Evidence for vacancy migration at 220 k in iron, given by high voltage electron microscopy

The growth of interstitial loops has been studied by 1 MeV electron microscopy in pure iron samples, irradiated and observed between 180 K and 800 K. It is shown that, below 220 K, the increase of the interstitial loop diameter obeys a tn law, with n<1 (t being the observation time). Above 220 K, a linear t law is observed. Moreover, the evolution of the loop density is strongly modified below and above 220 K. Both results lead to the conclusion that vacancies migrate at 220 K in iron. The discrepancy with previous results obtained by other physicists is explained by a model in which mobile vacancies interact with residual impurities.