The Influence of Silicon on Age Hardening Kinetics and Phase Precipitation in Al-Mg-Zn Alloys
Solid solution treatment at 450°C and 550°C and subsequent two step age hardening at 100°C and 150 °C up to 144 hrs. have been carried out for two conventional and four experimental 7xxx type of alloys with different Mg, Zn, Fe and Si content. The influence of silicon on phase and kinetics of age hardening zones and particles has been followed. Increase in silicon required higher solid solution temperature in order to achieve reasonable age hardening response. High silicon alloys, solid solution treated at high temperature, have tendency to recrystallize during aging. The GP-zone formation is affected by the ratio between Mg, Zn and Si. In alloys with Mg/Zn ratio in the range 1:2 GP(I)-type zones are formed, at higher solid solution temperature also GP(II); low Mg-content favor GP(II)- zones. In high silicon alloys GP-zones of b’’’-type (from the Al-Mg-Si) system contribute to age hardening. The precipitation kinetics of the main hardening phase h’, is influenced by the preceding GP-zone stage.
W.J. Poole, M.A. Wells and D.J. Lloyd
V. Hansen et al., "The Influence of Silicon on Age Hardening Kinetics and Phase Precipitation in Al-Mg-Zn Alloys", Materials Science Forum, Vols. 519-521, pp. 579-584, 2006