Interstitial-type defects formed by the clustering of self-interstitials produced under irradiation have rather peculiar properties in alpha-iron. Very small interstitial clusters are formed of <110> dumbbells, whereas larger clusters have either a <111> or a <100> orientation. This contrasts with other BCC metals where they are predominantly <111>. The competition between these different orientations raises the question of their relative stabilities as function of size and temperature, and of the transformation mechanism from <111> to <100> orientations observed experimentally. Their mobilities are also a key issue in the interpretation of experiments. We have addressed these questions by combining first principles and empirical potential approaches.

We have determined the stability and mobility of small self-interstitial defects by ab-initio calculations performed on cells containing up to 250 atoms using the SIESTA code. We then used these results to fit and validate a new semi-empirical potential for iron. This potential allows to access dynamical properties and larger sizes. (i) Using the activation-relaxation technique (ART) we performed on small clusters a systematic search of configurations and migration/rotation mechanisms. (ii) Using lattice-dynamics we studied the vibrational properties of small clusters: low frequency modes have been evidenced in <111>-type defects ; the associated large vibrational entropy is shown to have a significant effect on the relative stabilities at finite temperature. (iii) In connection with experiments we have investigated the properties of loops with up to 1000 defects: the relative stabilities as function of Burgers vector, their migration energies, and the loop-loop and loop-surface interactions. We discuss the dependence on the potential of these results.

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1. Kinetics of point defects in hcp-Zr from first principles