On the critical temperature required for grain boundary sliding in Nickel bicrystal: A micromechanical approach

Despite several decades of research on grain boundary sliding and its fundamental mechanisms, two questions remain: What is the critical temperature at which a grain boundary begins to slide, and what is its physical significance? Here, we determine the critical temperature for grain boundary sliding in a high-angle Ni grain boundary by employing an in situ high temperature micropillar compression in a scanning electron microscope (SEM). Regardless of the high melting point of Ni, grain boundary sliding was observed to initiate in the range of 250–300 °C (0.30–0.33 T$_m$) for 1 μm-diameter micropillars. The size-dependent mechanical response of micropillars indicates that the observed grain boundary sliding is a dislocation-mediated process. A comparison with nanocrystalline Ni from the literature suggests that the observed sliding temperature can be considered as the critical temperature required for unconstrained sliding. We propose that this critical temperature corresponds to the onset of lattice dislocation dissociation into grain boundary dislocations.