Mechanical milling of magnesium powder

The evolution of microstructure during mechanical milling of magnesium powder has been studied. Commercially pure (99.6%, −325 mesh) magnesium powder was mechanically milled in a modified SPEX 8000 shaker mill in an inert atmosphere. The optical microscopy of the powder in the early stage of milling showed deformation by twinning and re-twinning within the grains developing sub-grain boundaries, which eventually defined nanometre-sized grains. The grain size reduction examined using XRD revealed a rapid decrease and then saturation of the grain size at approximately 42 nm. A relatively large final grain size compared to other mechanically milled metals was obtained due to the high recovery rate of magnesium. The corresponding internal strain was also observed to be low, confirming that enhanced recovery had occurred during milling. The internal strain during milling showed inverse grain size dependence. Moiré fringe patterns of TEM micrographs showed absence of dislocations within the grains of as milled magnesium powder.