Deformation behavior of magnesium in the grain size spectrum from nano- to micrometer

This study investigates the deformation behavior of magnesium produced by hot extrusion of ball-milled powders in grains ranging from 120 μm down to 60 nm in size. For microcrystalline magnesium, lattice dislocation interactions with grain boundaries and/or twin boundaries provide a Hall-Petch relationship between the flow stress and the grain size. The Hall-Petch slope is negatively deviated as the grain size is reduced below 1 μm since twinning offers an additional deformation mode. As the grain size is further reduced below 100 nm, twinning is significantly suppressed and a portion of grain boundary sliding for plastic deformation increases, providing an inverse Hall-Petch relationship. Microstructure observation, a negligible strain hardening rate, a relatively high index of strain rate sensitivity, and a low activation volume in compression tests also demonstrate the particular deformation behavior of nanocrystalline magnesium.