Heterogeneous disconnection nucleation mechanisms during grain boundary migration

Shear-coupled grain boundary (GB) migration has been evidenced as an efficient mechanism of plasticity in absence of dislocation activity. The GB migration occurs through the nucleation and motion of disconnections. Using molecular simulations, we report a detailed study of the elementary mechanisms occurring during heterogeneous disconnection nucleation. We study the effect of a pre-existing sessile disconnection in a symmetric Σ17(410) [001] tilt GB on the GB migration mechanism. Shearing this imperfect GB induces its migration and reveals a new GB migration mechanism through the nucleation of a mobile disconnection from the sessile one. Energy barriers and yield stress for the GB migrations are evaluated and compared to the migration of a perfect GB. We show that the migration of the imperfect GB is easier than the perfect one and that a sessile disconnection can operate as a source of disconnection driving the GB migration. This GB migration mechanism has been observed on two other high-angle GB. The plastic deformation of polycrystalline solids is generally due to the mobility of dislocations. Grain boundaries (GB) are considered as static obstacles to mobile dislocations as drawn by the Hall-Petch law [1]. Consequently, in nano-crystalline materials (grain sizes < 100 nm), the dislocation-mediated plasticity is reduced or even absent. Recent results have evidenced that in such cases, GB under stress can migrate and participate to the plastic deformation [2-5]. Among the possible GB-based mechanisms [6], both experiments [2, 7-10] and molecular dynamics simulations [11-13] have evidenced the shear-coupled GB migration (SCGBM) as an efficient plastic mechanism at low temperature for low-and high-angle GB. The application of a shear stress on the GB induces its migration (over a distance m) coincidently with the relative in-plane translation d of the two grains forming the GB. The coupling factor β = d m characterizes the migration. The SCGBM has been abundantly studied using atom-istic simulations investigating for instance the dependence of the GB mobilities on the GB misorientation or structure [11, 13-15]. The elementary mechanisms of SCGBM have been numerically investigated on flat perfect model GB: the GB migration results from the nucleation of two mobile disconnections with opposite Burgers vectors (BV) and their migration in opposite directions [16-23]. Disconnections are GB line defects that have both step (height) and dislocation character [2