STRESS CORROSION CRACKING OF AN AL-ZN-MG-CU ALLOY AFTER DIFFERENT PRECIPITATION HARDENING TREATMENTS

Abstract

Stress corrosion cracking (SCC) was tested in a high strength 7000 series aluminium alloy applied for weapons and military equipment. The effect of one step- and two-step precipitation hardening (aging) on SCC was investigated. The slow strain rate test (SSRT) and the fracture mechanics (FM) method were used for SCC testing. The measurements of electrical resistivity allowed evaluating an aging degree in the tested alloy. Both testing methods have shown that the alloy after two-step precipitation hardening is significantly more resistant to SCC. The critical stress intensity factor for SCC, KISCC is significantly higher, and the crack growth rate on the plateau, vpl is more than one order of magnitude lower for the alloy in this state. Processes that take place at the crack tip are proposed and the effect of copper was analyzed. The effect of the testing solution (NaCl) temperature on the vpl was determined. Two values of the apparent activation energy, Ea, were obtained; one value (46.6 kJ mol-1) refers to higher temperatures, while the other (70.4 kJ mol-1) refers to lower test temperatures. These values of Ea correspond to the different processes that control the vpl, at higher and lower temperatures.