Effect of indentation size on plastic deformation processes in an ultrafine-grained Al-3% Mg alloy

An Al-3% Mg alloy was processed by Equal Channel Angular Pressing (ECAP) for 8 passes in route B_C thereby reducing the grain size of the annealed sample from ∼400 μm to ∼300 nm. The as-pressed ultrafine-grained material was examined using depth-sensing indentation tests over different length scales with the overall objective of identifying the different deformation processes taking place in the bulk submicrometer crystalline material. At small indentation sizes with indentation depths of <1000 nm, plastic instabilities in the form of the Portevin-Le Châtelier (PLC) effect were observed in the as-pressed material. At the macroscopic scale, however, the alloy exhibited no PLC instabilities thereby suggesting that dislocation motion then makes only a limited contribution to the plastic deformation and the flow is controlled instead by another mechanism associated with the presence of a very large number of grain boundaries.