Change in microstructure and mechanical properties of ultra-fine grained aluminum during annealing

Ultra-fine grained 1100 commercial purity aluminum sheet was produced by six cycles of an accumulative roll-bonding process (with an equivalent strain of 4.8) at 473 K. The ultra-fine grains in the as-ARB processed sheet had pancake-like morphology 690 nm in diameter and 270 nm in thickness. TEM/Kikuchi-line analysis clarified that the ultra-fine grains are not subgrains but polycrystals having large misorientations with respect to each other. Changes in microstructure and mechanical properties of the ultra-fine grained aluminum by annealing were investigated. As-AREB processed sample showed tensile strength of 275 MPa which is 3.3 times higher than that of starting material, while elongation was limited to 9%. The grain size increased with increasing annealing temperature. The pancake-like ultra-fine grains changed into equiaxed shape firstly, and then rapidly grew. Strength continuously decreased with increasing grain size, but elongation did not recover until the grain size became about 1 mu m. Especially, the ultra-fine grained material showed very small uniform elongation. As a result, only limited grain sizes provided both high strength and enough ductility. It was clarified that proof stress increases while strain-hardening exponent decreases with decreasing grain size.