Point defect diffusion in α-Zr

Abstract

The present review of defects in α-Zr is concerned with atomic and vacancy diffusion and with solute effects on radiation damage. The primary aim, in addressing atomic diffusion, is to try to expose the characteristics and systematics of intrinsic diffusion. This is done, both in a general context and in terms of problems and features more specifically associated with α-Zr. Among the latter are the anisotropic crystal structure, the limited temperature interval of α-phase stability, imposed by the α-β (hcp-bcc) transformation, and the tendency for Zr to have high residual impurity levels. It is emphasized that reliable, intrinsic, diffusion data can only be obtained through the use of high-purity single-crystal material. Vacancy diffusion is considered in terms of experimental data on irradiation damage. Solute effects on irradiation damage are discussed on the basis of the interactions of solutes with vacancies and self-interstitial atoms (SIA). A consideration of atomic diffusion data indicates that self-diffusion in α-Zr is intrinsically normal and that self-diffusion data associated with abnormal behaviour are probably expressive of diffusion dominated by extrinsic mechanisms: both structural and impurity-related defects appear to be important here. Intrinsic diffusion characteristics (diffusion coefficients ( D) and associated activation energies and pre-exponential factors) tend to scale in a remarkably simple manner with atomic size, with diffusion taking place by both interstitial and substitutional mechanisms. Small atoms diffuse mainly by interstitial paths, with associated D values which may be many orders of magnitude higher than those associated with substitutional solutes. Measurements of diffusion anisotropy show that diffusion is generally faster parallel to the c- axis than perpendicular to it: this is well established for fast-diffusing, interstitial-like solutes but not well known for substitutional solutes. The vacancy migration characteristics of α-Zr present an ongoing enigma. Experimental data from electron microscopic and positron annihilation spectroscopic (PAS) measurements of irradiation damage recovery are readily interpreted in terms of abnormally facile vacancy migration, however there does not appear to be any evident theoretical support for such a phenomenon as an intrinsic process. This raises the possibility of an extrinsic mechanism. In addition, there seems to be sufficient ambiguity associated with the data to allow their interpretation in terms of a model wherein vacancy migration is inherently normal. Solute effects on irradiation damage and recovery are discussed in a general context with a consideration of the attraction of SIAs for small solute atoms and of solute-vacancy interactions in terms of electrostatic and bond-strength approaches. Specific attention is given to the role of Sn, since it is shown, from PAS measurements, that radiation damage and recovery in Zr can be profoundly altered by alloying additions of this element. Some understanding of these results can be found in terms of defect interactions with clustered configurations of solute atoms. The results suggest the basis of a general approach towards the development of irradiation-damage resistant alloys.