The effect of pre-annealing on defects, microstructure and recrystallization of ultra-fine grained Al produced by high-pressure torsion

The defect accumulation, thermal expansion, microhardness and microstructure are investigated for severely plastically deformed technically pure Al (Al-base alloy of the 1050 series) as a function of initial state and post-deformation annealing treatments. Coarse grained as-cast and heat-treated states are deformed via high-pressure torsion. The excess volume release upon subsequent annealing is measured by dilatometry employing a constant heating rate and two characteristic sub-stages are revealed. The corresponding microstructure changes are further investigated by transmission electron microscopy. A strong impact of the initial state on the microstructure and the properties of ultrafine grained Al is established. The dilatometric length changes of ultrafine grained Al under annealing are related to the formation of Fe-rich precipitates as well as to the annihilation of deformation-induced defects.