Effect of Cooling Rates on the Microstructure Evolution and Eutectic Formation of As-cast Mg-Al-Ca Alloys

A Mg-5.0wt.%Al-2.0wt.%Ca alloy (AC52) was cast at different cooling rates varying from 0.5 to 65 °C/s. The dendrites was characterized by determining the secondary dendrite arm spacing (SDAS) and the volume fraction of secondary eutectic phases with the linear intercept and point counting methods, respectively. The SDAS decreases significantly with increasing cooling rates, while the volume fraction of the eutectic phase increases from 10.8 ± 1.44 vol.% at 0.5 °C/s to 20.4 ± 1.52 vol.% at 20 °C/s. However, a further increase in cooling rate beyond 20 °C/s has limited influence on the volume fraction of eutectic phases. A large number of dispersed eutectic phases were observed in the primary α-Mg of the alloys cast at low cooling rates. Although, at the microscale, there were no dispersed eutectic phases in alloys cast at a high cooling rate of 30 °C/s, nanoscale eutectic phases were found by TEM observation. The dimensions and morphology of the Al₈Mn₅ phase were also dependent on the cooling rates. When cooling rates were high, the size of Al₈Mn₅ particles was relatively small compared to that observed at low cooling rates. The majority of Al₈Mn₅ particles were found to be attached to the eutectic phases, which implies the possible nucleation of the eutectic phase on the Al₈Mn₅ particles present in the melt.