Effect of cooling rate and Al content on solidification behavior and microstructure evolution of as-cast Mg–Al–Ca–Sm alloys

Abstract

In the present study, the Mg–xAl–2Ca–2Sm (x = 3, 5, 9 and 15) alloys were fabricated in sand mold with stepped type, and the cooling rates varied from 0.3 to 3.5 °C s−1. The solidification behavior and microstructure evolution of Mg–xAl–2Ca–2Sm alloys were carried out by computer-aided cooling curve thermal analysis method, optical microscope, scanning electron microscope, X-ray diffraction analysis and transmission electron microscope, respectively. The experimental results showed that the nucleation temperature and grain size of α-Mg phase decreased with increasing the cooling rate and Al content. However, the volume fraction of precipitated phase increases with cooling rate and Al content. In the Mg–3Al–2Ca–2Sm alloy cooled at 3.5 °C s−1, there were Al2Sm (rod and particle structures), Al11Sm3 (acicular structure) and (Mg, Al)2Ca (lamella structure) phases. But the Al11Sm3 phase and (Mg, Al)2Ca phase will gradually transform to the Al2Sm phase and Al2Ca phase, respectively. In addition, there were both Al2Sm (particle structures), Al2Ca (fish-bone structure) and β-Mg17Al12 (reticular structure) phases in Mg–15Al–2Ca–2Sm alloy cooled ranging from 3.5 to 0.3 °C s−1. In the Mg–xAl–2Ca–2Sm alloys, the Al2Sm phase mainly distribute in the grains, but all the Al11Sm3, (Mg, Al)2Ca, Al2Ca and β-Mg17Al12 phases distributed in the boundary of grains. The solidification sequence of precipitated phase in Mg–xAl–2Ca–2Sm alloys can be obtained: Al2Sm → α-Mg → Al2Ca → β-Mg17Al12.