Investigation on strength, toughness and microstructure of cryogenically-deformed 7A85 aluminum alloy under various aging tempers

Abstract

The strength, fracture toughness, electrical conductivity, and microstructure evolution of cryogenically-deformed 7A85 aluminum alloy at different aging temperatures and aging times of single-step (T6), double (T74), and retrogression and re-aging (RRA) treatments were investigated. The results showed that the high-strength samples under T6 had low electrical conductivity and fracture toughness, which were significantly improved under T74 and RRA. Upon extending the aging time, the strength and intergranular fracture proportion increased and the fracture toughness decreased under T6, while the strength decreased and the transgranular fracture proportion and fracture toughness increased under T74 and RRA. The electrical conductivities were improved under T6, T74, and RRA by prolonging the aging time. The recommended single-step, double, and RRA aging tempers for cryogenically-deformed 7A85 aluminum alloy were T624 (120 °C/24 h), T742 (120 °C/6 h + 170 °C/2 h), and RRA0.5 (120 °C/24 h + 180 °C/0.5 h + 120 °C/24 h), respectively. By comprehensively considering the conductivity, fracture toughness, and mechanical properties, the optimal aging temper for the cryogenically-deformed 7A85 aluminum alloy was RRA0.5. Compared with T624, the fracture toughness of T742 and RRA0.5 increased by 38.7% with a certain strength loss and by 10.4% without strength loss, respectively. For the RRA treatment, the matrix precipitates were similar to the T6 state and the grain boundary precipitates were similar to the T74 state. The effect of heat treatment on the strength and fracture toughness could be explained by the cutting mechanism, the Orowan mechanism, and the strength difference between the matrix and grain boundaries.