[SY-I6] The influence of normal stress on the structural transformation and migration of grain boundaries
Invited
It is now widely accepted that stress plays important roles on the kinetics of grain boundaries. Past studies on grain boundary kinetics by atomistic simulation mainly focused on those under shear deformation by recognizing that shear coupling is one of the most fundamental modes of grain boundary motion. However, grain boundaries in polycrystalline materials under realistic loading conditions rarely experience pure shear deformation and stresses with both shear and normal components are more common. In this work, we used molecular dynamics simulations to investigate (1) how normal stress would influence the ground state structures of a few types of special CSL grain boundaries and (2) how such structural change would further influence the modes and energy barrier for migration in those grain boundaries.
We found that while both constant and cyclic normal stresses can facilitate the transition in grain boundary structure among its metastable states, the influences of tension and compression are not the same and vary among different types of grain boundaries. It is also found that the macroscopic strain caused in the materials due to the structural transformation in grain boundaries under cyclic stress can be well described by Coble creep, which implies a possible new mechanism of Coble creep that has been overlooked before. Furthermore, the grain boundary structural transformation can either lower or raise the energy barrier for grain boundary migration depending on the grain boundary type, which can be explained by the relative easiness of atomic shuffling in each grain boundary based on their DSC lattice.
We found that while both constant and cyclic normal stresses can facilitate the transition in grain boundary structure among its metastable states, the influences of tension and compression are not the same and vary among different types of grain boundaries. It is also found that the macroscopic strain caused in the materials due to the structural transformation in grain boundaries under cyclic stress can be well described by Coble creep, which implies a possible new mechanism of Coble creep that has been overlooked before. Furthermore, the grain boundary structural transformation can either lower or raise the energy barrier for grain boundary migration depending on the grain boundary type, which can be explained by the relative easiness of atomic shuffling in each grain boundary based on their DSC lattice.