The effect of Ba addition on microstructure of in situ synthesized Mg2Si/Mg–Zn–Si composites

In order to modify in situ synthesized Mg2Si phase in Mg–6Zn–4Si alloy, Ba addition of 0.1–3.0 wt.% had been explored. The effect of Ba addition on the microstructure of Mg2Si/Mg–Zn–Si composites was investigated by means of optical microscope, scanning electron microscope, X-ray diffraction and energy dispersive spectrometer. The results indicate that the morphology of primary Mg2Si in the composites changes from large dendrite to fine polygon with the increasing Ba content. The average size of primary and eutectic Mg2Si sharply decreases with increasing Ba content up to 1.0 wt.% and then slowly increases. Theoretical calculation and experimental analysis show that tiny BaMg2Si2 particles, formed by adding Ba, act as the heterogenous nucleation substrates for primary Mg2Si. Therefore, Si consumption due to BaMg2Si2 formation and primary Mg2Si nucleation results in the inhibition of Mg2Si growth. It is also found that the BaMg2Si2 phase in primary Mg2Si is obviously coarsened as Ba content exceeds 1.5 wt.% and some needle-like Ba2Mg3Si4 is found in alloy with 3.0 wt.% Ba. These are responsible for the over modification effect. Therefore, it can be concluded that proper Ba addition can effectively modify and refine primary Mg2Si and decrease the amount of eutectic Mg2Si.