Effect of Sc on Aging Kinetics in a Direct Chill Cast Al-Zn-Mg-Cu Alloy

The effect of Sc additions on precipitation strengthening in a direct chill (DC) cast Al-Zn-Mg-Cu alloy was studied after natural and artificial aging. The microhardness, room temperature (RT) mechanical properties, and phase composition of the alloys were determined after different steps of aging. The strengthening mechanisms were discussed. It was shown that minor additions of Sc increased the strength of the Al-Zn-Mg-Cu alloy after casting and solution heat treatment, due to the precipitation of fine coherent Al₃(Sc,Zr) particles. An analysis of the aging kinetics revealed that Sc had no effect on the natural aging, which was controlled by the formation and growth of Guinier–Preston (GP) I zones. On the other hand, the Sc additions accelerated the aging process at 120 °C and 150 °C within a period of time of the formation and growth of GP II zones and η′ particles. It was concluded that the presence of Sc accelerated the formation and growth of GP II zones in the Al-Zn-Mg-Cu alloys, which led to the earlier precipitation of the η′ phase. However, at longer aging times at 120 °C and 150 °C, the aging response of the Sc-containing alloys slowed down, due to faster coarsening of the η′ particles and their transformation into η particles. A model of the formation of vacancy-rich clusters (VRCs), precursors to GP zones, in the Al-Zn-Mg–based alloys was proposed. According to this model, the observed effects of Sc on aging are the result of the Sc-induced increase in the number density of the GP II clusters and the concentration of quenched-in solute-bound excess vacancies.