Effects of Zn/Mg Ratio on Aging-Precipitation Sequence of Al-Zn-Mg Alloys

The age-hardening behaviors of Al-6.2Zn-2.3Mg and Al-5.0Zn-3.0Mg alloys after solution treatment at 470 degrees C for 2 h, and aging treatment at 120 degrees C were studied by hardness measurement. The valence electron structure of the alloy matrix was calculated, and the micro-mechanism of the formation of atomic clusters in the matrix during the solid solution and initial aging was studied. Then the effect of Zn/Mg ratio on precipitation sequence of the Al-Zn-Mg alloy was analyzed. Results show that the age-hardening behavior of the Al-6.2Zn-2.3Mg alloy exhibits an obvious two-peak characteristic because the solid solution alpha-Al-Zn-Mg preferentially formed in the initial aging process provides the conditions only for the formation of GP zone of eta precipitation sequence. However, the two solid solutions alpha-Al-Zn-Mg and alpha-Al-Mg-Zn-Mg-Al formed during the initial aging of Al-5.0Zn-3.0Mg alloy provide the conditions for the GP zone formation of the precipitation sequence of eta and T, respectively. Therefore the two precipitation sequences, which have many differences in the aging progress, transforming time, strengthening phase precipitation and strengthening effects, start simultaneously during aging, so the age-hardening behavior of Al-5.0Zn-3.0Mg alloy does not show obvious two-peak characteristics.