Effects of strain rate on tensile properties of pure Mg and in situ AlN reinforced Mg–5Al composite

Bulk pure Mg and Mg–5Al–1AlN composite samples with various grain sizes were synthesized by consolidation of powders mechanically milled (MMed) for over various milling durations, followed by sintering and extrusion. Both materials displayed distinct tensile behaviors at different tensile strain rates (3.33 × 10−3 s−1, 3.33 × 10−4 s−1 and 3.33 × 10−5 s−1) with strain-rate sensitivity index (m) of 0.015–0.088 for composite samples and 0.014–0.118 for pure Mg samples. Enhanced strain-rate sensitivity at longer milling duration indicates significant time dependent plasticity or creep at room temperature. Unusually small apparent activation volume and the apparent activation energy of 50 kJ mol−1 which is close to the activation energy for grain boundary diffusion of Mg suggest that below a critical nanograin size, the deformation is controlled by grain boundary sliding with simultaneous grain boundary diffusion.