Effect of Zn/Mg ratio on precipitation and strengthening of Al–Zn–Mg alloys after aging

The influence of Zn/Mg ratio (1.2-3.5) on the precipitation and strengthening effect of Al-Zn-Mg (Zn+Mg=10wt.%) alloys aged at 130°C was investigated by hardness test, tensile test at room temperature combined with microstructure examination by using optical microscopy, transmission electron microscopy and high resolution transmission electron microscopy. The hardness of the peak-aged alloys tends to increase with the increase of Zn/Mg ratio. The yield strength of the aged alloys increases when Zn/Mg ratio increases from 1.2 to 2.3, with an increased rate of about 6.2%, and then exhibits little change after 2.3. T′ (Al 2 Mg 3 Zn 3 ) and η′ (MgZn 2 ) strengthening phase co-exist in the aged alloys; with the increase of Zn/Mg ratio, their sizes exhibit little change, the quantity decreases for T′ phase but increases for η′ phase. The calculation based on dislocation by-passing mechanism shows that T′ phase and η′ phase makes main and minor contribution to precipitation strengthening, respectively, and the total precipitation strengthening exhibits a first increase and then slight decrease trend with the increase of Zn/Mg ratio. The increments of yield strength after aging were calculated considering both precipitation strengthening and solution strengthening, and the results are in good agreement with the experimental ones.