Abstract
Fine-scale precipitation of the eta' 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 degrees C and 135 degrees 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 degrees C, corresponding to the maximum value of hardness, small TV precipitates were observed in the alloy matrix. After two-step ageing at 70 degrees C plus at 135 degrees 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. (c) 2009 Elsevier Ltd. All rights reserved.