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.

Influence of the Al2Ca phase on microstructure and mechanical properties of Mg–Al–Ca alloys

“…Based on the experimental results described above, the grain refinement mechanism may be considered to focus on the two critical aspects as following: One may argue that the effect of solute addition on grain size, which has been explained in terms of the growth restriction factor (GRF) (also known as Q) [16,17]. The larger the Q value, the more powerful the growth restriction is expected to be.…”

“…The larger the Q value, the more powerful the growth restriction is expected to be. Particularly the calcium has a high Q-value of 11.94 [16]. At the initial stage of solidification, the strong segregation ability possessed by Ca would form intensified constitutional undercooling in a diffusion layer ahead of the advancing solid/liquid interface which restricts grain growth and promotes nucleation in the melt.…”