Invited Lecture
In situ TEM and simulation study of shear-migration coupling of grain boundaries

Nanocrystalline metals (d≤100 nm) possess a much greater resistance to plastic deformation than those with conventional grain size. One of the specificities of these nanocrystals is that they contain a large proportion of grain boundaries (GBs) but virtually no dislocations. Plastic deformation is then thought to occur using alternative paths. Several experiments have shown that in these small-grain materials, the plastic deformation is carried out predominantly by grain boundaries. The dominant mechanism is the so-called shear-migration coupling. Despite a recent increase in simulations studies, its experimental characterization remains very scarce. Aside from experimental obstacles, the problem is very vast as real grain boundaries possess at least 5 degrees of freedom and contains a potentially infinite number of disconnections, a specific defect that combines a step and dislocation character.

We have conducted both in-situ TEM experiments and molecular dynamic simulations using the NEB technique (Nudge Elastic Band). We could show that shear-migration coupling involves and is imposed by the displacement of these disconnections whose origin will be discussed. The question that will finally be addressed here is to whether we should still consider a given GB as a crystalline defect or a network of its own, which mechanical properties (mobility, shear coupling) are governed by its nature or by its defects.

Figure 1: Disconnection observed using a Geometric Phase Analysis of a high resolution TEM micrograph in an Al bicrystal.