Computer simulation of radiation damage in hexagonal close-packed metals.
Two HCP metals, titanium and zirconium, have been modelled using molecular
dynamics and recently developed many-body potentials. These two metals have
similar lattice parameters, c/a ratios, melting temperatures, elastic and dislocation
properties and, more importantly, responses to radiation damage(Griffith 1988,1989
& 1991, Hood 1988 & 1993), but differ by nearly a factor of two in atomic mass,
thereby allowing the direct investigation of the effect of mass on radiation damage in
the HCP system. Using the MOLDY code, successfully modified for the HCP
structure, these two models w re rigorously investigated with respect to their point
defect properties, displacement threshold energy response, and cascade processes. A
marked preference for interstitial sites within the basal plane was found, in accordance
with previous static studies on HCP metals. The displacement threshold energy
showed a complex dependence on orientation within the HCP structure, but at higher
energies this effect was swamped by structural disruptions during cascade
development. The effect of mass was exhibited as a proportional increase in the mean
displacement threshold energy, which carries over into cascade generation.
Cascade morphology was seen to undergo a transition at energies of -1 keV,
associated with the onset of true cascade conditions. This transition was reflected
most markedly in the relaxation time for the recombination phase beyond the cascade
peak, and explanation is presented for the transition in terms of ballistic, energetic and
temporal effects. The dissimilarities between the two models were found to be mainly
attributable to the mass difference. The condition of the cascade core at the peak was
seen to be close to that of a liquid, with some discrepancies which indicate a lack of
true melting, and an absence of the vacancy clustering often associated with a molten
cascade core. The approximation of liquid-like structure was supported by the
isotropy of the cascade-induced atomic mixing, despite the preference for basal-plane
movement in the solid state. In agreement with modelling of other metals, the defect
production efficiency for true cascade conditions was well below the NRT estimate,
and an empirical relationship between final Frenkel-pair numbers and PKA energy is
presented. SIA clustering occurred to a similar extent in both models, and small
clusters were highly mobile and confined to single <1120> rows in the basal planes.
The implications of these findings for microstructural evolution are discussed, along
with comparisons of the results with other systems.