Solidification microstructures of the undercooled Co–24 at%Sn eutectic alloy containing 0.5 at%Mn

•Coupled growth of the two eutectic phases always takes place.•Small addition of Mn influences the eutectic interface morphology evolution.•The growth interface morphology depends on the interface energy anisotropy.•0.5 at%Mn addition changes the interface energy anisotropy.•0.5 at%Mn addition introduces divorced eutectic mechanism for anomalous eutectic.

Solidification samples of undercooled Co–24 at%Sn eutectic alloy containing a small amount of Mn (<1.0 at%) were prepared by the glass fluxing technique. The surface and internal solidification microstructures of the samples were observed by a scanning electron microscope (SEM) and an optical microscope (OM), respectively. The experiment results revealed that the addition of 0.5 at%Mn remarkably changed the solidification behaviors of the undercooled Co–24 at%Sn eutectic alloy. The addition of 0.5 at%Mn influenced the morphological selection of eutectic growth interface by increasing the interface energy anisotropy during the solidification of the undercooled Co–24 at%Sn eutectic melt. As undercooling increases, the coupled eutectic growth interface morphology successively experienced dendritic pattern, factual seaweed pattern and compact seaweed pattern. Besides, the addition of 0.5 at%Mn decreased the critical undercooling for the formation of anomalous eutectic by introducing a new formation mechanism of anomalous eutectic, i.e. divorce eutectic mechanism.

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