Theory of Frank loop nucleation at elevated temperatures

Abstract

The nucleation of Frank loops from immobile primary interstitial clusters produced by irradiation in collision cascades is considered. Owing to the thermal instability of primary vacancy clusters at elevated temperatures, interstitial clusters receive, on the average, a net vacancy flux acting against their further growth. However, it has been shown previously that, under the combined action of the dislocation bias and continuous loop coalescence, sufficiently large interstitial loops manage to grow despite the net flux of vacancies. The nucleation of a growing interstitial loop, then, constitutes the growth, beyond a critical loop size, of an individual small cluster in an ensemble of clusters that are shrinking on the average. In this paper, the rate of interstitial loop nucleation based on this theory is developed analytically. We show that fluctuations in point-defect fluxes produced by the random (in time and space) generation of point defects in packets during cascade irradiation may significantly increase, that is by orders of magnitude, the probability of a small cluster to survive and grow. It is found that this flux is sufficient to explain the loop number densities experimentally observable in stainless steels at elevated temperatures, as well as to provide through the dislocation climb the steady-state swelling rate of about 1% (NRT)⁻¹ (displacements per atom)⁻¹.

Acknowledgement

The authors are grateful for funding support from the Hong Kong Research Grant Council (research grants 5173/01E, 5167/01E and 5177/02E).

Notes

^a  Stoller and Odette (1987).