Quasi-aperiodic grain boundary phases of {\Sigma}5 tilt grain boundaries in refractory metals

Abstract

We report new ground-state structures and phase transitions in $\Sigma$5[001] tilt grain boundaries (GBs) in body-centered cubic (BCC) refractory metals Nb, Ta, Mo, and W. $\Sigma$5 tilt GBs have been extensively investigated over the past several decades, with their ground-state structure -- composed of kite-shaped structural units -- previously thought to be well understood. By performing a rigorous GB structure search that optimizes the number of atoms in the boundary core, we predict new quasi-aperiodic "split kite" phases analogous to those previously found in GBs in face-centered cubic metals. Our results suggest that complex aperiodic phases of GBs appear to be a general phenomenon, as validated through density functional theory calculations. Moreover, the atoms in the split kite phase demonstrate distinct collective diffusion dynamics. Phase-contrast image simulations of split kites show better agreement with experimental observations, offering an alternative explanation for previous microscopy results and motivating future atomically resolved imaging of the GB structure.