Effects of the initial dislocation density on size effects in single-crystal magnesium

Single-crystal magnesium micropillars, ranging in diameter from approximately 600 nm to 10 μm, are fabricated using focused ion beam machining and are loaded in uniaxial compression along either the [0 0 0 1] or [23¯14] axis. The influence of initial dislocation density on size effects is investigated for compression along the [0 0 0 1] axis using two distinct initial dislocation densities. Separately, at fixed low dislocation density, the influence of orientation on size effects is examined by comparing compression along the [23¯14] and [0 0 0 1] directions. Our microcompression results show that decreasing the initial dislocation density results in a stronger size effect in terms of both increased strength and stochasticity with decreasing pillar size. Comparison with a probability-based model shows good agreement between theoretical predictions and experimental observations. Our results demonstrate that the properties of magnesium micropillars depend on the specimen diameter, the initial dislocation density and the orientation of the basal planes with respect to the loading axis.