Multiscale modelling of crowdion and vacancy defects in body-centred cubic transition metals

• P.M. Derlet
• D. Nguyen-Manh
• S.L. Dudarev

We investigate the structure and mobility of single self-interstitial atom and vacancy defects in body-centred cubic transition metals forming groups 5B (vanadium, niobium and tantalum) and 6B (chromium, molybdenum and tungsten) of the Periodic Table. Density-functional calculations show that in all these metals the axially-symmetric h111i self-interstitial atom configuration has the lowest formation energy. In chromium the difference between the energies of the h111i and the h110i self-interstitial configurations is very small, making the two structures almost degenerate. Local densities of states for the atoms forming the core of crowdion configurations exhibit systematic widening of the ‘local’ d-band and an upward shift of the anti-bonding peak. Using the information provided by electronic structure calculations, we derive a family of Finnis-Sinclair-type interatomic potentials for vanadium, niobium, tantalum, molybdenum and tungsten. Using these potentials, we investigate thermally activated migration of self-interstitial atom defects in tungsten. We rationalize the results of simulations using analytical solutions of the multi-string Frenkel-Kontorova model describing non-linear elastic interactions between a defect and phonon excitations. We find that the discreteness of the crystal lattice plays a dominant part in the picture of mobility of defects. We are also able to explain the origin of the non-Arrhenius diffusion of crowdions observed at elevated temperatures.