Precipitation behavior and properties of a new high strength Al-Zn-Mg-Cu alloy

The microstructure and the associated hardness, strength and electrical conductivity of a new Al-Zn-Mg-Cu alloy during one-step ageing treatment were systematically studied. The results show that the electrical conductivity of the alloy increased continuously with increasing ageing temperature and ageing time. At the early stage of ageing, the hardness and strength of the alloy increased rapidly and then reached the peak value. When aged at 120 degrees C, the hardness and strength maintained at high level for a long time after the peak value. The main precipitations are GPI zones, GPII zones and metastable eta' phase. GPI and GPII zones are found in the alloy after ageing for 24 h at 120 degrees C, which indicates that some stable GP zones can exist through the ageing process. When aged at 160 degrees C, the hardness and strength decreased rapidly after the peak value. The precipitation process is significantly promoted compared with that aged at 120 degrees C. Both GPI zones and GPII zones disappeared after ageing for 1 h at 160 degrees C. The main precipitates are eta' phase when aged at 160 degrees C for 1 h. The main precipitates are eta phase when the ageing time prolongs to 24 h.