Microstructure evolution of undercooled binary Co–Sn near eutectic alloys under high magnetic field

The non-equilibrium solidification behavior of deeply undercooled Co–Sn binary alloys (hypoeutectic Co_79.5Sn_20.5 at.%, eutectic Co₇₆Sn₂₄ at.% and hypereutectic Co₇₂Sn₂₈ at.%) has been investigated by the glass fluxing method under high magnetic fields up to 12 T. Analysis of the results showed that anomalous eutectics consistently manifest in the terminal microstructure of Co–Sn binary alloys regardless of whether a magnetic field is applied. The application of a high magnetic field reduces the critical undercooling for the formation of anomalous eutectics without changing the primary solidification pathway. It was found that at low undercoolings, regular eutectics formed preferentially with a mutual transformation between regular and anomalous eutectics subsequently. At large undercoolings, regular and anomalous eutectics formed nearly simultaneously, further analysis revealed that the two eutectic phases can transform from either coupled or uncoupled growth states into anomalous eutectics. The present research reveals the microstructural evolution characteristics of Co–Sn binary alloys and explains the origin of anomalous eutectics. It enriches the theory of high magnetic field effects on alloy solidification and offers practical guidance for optimizing the microstructures of eutectic alloys.