Effect of Calcium and Silicon on the Character of Solidification and Strengthening of the Al–8% Zn–3% Mg Alloy

The phase composition, character of nonequilibrium solidification, structure, and strengthening of alloys based on the Al–8% Zn–3% Mg (wt %) composition, which are cooperatively and separately alloyed with silicon and calcium, are studied in this work using calculations and experimental methods. In the Al–Zn–Mg–Ca–Si system, the (Al,Zn)₄Ca, Al₂CaSi₂, Mg₂Si and T (Al₂Mg₃Zn₃) phases can be in equilibrium with the (Al) solid solution. Conditions of the two-stage annealing, which is used for the dissolution of eutectic inclusions of the Т and Mg₂Si phases formed upon nonequilibrium solidification, are substantiated. The Al₂CaSi₂ phase is characterized by needle-like morphology and will not take on a spherical shape upon heating up to 520°С. Upon the individual introduction of silicon (0.5%) and calcium (1 and 2%), the effect of precipitation strengthening upon aging is seen to decrease. This is due to the decrease in the magnesium and zinc concentrations in the (Al) solid solution because of fixing these elements to form the Mg₂Si and (Al,Zn)₄Ca, phases, respectively. Upon the cooperative introduction of Si and Ca, the Al₂CaSi₂ phase forms; this allows us, to a considerable extent, to retain unchanged the initial composition of the (Al) solid solution and, therefore, the effect of precipitation strengthening.