Microstructure evolution of as-cast Mg–5Sn alloy with Ba addition

• Morphology of Mg2Sn phase is distinct from irregular shape reported elsewhere.
• With Ba addition, new BaMg2Sn2 phase forms distributed along the grain boundaries.
• The increased Ba content refines the Mg2Sn phase and dendrite arm spacing of α-Mg.
• The feather-shaped BaMg2Sn2 phase comprises nanocrystals rather than a monocrystal.

This work lays great emphasis on the effect of Ba addition on microstructure evolution of magnesium alloy Mg–5Sn. It is found that the Mg2Sn phase in Mg–5Sn alloy adopts nearly spherical or plate-like morphology, the morphology distinct from the previously reported irregularshape. One thing to be noted is that most of those spherical particles have a distinct defect contrast in the interior. Our results reveal that with trace addition of Ba, a new BaMg2Sn2 phase forms distributed along the grain boundaries, and both the Mg2Sn phase and the secondary dendrite arm spacing of primary α-Mg phase are refined. Even more surprising is that the feather-shaped or needle-like BaMg2Sn2 phase is composed of numerous nanocrystals with a radius less than 50 nm rather than a monocrystal, in contrast to a large extent with Mg2Sn phase, a phase to be only a monocrystal. This unusual phenomenon seems to be greatly tied to the oriented attachment mediated growth mechanism of BaMg2Sn2 phase.

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