On incipient plasticity in the vicinity of grain boundaries in aluminum bicrystals: Experimental and simulation nanoindentation study

International audienceThe local mechanical behavior near symmetric tilt grain boundaries in aluminum bicrystals were studied by the nanoindentation technique as well as by computer simulations. Experimentally, grain boundaries with misorientation angles 8.7°, 13.8° and 18.8° were examined. Two well pronounced pop-in events were observed on the load – penetration depth curves measured during indentation in the close vicinity of the studied boundaries. The load of the pop-ins, observed close to the boundaries, practically did not differ from that obtained in the grain interior. This was interpreted as evidence that the boundaries with misorientations in the examined angular range do not represent specific sites for sources of lattice dislocations. The load, at which the second pop-ins took place, substantially increased with increasing misorientation angle of the examined boundaries. Quasistatic molecular dynamics simulations were performed to identify the details of the interaction between grain boundary and dislocations generated during indentation. For this purpose, the bicrystals with similar geometry and misorientation angles, as investigated experimentally, were computed. The simulation results showed that the direct transmission of incoming dislocations across the grain boundary was the primary mechanism for the plastic flow transfer past the boundary. The analysis of the results of both experiments and simulations provided evidence that the capability of grain boundaries to act as a barrier for the motion of incoming dislocations depends crucially on grain boundary structure

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