The nucleation of Frank loops from immobile primary interstitial clusters in collision cascades, produced by irradiation, was considered. Due to the thermal instability of primary vacancy clusters at high temperatures, the interstitial clusters tended to receive a net vacancy flux which acted against their further growth. It had been shown previously that, under the combined action of dislocation bias and continuous loop coalescence, sufficiently large interstitial loops nevertheless managed to grow in spite of the net flux of vacancies. Nucleation of a growing interstitial loop then reflected the growth, beyond a critical loop size, of an individual small cluster in an ensemble of clusters that was shrinking on average. The rate of interstitial loop nucleation, based upon this theory, was developed analytically here. It was shown that fluctuations in point-defect flux, produced by the random (temporally and spatially) generation of point defects in packets during cascade irradiation, could increase (by orders of magnitude) the probability of a small cluster surviving and growing. It was found that this flux was sufficient to explain the loop number-densities which had been observed experimentally in stainless steels at high temperatures. It also provided, via dislocation climb, the steady-state swelling rate of about 1%/NRT.dpa.

Theory of Frank Loop Nucleation at Elevated Temperatures. A.A.Semenov, C.H.Woo: Philosophical Magazine, 2003, 83[31], 3765-82