A molecular dynamics study of grain boundary structures and their impacts on dislocation nucleation mechanisms

It is well known that grain boundaries play a significant role in determining the mechanical properties of polycrystalline materials. The study of the relationships between grain boundary (GB) structure and its associated deformation mechanisms is therefore of importance. This thesis investigates GB structures and GB energies over a wide range of GB misorientation angles and it investigates their influences on deformation mechanisms. In the first part of this thesis, 1184 molecular statics (MS) simulations were conducted to study the structures and energies of the GBs of Cu and Al, taking into account their 66 tilt axes and various misorientation angles. The effects of GB tilt axis and misorientation angle on GB structure and GB energy were systematically investigated. In the second part of this thesis, molecular dynamics (MD) simulations were performed to study the deformation mechanisms of symmetric tilt GBs with tilt axes of [0 0 1] and [1 1 0] in Cu under both ‘free’ and ‘constrained’ boundary conditions. The results indicate that stress states can have a profound effect on dislocation mechanisms. Dislocation nucleation was found to be independent of intrinsic GB structures. An automatic analysis of MD simulations provided detailed information on the dislocation nucleation and emission of GBs. The results in this thesis contribute to our understanding of GB structure, GB energy and the dislocation nucleation mechanism. The GB energy data obtained in this thesis will be included in a crystal plasticity finite element model to improve predictions of texture evolution and grain refinement during plastic deformation.