Effect of grain size on slip activity in pure magnesium polycrystals

The aim of this work is to investigate the effect of grain size on slip activity, a critical issue for materials design that remains unsolved. Three pure magnesium polycrystals with average grain sizes (d) of 36, 19 and 5 μm and with very similar textures and grain boundary (GB) misorientation distributions were designed by selecting appropriate rolling and annealing conditions. The three microstructures were tested in tension along the rolling direction (RD) at 50 °C and at an strain rate of 10−3 s−1 and the incidence of the different deformation mechanisms was evaluated by electron backscattered diffraction (EBSD)-assisted slip trace analysis. A clear transition from non-basal to basal slip-dominated flow takes place when d decreases below 36 μm, i.e. at grain size ranges that are very relevant for most structural applications. This change in slip activity is shown to be consistent with an increase in the CRSSnon-basal/CRSSbasal ratio with decreasing d, i.e. with a more potent GB strengthening of non-basal than basal systems. Accordingly, a transition in the mechanisms responsible for the accommodation of local stress concentrations at GBs takes place with decreasing grain size. Intergranular compatibility becomes increasingly difficult with grain refinement due to the progressively lower availability of active deformation mechanisms. Macroscopically, these phenomena translate to a reduction of work hardening and ductility with decreasing grain size.