Modeling irradiation growth of zirconium and its alloys

Abstract

This paper outlines the issues and methodology involved in the modeling of irradiation growth in Zr and its alloys, emphasizing on the effects of production bias and of anisotropic diffusion of the point defects. Due to the anisotropy of the crystal structure, the usual rate theory model for cubic metals, which assumes the isotropic diffusion of point detects, do not accurately reflect the reaction kinetics of point-defects in Zr. A model based on the reaction rate theory of anisotropically diffusing reactants predicts that the duffisional anisotropy difference (DAD) between vacancies and interstitials can produce a large bias. This bias is a zero-order effect and completely dominates the conventional dislocation bias caused by the first-order elastic interaction between the point-defect and the sink. Thus, contrary to the usual rate theory model, the bias for edge dislocations in Zr depends on their line directions, and need not be biased towards interstitials. Grain boundaries and surfaces are biased sinks also, and are biased towards the vacancies or interstitials or vacancies according to their orientations. This large variety of biases for sinks adds a new dimension to the complexity of deformation behaviour to non-cubic metals. In this paper, we show that the diffusional anisotropy can be obtained from loop-growth measurements under HVEM. Under cascade damage conditions, the production and evolution of primary clusters is taken into account within the production bias theory. The possibility of fast diffusing vacancy—impurity atom pairs is also considered. The intergranular compatibility requirement in the growth of polycrystalline samples is satisfied using the self-consistent model. Application of the present model to the growth behaviour of polycrystalline Zircaloy-2 yields results which agrees very well with the observation.

Key Words:

• Irradiation growth
• Zr
• Zircaloy 2
• Anisotropic diffusion
• Production bias