Ab initio investigation of radiation defects in tungsten: Structure of self-interstitials and specificity of di-vacancies compared to other bcc transition metals

The results of DFT calculations on radiation point defects in tungsten are presented. The lowest energy configuration of the self-interstitial has exactly the 〈1 1 1〉 orientation and no tilt from this direction is observed when using appropriate cell geometry and pseudopotential. The present DFT calculations confirm that in pure tungsten the interactions between two vacancies are unexpectedly repulsive until the fifth nearest-neighbor and that the second nearest-neighbor di-vacancy is the most repulsive. The electronic entropy contribution to the free energy makes the nearest-neighbor configuration attractive at high temperature. A comparison with other bcc metals shows that the binding energies between two vacancies are strongly metal dependent and that tungsten leads to the largest deviation from empirical potential predictions. In tungsten, the effect on vacancy properties of alloying by tantalum and rhenium has been investigated using the Virtual Crystal Approximation (VCA). The effect of these alloying elements is essentially to change the filling of the d-band and the vacancy formation energy is found to be maximal and the relaxation to be minimal when the Fermi level is at the minimum of the pseudo-gap, as predicted by previous tight-binding calculations. Di-vacancies are shown to become attractive at first and second nearest-neighbor upon tantalum alloying and even more repulsive upon rhenium alloying.