1.1%Li was added to 7075 alloys to obtain the Li-containing Al-Zn-Mg-Cu alloys. The microstructure and hardness of the alloys are investigated by transmission electron microscopy (TEM) and Vickers hardness. The hardness of the single-aged alloys is low. When the alloys were double-aged or multi-aged, the hardness is comparable to that of Al-Zn-Mg-Cu alloys at peak ageing. Two peaks were present in the hardness curves of the multi-aged Li-containing Al-Zn-Mg-Cu alloys. With the last-step temperature increases, two-peak phenomenon becomes prominent. The density and size of precipitates are influenced remarkedly by the ageing processes. Coarse grain boundary precipitates and PFZ (precipitate free zone)can be observed when the Li-containing Al-Zn-Mg-Cu alloys were multi-aged, and the higher the last-step ageing temperature, the wider the PFZ is.
Studies on the Precipitate-Free Zone in an Al-Zn-Mg Alloy