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Title: Computer simulation of radiation damage in hexagonal close-packed metals.
Author: Wooding, Stephen John.
ISNI:       0000 0001 3572 2466
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 1994
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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.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Titanium; Zirconium