Optimum scandium addition in A356 cast alloys: alloy design, microstructural evolution and mechanical properties

Abstract

The effect of scandium (Sc) addition on the microstructural evolution and mechanical properties of A356.2 cast alloys in both as-cast and T6 heat-treated conditions was investigated using a combined thermodynamic and experimental approach to identify an optimum composition for enhanced performance. Thermodynamic calculations and cooling curve analysis showed that Sc alters solidification behaviour through the formation sequence of Sc-containing intermetallic phases. Experimental results demonstrated that Sc additions up to 0.4 wt% significantly refined primary α-Al grains and reduced secondary dendrite arm spacing (SDAS), while modifying eutectic Si from coarse plate-like to fine fibrous morphology. The transformation of β-Al5FeSi into compact Al-Fe-Si-Sc phases and the formation of nanoscale Al-Si-Sc-(Ti) dispersoids further contributed to strengthening. In the as-cast condition, the A356-0.4Sc alloy exhibited the best combination of strength and ductility, whereas under T6 heat treatment, the A356-0.2Sc alloy showed superior performance due to enhanced precipitation strengthening and thermal stability of Sc-containing dispersoids. At higher Sc content (0.6 wt%), primary AlSc2Si2 formation led to microstructural coarsening and reduced properties. This study establishes an optimum Sc addition range of 0.2–0.4 wt% and provides mechanistic insight into the role of Sc in alloy design.