Microstructural Evolution of Mg-4Al-2.5Ca Alloy during Solidification

Abstract

In the present work, the microstructure evolution of Mg-4Al-2.5Ca alloy during solidification was investigated by means of scanning electron microscopy (SEM), energy dispersive spectrometer (EDS) and differential scanning calorimetry (DSC). The results showed that the as-cast microstructure was typical dendritic structure, consisting of α-Mg and the Al2Ca with a C15 structure formed at inter-dendritic regions. But during slow solidification, the sequence of phase formation of this alloy consisted of following stages: L→ L + α-Mg → L + α-Mg + (Mg, Al)2Ca → α-Mg + (Mg, Al)2Ca + Al2Ca → α-Mg + Al2Ca. Thermal analysis showed that the (Mg, Al)2Ca and Al2Ca phase separation temperature was about 520°C and 510°C respectively, which is different from thermodynamic calculation. With the temperature decreasing, the liquid (Mg, Al)2Ca phase with thick and continuous morphology transformed into lamellar-shaped eutectic phase, and parts of (Mg, Al)2Ca phase translated into Al2Ca phase with bone-shaped and needle-like.