Stress corrosion cracking of Al-Zn-Mg alloy AA-7039 by slow strain-rate method

Abstract

The stress corrosion behaviour of Al-Zn-Mg alloy AA-7039 in an aqueous 3.5 wt % Nad solution (pH=1) was studied with the specimens under constant strain rate as function of ageing state and cold working. The tests were carried out at temperatures of 30 and 45°C and strain rates between 7.6×10⁻⁷ and 7.6×10⁻⁶ sec⁻¹ and the apparent activation energy for mechanical deformation in oil and stress corrosion cracking (SCC) process in NaCl solution were determined. The fracture energy in NaCl solution under constant strain rate, as compared with experiments in oil, was lowered in the overaged specimens and markedly lowered in sequence of the peak-aged and the underaged specimens. The values of fracture energy for peak-aged and overaged specimens were nearly similar to those in oil at the higher strain rate of 7.6×10⁻⁶ sec⁻¹. The relative fracture energy was comparatively lowered in the fine-grained specimens, as compared to coarsegrained specimens. The apparent activation energy for mechanical processes in oil was found to be 103 kJ mol⁻¹ in the peak-aged and 96.5 kJ mol⁻¹ in the overaged specimens. Those for SCC processes in NaCl solution were 47.5 kJ mol⁻¹ in the peak-aged and 51.5 kJ mol⁻¹ in the overaged specimens. The results suggest that stress corrosion (SC) cracks are initiated by electrochemical dissolution of grain boundaries (gbs) and propagated by mechanical processes such as creep.