POINT-DEFECTS AND CLUSTERS IN THE HCP METALS - THEIR ROLE IN THE DOSE TRANSITION

The transition from low-dose to high-dose irradiation effects has only been studied extensively for zirconium, and hence a wide-ranging review of this subject for the hcp metals in general is not possible at present. A summary of the effects found in zirconium reveals the importance of the low symmetry of the hcp system in the mechanisms of transport of defects created by radiation damage and the ensuing development of damage microstructure. Solute atoms are also shown to be capable of having a strong influence on these phenomena. Hence, in order to gain an understanding of the complex nature of dose effects in the hcp metals, it is necessary to acquire a clear picture of the properties of point defects and their clusters in pure and impure metals. The present paper reviews the areas where Significant progress has been made in recent years, namely (i) intrinsic point defect properties found by computer modelling and inferred from experiment, (ii) the roles of interstitial and substitutional solute atoms in diffusion; (iii) the importance of diffusional anisotropy for intrinsic and extrinsic defects; (iv) the factors that influence the formation of vacancy dislocation loops by collapse of displacement cascades; (v) the evolution of interstitial and vacancy loops during irradiation; and (vi) the ways in which understanding of (i)-(v) can provide a model for the experimental observations of dose effects. It is concluded that although real advances have been made, there are many issues of fundamental importance that remain unresolved.