Structure and metastability of mesoscopic vacancy and interstitial loop defects in iron and tungsten

The most recent observations of dynamical time-dependent fluctuating behaviour of mesoscopic radiation defects in body-centred cubic metals (Arakawa et al 2006 Phys. Rev. Lett. 96 125506; 2007 Science 318 956-9; Yao et al 2008 Phil. Mag. at press) have highlighted the need to develop adequate quantitative models for the structural stability of defects in the mesoscopic limit where defects are accessible to direct in situ electron microscope imaging. In pursuit of this objective, we investigate and compare several types of mesoscopic vacancy and interstitial defects in iron and tungsten by simulating them using recently developed many-body interatomic potentials. We show that the mesoscopic vacancy dislocation loops observed in ion-irradiated materials are, without exception, metastable with respect to the transformation into spherical voids, but that the rate of this transformation and even the specific type of the transformation mechanism depend on the defect size and the properties of the material.