The enhanced aging hardening behavior in Si-containing Al-5Zn-1Mg-1Cu alloys

Abstract The ageing behaviour and phase transformation of a range of Si-microalloyed low Mg/Zn ratio Al-5.0Zn-1.0Mg-Cu alloys were investigated at a temperature of 150°C by using TEM and microhardness tests. Adding Si had a important influence on the proportion of precipitates and microhardness. As Si content increased, the precipitated phase underwent a gradual transition from the T+η precipitates to the fine GPB-II precipitates. Concurrently, the microhardness exhibited a notable enhancement from 120 HV to 150 HV when the alloys were subjected to 150°C aging. Existence of small and even GPB-II phases in the Si-adding alloy was identified as underlying cause of this result. This GPB-II phase exhibited a bilayer microstructure. In core region predominantly comprise Zn, Si and Mg, and the shell region primarily consist of Zn and Cu. This structure effectively inhibited phase growth, maintaining the phases at a smaller scale and enhancing the alloy’s hardening effect.