Atomic simulation of bcc niobium Σ5⟨001⟩{310} grain boundary under shear deformation

The shear behaviors of grain boundaries are investigated using molecular dynamics simulations. The Sigma 5 < 001 >{310} symmetric tilt grain boundary (GB) of body-centered cubic (bcc) Nb is investigated and the simulations are conducted under a series of shear directions at a wide range of temperatures. The results show that the GB shearing along [1 (3) over bar0], which is perpendicular to the tilt axis, has a coupled motion behavior. The coupling factor is predicted using Cahn's model. The critical stress of the coupling motion is found to decrease exponentially with increasing temperature. The GB under shear deformation along the [00 (1) over bar] direction, which is parallel to the tilt axis, has a pure sliding behavior at most of the temperatures investigated. The critical stress of sliding is found to be much larger than that of the coupled motion at the same temperature. At very low temperatures, pure sliding is not observed, and dislocation nucleating and extending is found on GBs. We observed mixed behaviors when the shear direction is between [1 (3) over bar0] and [00 (1) over bar]. The transition region between GB coupled motion and pure sliding is determined. If the shear angles between the shear direction and the tilt axis are larger than a certain value, the GB has a coupled motion behavior similar to the [1 (3) over bar0] direction. A GB with a shear angle smaller than the critical angle exhibits mixed mechanisms at low temperatures, such as dislocation, atomic shuffle and GB distortion, whereas for the [00 (1) over bar]-like GB pure sliding is the dominating mechanism at high temperatures. The stresses to activate the coupling and gliding motions are analyzed for shear deformations along different directions at various temperatures. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.