Cerium Effect on the Phase Composition and Crystallization Behavior of Aluminum Casting Alloys Based on the Al–Mg–Si System

Abstract

Using a Thermo-Calc software package (TCAl4.0 database), unexplored data concerning the phase composition and crystallization behavior of Al–Mg–Si–Ce alloys have been obtained in the composition range of two-phase cast aluminum-magnesium alloys such as (Al) + Mg2Si. It is shown that phases such as (Al), Al4Ce, Mg2Si, and Al8Mg5 can form in the course of crystallization. At a concentration of Mg amounting to 4% and at a concentration of (Si + Сe) = 1.5%, a simultaneous increase in Ce and decrease in Si from 0.2 and 1.3%, respectively, allow consecutive reactions L + (Al) + Al4Ce and L + (Al) + Al4Ce + Mg2Si to occur. This makes it possible to propose that the Al4Ce phase can hinder the growth of the eutectic Mg2Si phase. Moreover, at a temperature of 20°С, such a change in concentrations decreases the amount of Mg2Si and increases the fraction of the Al8Mg5 phase, which is also accompanied by a decrease in the amount of magnesium silicide. When 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% of Ce results in a 0.17% increase in the fraction of the Ce-containing intermetallic compound, whereas, at 0.7% of Ce, the fractions of the two phases become equal. In studying the phase composition upon characteristic annealing-temperature values amounting to 400 and 550°С, it has been revealed that, due to the Al8Mg5 phase dissolving, the (Al) solid solution becomes supersaturated. Every 0.1% of Ce leads to a 0.005% increase in the Mg content in (Al) at 400°С and to a 0.01% increase in that at 500°С, which could indicate the potentialities of a positive Ce effect on matrix strengthening. Based on the results, it has been concluded that it is worthwhile to add Ce in an amount of up to 0.7%, which leads to slightly decreasing liquidus temperature (~636–638°С), but results in a 30°С decrease in the nonequilibrium solidus temperature to 421ºС. At the same time, at a constant temperature of the Mg2Si phase formation (581°С), upon adding Ce, the crystallization range of the (Al) + Al4Ce eutectic expands, which can compensate for the deterioration of casting properties. The Al–4% of Ce–0.5% Si–0.7% Ce alloy has the following phase composition: Al4Ce 1.19%; ratio [Mg2Si/Al4Ce] = 0.89; and fraction of Al8Mg5 of 7.92% at 20°С and 3.22 and 3.36% Mg in (Al) at temperatures of 400 and 550°С, respectively. These results can be the basis for further experimental studies to justify the compositions and temperature modes required for obtaining aluminum-magnesium cast alloys containing Ce that exert a modifying effect on the eutectic Mg2Si inclusions.