Atomic-level physics of grain boundaries in bcc molybdenum

We present a systematic trend study of the symmetric tilt grain boundaries about the $〈110〉$ axis in molybdenum. Our results show that multiple structural phases, some incorporating vacancies, compete for the boundary ground state. We find that at low external stress vacancies prefer to bind to the boundaries in high concentrations, and moreover, that external stress drives structural phase transitions which correspond to switching the boundaries on and off as pipe-diffusion pathways for vacancies.