Mechanisms of plastic deformation in ultrafine-grained aluminium - In-situ and ex-post studies

The microstructure of a 1050 aluminium alloy produced by hydrostatic extrusion varies in terms of grain boundary characteristics and the dislocation substructure depending on the grain orientation. This leads to a variance of plastic deformation mechanisms under external load. In this paper, the microstructure of as-extruded samples was compared to extruded and deformed in a bulk compression test to follow the reaction of various grains to external strain. In-situ TEM straining experiments were performed to study the variance of mobile dislocation activities depending on the local dislocation substructure in as-extruded material to deduce the operative deformation mechanism. These experiments accompanied with an estimation of strengthening mechanisms allowed to explain the role of different grains in the plastic deformation of ultrafine grained aluminium treated as a heterogenous complex system. It is demonstrated that well-developed ultrafine grains are responsible for providing strength since no intergranular dislocation intersections were reported but the motion of grain boundary dislocations or dislocation annihilation in boundaries. At the same time, relatively large grains with well-developed dislocation substructures accommodate plastic strain by provoking a more complex reaction - unstable dislocation arrays collapse while more advanced structures evolve into new low angle boundaries. The results from in-situ experiments were also used to explain subgrain shape changes observed after bulk deformation.