Migration kinetics of the self-interstitial atom and its clusters in bcc Fe

Effect of Carbon on Void Nucleation in Iron

The study reports the significance of carbon presence in affecting void nucleation in Fe. Without carbon, void nucleation rates decrease gradually at high temperatures but remain significantly high and almost saturated at low temperatures. With carbon present, even at 1 atomic parts per million, void nucleation rates show a low-temperature cutoff. With higher carbon levels, the nucleation temperature window becomes narrower, the maximum nucleation rate becomes lower, and the temperature of maximum void nucleation shifts to a higher temperature. Fundamentally, this is caused by the change in effective vacancy diffusivity due to the formation of carbon-vacancy complexes. The high sensitivity of void nucleation to carbon comes from the high sensitivity of void nucleation to the vacancy arrival rate in a void. The void nucleation is calculated by first obtaining the effective vacancy diffusivity considering the carbon effect, then calculating the defect concentration and defect flux change considering both carbon effects and pre-existing dislocations, and finally calculating the void nucleation rate based on the recently corrected homogeneous void nucleation theory. The study is important not only in the fundamental understanding of impurity effects in ion/neutron irradiation but also in alloy engineering for judiciously introducing impurities to increase swelling resistance, as well as in the development of simulation and modeling methodologies applicable to other metals.

Direct observation of interstitial dislocation loop coarsening in α-iron

Interstitial loop coarsening by Ostwald ripening can provide insight into single point defects but is very difficult to observe in α-iron and many other metals where nanoscale vacancy clusters dissociate and annihilate loops. We show that by implanting helium in the samples at a carefully chosen energy, it is possible to observe Ostwald ripening of loops by transmission electron microscopy during in situ isochronal annealings. This coarsening of loops results in a sharp increase of the mean loop radius at around 850 K. Using cluster dynamics simulations, we demonstrate that loops evolve due to vacancy emission and that such experiments give a robust estimation of the sum of the formation and migration free energies of vacancies. In particular, our results are in good agreement with self-diffusion experiments and confirm that entropic contributions are large for the vacancy in α-iron.

Self-trapped interstitial-type defects in iron

Small interstitial-type defects in iron with complex structures and very low mobilities are revealed by molecular dynamics simulations. The stability of these defect clusters formed by nonparallel {110} dumbbells is confirmed by density functional theory calculations, and it is shown to increase with increasing temperature due to large vibrational formation entropies. This new family of defects provides an explanation for the low mobility of clusters needed to account for experimental observations of microstructure evolution under irradiation at variance with the fast migration obtained from previous atomistic simulations for conventional self-interstitial clusters.