production of Frenkel pairs declines with increasing ca scade energy in a similar fashion to that found recently for cubic met als. Some interstitial clusters are formed in the cascade process in t itanium and self-interstitial atoms were observed to migrate preferent ially 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 displacemen t of atoms in the final state of two 2 and 5 keV cascades shows that t he mixing in our model of alpha-titanium is essentially isotropic.