Cryogenic deformation behavior and failure mechanism of AA7075 alloy sheets tempered at different conditions

The enhancement of cryogenic plasticity for some aluminum alloys has attracted broad attention, which provides a new approach to the formability improvement of high-strength aluminum alloys. However, the hardening behavior and the ductility of aluminum alloys at cryogenic temperatures are very sensitive to the microstructure tempered at different conditions, with complex combination of GP zones, second phase precipitates, coarse and equilibrium phases. In this paper, three groups of tempered AA7075 aluminum alloy sheets are elaborately designed by means of different approaches, i.e., solution-treated (W), peak artificial aged (T6), and fast re-dissolved (FD) from T6, to investigate their cryogenic deformation behavior and failure mechanism. The effect on cryogenic plasticity of second phases, including the coarse particles in the matrix and the precipitate free zones (PFZs) accompanied by chain distributed particles along grain boundary, can be isolated by comparison among these three kinds of AA7075 alloys. The cryogenic mechanical properties of AA7075 sheets are evaluated by uniaxial tensile test, and the microstructure evolution observed by transmission electron microscope (TEM), scanning electron microscope (SEM) and laser confocal microscope (LCM). It shows that the enhancement of both ductility and continuous work hardening for W- and FD-tempered AA7075 sheets at cryogenic temperatures results from the uniformly distributed dislocations in the matrix of supersaturated solid solution (SSSS). The relatively low cryogenic ductility of AA7075-FD in comparison with AA7075-W results from the cracked η phases in the interior of grain, which has an increasing brittleness and high stresses on the particle due to the bypassing process at cryogenic condition. Furthermore, the undesirable early fracture of AA7075-T6 alloy during cryogenic deformation process is systematically illustrated from the aspect of interaction between dislocation and second-phase particles. Beside the cracked η phases in the interior of grains, defects also appear in the non-coherent interface between PFZ and the equilibrium η phase due to stain localization at grain boundary. As a result, AA7075-T6 alloy exhibits a ductile-brittle mixed fracture mode dominated by PFZ and the equilibrium η phase at grain boundary.