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Heterogeneous disconnection nucleation mechanisms during grain boundary migration

Nicolas Combe 1 Frédéric Mompiou 2 M Legros 2
1 CEMES-SINanO - Surfaces, Interfaces et Nano-Objets
CEMES - Centre d'élaboration de matériaux et d'études structurales
2 CEMES-PPM - Physique de la Plasticité et Métallurgie
CEMES - Centre d'élaboration de matériaux et d'études structurales
Abstract : 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 [24]. Various experiments have given strong clues of the role of these disconnections in the SCGBM [9, 10]. However, real GB are far from being flat and perfect, and the role of pre-existing defects as GB steps or triple junctions [25, 26] has been considered on the GB migration. In the presence of defects, the nucleation of disconnections will certainly be heterogeneous. In this study, we investigate the heterogeneous disconnection nu-cleation in presence of a pre-existing disconnection with a BV out of the GB plane. Indeed, bulk dislocations interact with GB through various processes [27, 28]: they can be transmitted or absorbed, totally or partially. In case of a total or partial absorption, a residual discon-nection remains in the GB. While glissile disconnections quickly move as they easily couple to a shear stress, dis-connections with a BV out of the GB plane are sessile and potentially affect the GB migration. We examine here this latter case and reveal a new SCGBM mechanism through the nucleation of mobile disconnections from a sessile one. As a representative and simple case, we study the migration of a symmetric tilt GB in response to an external shear deformation in a copper bi-crystal using the molecular dynamics (MD) simulation package LAMMPS [29] and an embedded-atom potential [30]. Figure 1a reports a sketch of the simulation cell under study. Initially, the simulation cell is prepared with two symmetric grains of a perfect fcc copper crystal disorientated relatively to each other by an angle θ = 28.07 • around the [001] direction and separated by a perfect Σ17(410)[001] GB. Periodic boundary conditions (PBC) are applied in the [ ¯ 140] (y-axis) and [001] (z-axis) directions. The simulation cell x (x direction along [410]), y and z sizes are, respectively, 32.4 nm, 11.9 nm, and 2.9 nm. The cell contains 94216 atoms. Two 1.5 nm thick slabs at the top and bottom of the cell that contain atoms with relative positions frozen to the perfect lattice ones are used to impose a shear stress on the GB. Equilibrium structures of the system are obtained by minimization of the potential energy at 0 K using a conjugated gradient algorithm. In order to create the pre-existing disconnection with a realistic structure, a 1/2[110] edge bulk dislocation is artificially introduced in grain 1 (Fig. 1a). This dislocation is moved towards the GB by the application of a stress and is fully absorbed in the GB: it creates a disconnec-tion, referred as δ in the following. [31] Fig. 1c-i) reports
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Nicolas Combe, Frédéric Mompiou, M Legros. Heterogeneous disconnection nucleation mechanisms during grain boundary migration. Physical Review Materials, American Physical Society, 2019, 3 (6), ⟨10.1103/PhysRevMaterials.3.060601⟩. ⟨hal-02117724⟩



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