Influence of cerium on the phase composition and crystallization behavior of cast aluminum alloys based on the Al—Mg—Si system

This study was conducted with calculations made in Thermo-Calc software (TCAl4.0 database) to find out the unexplored data on the phase composition, crystallization behavior of Al—Mg—Si—Ce alloys as regards the compositions of two-phase (Al) + Mg₂Si cast aluminum-magnesium alloys. It was shown that (Al), Al₄Ce, Mg₂Si, Al8Mg5 phases may form during crystallization. At 4% Mg and (Si + Ce) concentrations of 1.5 %, a simultaneous increase in Ce and decrease in Si contents from 0.2 % and 1.3 % points promote consistent reactions L + (Al) + Al₄Ce and L + (Al) + Al₄Ce + Mg₂Si. This suggests that the Al₄Ce phase may hinder the growth of Mg₂Si phase eutectic inclusions. Moreover, at 20 °C such a change in concentrations promotes a simultaneous decrease in the contents of Al₄Ce and Al₈Mg₅ phases, along with a decrease in the amount of magnesium silicide. While adding Ce in the Al—4%Ce—0.5%Si alloy, the fraction of Mg₂Si is approximately constant throughout the entire crystallization range (1.34 %), but each 0.1% Ce increases the Ce-bearing intermetallic fraction by 0.17 %, and at 0.7 % Ce the proportions of two phases are equal. When studying the phase composition at representative annealing temperatures of 400 и 550 °C, it was revealed that the (Al) solid solution becomes supersaturated as a result of Al₈Mg₅ phase dissolving. Each 0.1% Ce increases the Mg content in the (Al) solid solution by 0.005 % in the first case and by 0.01 % in the second one. This indicates a potentially positive influence of Ce on matrix strengthening. Based on the results, it was concluded that it is advisable to add Ce in an amount of up to 0.7 %, which slightly reduces the liquidus temperature (to ~636+638 °C), but reduces the non-equilibrium solidus temperature by ~30 °C to 421 °C. At the same time, at a constant Mg₂Si phase formation temperature (581 °C), the eutectic crystallization range (Al)+Al₄Ce expands with Ce addition, which can compensate for the decrease in casting properties. The Al—4%Ce—0.5%Si—0.7% Ce alloy has the following phase composition: Al₄Ce 1.19 %, the [Mg₂Si/Al₄Ce] ratio = 0.89, Al₈Mg₅ fraction is 7.92 % at 20 °C, Mg concentrations in the (Al) solid solution are 3.22 % and 3.36 % at temperatures of 400 °C and 550 °C, respectively. The presented results serve as the basis for subsequent experiments and justify compositions and temperature conditions for obtaining cast aluminum-magnesium alloys with cerium having a modifying effect on Mg₂Si eutectic inclusions.

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