Varied Modification Mechanisms of Sr and Sb Under Diverse Cooling Rates on Primary Mg₂Si in an Al-20Mg₂Si Alloy

Abstract

Unraveling the varied modification mechanisms of trace elements under diverse cooling rates is of great importance to regulate the morphology of primary Mg₂Si in hypereutectic Al-Mg₂Si alloys. In the present work, the modification mechanisms of Sr and Sb in Al-20Mg₂Si alloys under different cooling rates are discussed in detail. Increasing the cooling rates from ~ 50 to 76 °C/s to ~ 213 to 230 °C/s leads to the morphology transition of primary Mg₂Si from hoppers to dendrites, which is attributed to a smaller constitutional zone with the increase of temperature gradients introduced by high cooling rates. The addition of 0.15 wt pct Sr modifies the morphology of primary Mg₂Si phase to cubes at cooling rates of ~ 50 to 76 °C/s and ~ 213 to 230 °C/s with the same modification mechanism, i.e., by increasing nucleation driving force for primary Mg₂Si and the adsorption-poisoning effect. In contrast, the modification mechanisms of Sb are different under varying cooling rates. At low cooling rates of ~ 50 to 76 °C/s, Sb modifies the morphology of primary Mg₂Si to be truncated octahedrons by introducing Mg₃Sb₂ particles as heterogeneous nuclei and the substitution of Sb to Si in Mg₂Si crystals. At high cooling rates of ~ 213 to 230 °C/s, primary Mg₂Si grows coarser with a dendrite morphology due to solute trapping, which changes the modification mechanism to substitution alone. Moreover, it is interesting that at cooling rates of ~ 213 to 230 °C/s, primary and eutectic Mg₂Si particles in the Al-20Mg₂Si-0.15Sr alloy are refined simultaneously, which greatly suppresses the crack formation and propagation. Accordingly, the ultimate tensile strength and elongation to failure are increased to ~ 242 MPa and ~ 5.6 pct compared to the unmodified alloy (~ 188 MPa and ~ 1.4 pct, respectively).