Characterization of the mechanical properties changes in an Al–Zn–Mg alloy after a two-step ageing treatment at 70° and 135 °C

Fine-scale precipitation of the η′ phase and its precursors are essential for the mechanical properties of Al-4.6 wt%Zn-1.2 wt%Mg alloy. This paper deals with an investigation of precipitation in an industrial Al–Zn–Mg alloy at various stages of a conventional two-step ageing treatment at 70 °C and 135 °C. The effect of microstructure on the mechanical properties was performed using microhardness and tensile tests, together with optical, scanning and transmission electron microscopy. After ageing at 135 °C, corresponding to the maximum value of hardness, small η′ precipitates were observed in the alloy matrix. After two-step ageing at 70 °C plus at 135 °C, the volume fraction of this precipitate becomes higher. Consequently, the yield strength of the material increases and it maintains its ductility. This high precipitate density slows the dislocation movement and thus a higher stress was required for its bowing.