Computer simulation of vacancy and interstitial clusters in bcc and fcc metals

Abstract

Interstitial clusters in bcc-Fe and fcc-Cu and vacancy clusters in fcc-Cu have been studied by computer simulation using different types of interatomic potentials such as a short-ranged empirical pair potential of Johnson type, short-ranged many-body potentials of Finnis-Sinclair type and long-ranged pair potentials obtained within the generalized pseudopotential theory. The stability of a self interstitial in bcc-Fe was found to be dependent on the range of potential but not on the type. Thus, both short-ranged potentials simulated 〈110〉 dumb-bell as a stable configuration while in the case of the long-ranged potential the stable configuration is the 〈111〉 crowdion. Nevertheless the structure and properties of interstitial clusters were found to be qualitatively the same with all the potentials. Up to 50 interstitials, the most stable clusters were found as perfect dislocation loops with Burgers vector b = 1 2 〈11 1〉 . The stability of interstitial clusters in Cu also does not depend on the potential and for the same sizes the most stable configurations are faulted Frank loops 1 2 〈111〉{111} and edge loops in the {110} plane. The structure and stability of vacancy clusters in fcc-Cu were found to be dependent mainly on both the range of potential and equilibrium conditions. Thus for long-ranged non-equilibrium pair potentials vacancy clusters in the {111} plane collapsed and formed vacancy loops or stacking fault tetrahedra depending on the shape of the initial vacancy platelet. For the short-ranged equilibrium many-body potential vacancy clusters do not collapse into loops or tetrahedra. The process of vacancy clustering in the cascade region has been studied by molecular dynamics. This study has been done for the case of a PKA energy of about 20–25 keV. We found that the processes simulated with the short-ranged many-body potential and the long-ranged pair potential are qualitatively different. Thus for the many-body potential we have observed melting and crystallization of the central part of the cascade region, sweeping of vacancies inside due to the moving of the liquid-solid interface and increasing of vacancy concentration in the centre of the cascade region; however no significant clustering was observed. Contrarily for the long-ranged pair potential we have observed a very fast diffusion in the solid crystallite and the formation of stacking fault tetrahedra. The results obtained have been discussed and compared with the experimental data.