Evolution of precipitate microstructure during stress aging of an Al-Zn-Mg-Cu alloy

The effect of elastic tensile stress on the microstructure of 7075 aluminum alloy aged at 433 K for 1 h has been investigated in this paper. It was found that double peaks occur in hardness after various aging treatments. The peak microhardness of the stress-aged specimens reaches 178 HV, much higher than that of the conventional aged sample. The stress aging exhibits a diverse microstructure: the GPII zones and various sizes of η precipitates are just identified in the stress-free aged sample; the main hardening η′ precipitates, with the highest and lowest degree of dispersion, are formed after 25 and 50 MPa stress-aged respectively; a finer aging precipitate size distribution, a larger grain boundary precipitate size and spacing and a wider precipitate free zone are represented in the stress-aged specimens. After the 25 MPa stress aging treatment, a preferential growth orientation of the larger-size MgZn2 phase is observed in 7075 aluminum alloy. During aging, the external stress accelerates the growing of the larger-size MgZn2 phase, promotes the formation of the η′ precipitate and inhibits the formation of the η phase. Our study provides a new process to improve the comprehensive properties of Al-Zn-Mg-Cu alloys.