Investigation of the role of disconnections in the shear coupled grain boundary migration mechanism

The grain size reduction in metals (Al, Cu) under less than a few hundreds of nanometers tends to suppress usual intra-granular plasticity, leading to the activation of grain boundary (GB) mechanisms. Among them the shear coupled GB migration has been extensively studied because of its ability to relax strain. It has been shown that this mechanism is the result of the nucleation and propagation of GB defects called disconnections. In order to investigate the role of these disconnections we combine both molecular dynamics (MD) simulations and in-situ transmission electron microscopy (TEM) straining experiments of aluminum bicrystals.

Experimentally, aluminum bicrystal thin foils with Σ3[110](1-1-1) and Σ41[001](540) tilt GB were strained in-situ in the TEM at 400°C in order to favor shear migration coupling. The observations show the propagation of dislocations with a step character along the GB, i.e. disconnections. Moreover lattice dislocations interacting with the GB were frequently observed.

Theoretically, the Σ41[001](540) aluminum bicrystal has been studied using MD simulations. We have first investigated the migration of a perfect GB by calculating the minimum energy path (MEP) using the nudged elastic band method. The configurations along the MEP shows the nucleation and propagation of a pair of opposite disconnections. Because homogeneous nucleation is unlikely to occur in real crystals, we have also investigated the shear coupled migration of the same GB where a simple defect, a row of vacancies, is introduced. We show that the presence of this defect decreases both yield stress at 0 K and energy barriers for the GB migration. We reveal the migration mechanism at the atomic scale of an imperfect GB containing a row of vacancies.