Solidification and microstructural formation of a ternary eutectic Al-Cu-Si cast alloy

A novel ternary eutectic Al-27%Cu-5%Si cast alloy was investigated to understand its solidification behavior and microstructural evolution. The studied alloy was cast, remelted and subjected to a rapid cooling in ice-water mixture, and also cooled slowly in air. The solidification sequence of the alloy was studied using Thermo-CalC software with a subsequent DSC measurement. Micro- and nano-hardness tests were conducted at room temperature to assess the change of hardness associated with the microstructural changes due to the different cooling paths the alloy was subjected to. Thermodynamic analysis showed that the equilibrium solidification path of the eutectic alloy consists of three stages: precipitation of Al2Cu primary phase at 527.6 °C, evolution of binary eutectic (α-Al + Al2Cu) at 525.9 °C and the formation of ternary eutectic (α-Al + Al2Cu + Si) at 521.7 °C. DSC analysis of the alloy suggests only one endothermic peak at 536.4 ± 0.2 °C due to the melting of the alloy, which also corresponds to the ternary eutectic transition. The quenched alloy has a well-refined and uniform distribution of bimodal eutectic microstructure when compared with the slowly cooled alloy. The hardness of the quenched alloy was observed to be 16% higher than the slowly cooled alloy. The increased microhardness of the quenched alloy is attributed to the solid solution strengthening of the alloying elements and homogeneous dispersion of fine Si particles, and partly due to the higher volume fractions of both θ-Al2Cu and Si phases. © 2020 The Author