Roll-bonded aluminium alloy diffusion couple annealed for 60 minutes at 600°C.

This micrograph shows the grain structure of a roll-bonded aluminium alloy diffusion couple annealed for 60 minutes at 600°C. The upper half of the couple is Al-0.5Fe-1.0Mn (wt.%), the lower half is Al-1.0Si (wt.%). Annealing has recrystallised the microstructure. The Al-Fe-Mn alloy has many intermetallic particles (the dark phase). These help create recrystallisation nuclei, hence this alloy has a high density of small grains relative to the Al-Si alloy.The Barker's etch produces a thick oxide layer on the grains of aluminium (anodising). When viewed in cross-polarised light, interference in the oxide layer produces colours which depend on grain orientation, hence the grain structure is imaged. Strain of the aluminium matrix during rolling around the intermetallic particles in the Al-Fe-Mn alloy produces a high density of dislocations. Because recovery of the dislocations creates nuclei for recrystallisation, this high defect density results in a much finer recrystallised grain structure than in the particle-free Al-Si alloy