An almost unifying theory for grain boundary‐based plasticity

Revealed in metallic nanocrystals, or thin films (Fig. 1) grain boundary (GB)-based plasticity has been studied for many years under various names: stress-assisted grain growth, grain rotation, grain boundary sliding or shear-coupled grain boundary migration. Based on MD simulations, TEM and in-situ TEM approaches, we will show that a key player in these mechanisms is the disconnection [1, 2]. This defect combines a step and a Burgers vector character, and belongs to GBs, especially real GBs. The motion of these defects can explain most of the above-mentioned mechanisms depending on the amplitude of both its step and dislocation components. But not all of them. Some observations suggest that local atomic shuffling also plays a role as clear non-conservative behaviours are detected, probably postponing the expected happy ending of a complete GB-based plasticity understanding. Please click Additional Files below to see the full abstract

Étude expérimentale et théorique de la migration de joints de grains, couplée à un cisaillement

Contrary to conventional coarse-grained metals where plastic deformation is carried out by intragranular dislocation motion, specific grain boundary-based mechanisms are involved in deformation of nanocrystalline metals. Among them, the shear-coupled grain boundary migration, i. E. The motion of the grain boundary perpendicular to its plane in response to a shear strain, has been found to be efficient to accommodate the deformations observed in several small-grained metals and under different mechanical solicitations. Despite many experimental and theoretical efforts in recent years, the elementary mechanisms of the shear-coupled grain boundary migration are still poorly known. The major purpose of the present work is thus to investigate these elementary processes both experimentally by transmission electron microscopy (TEM) and theoretically by atomistic simulations. In-situ TEM straining experiments on Al bicrystal at 400C show the migration of a S41{540}grain boundary by the collective motion of macro-steps moving along the interface. These macrosteps are characterized by measuring the deformation (related to the coupling factor) that they induce by their motion. This deformation can include both parallel (shear-coupled migration) and perpendicular (involving climb) components to the interface. Moreover, different macrosteps carry different deformations indicating the multiplicity of the possible coupling modes for a given grain boundary. Moving elementary steps, presumably composing the macro-steps are also observed in the grain boundary. By high resolution TEM (HRTEM) observations the elementary steps are identied as disconnections and are characterized by their step heights and Burgers vectors. The TEM in-situ straining experiments at ambient and 400C show also the rapid decompositions of lattice dislocations in the grain boundary, suggesting a possible mechanism for the creation of disconnections. The possible decomposition reactions are hence considered in order to determine the nature of the produced disconnections. The disconnections with small step heights and Burgers vectors and small climb components are thought to be mobile and the potential deformation that they would carry is in agreement with the experimentally measured coupling factors. The shear-coupled grain boundary migration is studied by atomistic simulation in a Cu bicrystal containing a symmetrical S13{320} grain boundary, at 0 K. The minimum energy path (MEP) of the grain boundary migration is determined by the Nudged Elastic Band (NEB) method. The structural evolution of the grain boundary along the MEP shows that the grain boundary migration occurs through the nucleation and motion of grain boundary steps, identified as disconnections, in agreement with experimental observations. The energy barrier for the nucleation of the disconnections is found to be about 11 times larger than the energy barrier for their motion. Hence, in the absence of pre-existing disconnections in the grain boundary, the nucleation of the disconnections is the limiting step of the shear-coupled grain boundary migration.La déformation plastique des matériaux est généralement induite par le déplacement de dislocations. Cependant dans les matériaux nanocristallins, ces déplacements sont inhibés et les mouvements des joints de grains jouent alors un rôle important dans la plasticité. La migration des joints de grains couplée au cisaillement est un mécanisme efficace mis en évidence lors de sollicitations mécaniques dans différents matériaux: le joint de grains se déplace perpendiculairement a son plan en réponse a une contrainte de cisaillement. Malgré de nombreux efforts expérimentaux et théoriques ces dernières années, les mécanismes élémentaires de la migration des joints de grains couplée au cisaillement restent méconnus. Le présent travail se propose d'étudier expérimentalement par microsopie électronique en transmission (MET) et théoriquement par simulation atomistique ces mécanismes élémentaires. Des expériences de traction in-situ en MET portant sur des bicristaux d'Aluminium chauffés à 400C montrent qu'un joint de grains S41{540} se déplace par le mouvement collectif des macro-marches le long de l'interface. Ces macro-marches sont caractérisées par la mesure de leur déformation. Cette déformation peut comporter à la fois une composante parallèle (couplée au cisaillement) et une composante perpendiculaire à l'interface (montée). Différentes macromarches caractérisées par différentes déformations ont été observées sur un même joint de grains: différents modes de couplage peuvent ainsi permettre à un joint de grains de migrer. Des marches élémentaires, composant vraisemblablement les macro-marches ont été également observées. L'observation par MET haute résolution de ces marches élémentaires a permis de les identifier comme des disconnections. Elles sont caractérisées par leurs hauteurs et leurs vecteurs de Burgers. Des expériences de traction in-situ en MET montrent que des dislocations du réseau se décomposent dans le joint, suggérant un mécanisme possible de création de ces disconnections. Les réactions de décomposition les plus probables ont été analysées pour déterminer la nature des disconnections produites. Les disconnections produites ayant des hauteurs de marches faibles et de faibles Vecteurs de Burgers, si elles sont supposées mobiles, sont caractérisées par des facteurs de couplage en bon accord avec les facteurs de couplage mesurés expérimentalement. La migration des joints de grain couplée au cisaillement est étudiée par simulation atomistique dans un bicristal de Cuivre contenant un joint de grains symétrique S13{320}. Le chemin de moindre énergie lors de la migration est déterminé en utilisant la méthode Nudged Elastic Band. L'évolution du joint de grains lors de sa migration montre la nucléation et le déplacement des marches, identifiées comme des disconnections, en bon accord avec les résultats expérimentaux. La barrière d'énergie pour la nucléation des disconnections est environ 11 fois plus élevée que le barrière d'énergie pour leur déplacement. Ainsi, en l'absence de disconnection pré-existante dans le joint, la nucléation des disconnections est l'étape limitante de la migration du joint de grains couplée au cisaillement

