Section of a metal matrix composite (MMC) after extrusion and annealing

Commercial purity aluminium has atomised, mixed with 13 micron spherical alumina particles, ball milled, tumbled, cold compacted (20MPa) and then extruded (with the working direction vertical in the image). It has then been annealed at 630 degrees C. Vertical stringers of fine oxide particles are visible. Note the small yellow 'island' grain which has formed within one of the recrystallised grains and is likely to have nucleated on the adjacent reinforcement particle. Note also the boundary facets of some grains. These indicate a preferred habit plane and are probably coincidence site lattice (CSL) boundaries.