Stress and temperature dependence of stress relaxation ageing behaviour of an Al–Zn–Mg alloy

Abstract

The stress (from elastic to plastic) and temperature dependence of stress relaxation ageing (SRA) behaviour of an Al–Zn–Mg alloy, AA7B04–P, has been experimentally investigated in this study. A series of SRA tests have been carried out under various initial stress levels in both elastic and plastic regions and at different temperatures. Corresponding microstructural evolution during SRA has been characterised using transmission electron microscopy (TEM). It is found that increasing the initial stress and/or temperature enhance the stress relaxation in both elastic and plastic regions. The dislocation creep mechanism plays the dominant role at the investigated temperatures during SRA, with the stress exponent n ranging from 3 to 8, decreasing with increasing temperature. External stresses accelerate the coarsening of GP zones and η’ precipitates and, when loaded to the plastic region, promote the formation of large rod-shaped η precipitates within 2 h of SRA tests, due to the high energy sites provided by dislocations from plastic loading. Yield strength shows a much higher sensitivity to the temperature than creep strain has during SRA tests. A temperature below 165 °C is suggested for SRA of AA7B04–P, so that a high stress relaxation level with less than 15% strength loss can be obtained after 16 h forming.