Comptes Rendus Physique

International audienceShear-coupled migration of grain boundaries: the key missing link in the mechanical behavior of small-grained metals? Migration couplée au cisaillement des joints de grains : le chaînon manquant dans le comportement mécanique des métaux à petits grains

The role of disconnections in deformation-coupled grain boundary migration

cited By 26International audienceGrain boundary (GB) migration under stress has been recognized in recent years as an important plastic deformation mechanism especially in small-grained materials. It is believed to occur via the motion of disconnections along the interface. However, the origin of these disconnections is a key point for a deeper understanding of this mechanism. In this paper, we consider that GB migration under stress can occur both due to the motion of pre-existing disconnections and due to disconnections resulting from decomposition of lattice dislocations interacting with the GB. High-resolution transmission electron microscopy experiments carried out on an aluminum bicrystal with a Σ41 540 GB indeed confirm the existence of different kinds of disconnections and pure steps prior to deformation. In situ straining experiments performed in the same bicrystal at room and high temperatures reveal the rapid decomposition of lattice dislocations in the GB plane. Theoretical investigation of the possible decomposition reactions shows that different types of disconnections with Burgers vector having both glide and climb components, i.e. parallel and perpendicular to the GB plane, can be produced. Disconnections with a small climb component are likely to move along the GB under stress and induce deformation parallel and perpendicular to the GB plane. Concomitant motion of disconnections with Burgers vectors at right angles to the GB plane is believed to produce GB migration coupled with grain rotation. It is also shown that disconnection interactions in the GB lead preferentially to purely glissile disconnections producing a coupling factor in agreement with the observed coupling factor measured in experiments on macroscopic bicrystals. The idea that shear-coupled GB migration can occur by the continuous feeding of lattice dislocations decomposing in the GB during the migration is also investigated. This process is thought to play a role during recrystallization