Stress-corrosion cracking susceptibility of the superplastically formed 5083 aluminum alloy in 3.5 pct NaCl solution

The slow strain rate test (SSRT) method was employed to study the stress corrosion cracking (SCC) susceptibility of the superplastic 5083 Al alloy in a 3.5 pct NaCl solution after superplastic forming and various heat treatments. Experimental results showed that both superplastically formed specimens and specimens subject to the same thermal processes as that used in superplastic forming suffered severe SCC susceptibility, and obvious intergranular fracture surfaces were also observed. Furthermore, scanning transmission electron microscopy (STEM) and energy-dispersive X-ray spectroscopy (EDS) analyses demonstrated that the thermal processes of superplastic forming led to continuously distributed precipitation layers of β phase (Mg₂Al₃) at grain boundaries, i.e., sensitization had occurred. However, postforming annealing treatment at 345 °C for 1 hour eliminated the sensitization effect of both specimens. In this case, the SCC susceptibility was alleviated, and the fracture surfaces changed to a transgranular dimpled structure, characteristic of that found in the as-received specimen. From the metallographic observations, it was also seen that a number of cavities appeared at the grain boundaries of the superplastically formed specimen. However, the cavitation effect on SCC susceptibility is minor in comparison with the sensitization effect.