**4. Conclusions**

In summary, the broad temperature range of binary Co-4.54%Sn alloy was rapidly solidified using the fluxing method and the maximum undercooling attained 208 K. XRD analyses show that three phases of αCo, βCo3Sn2 and εCo were displayed during the solidification, which corresponds with the equilibrium phase diagram. The microstructure of the primary αCo phase remarkably refines and the volume fraction of βCo3Sn2 phase of interdendritic structure decreases evidently with undercooling. The dendritic growth velocity presents an increasing tendency at the beginning stage and the maximum velocity reaches 0.95 m/s when the undercooling is lower than 175 K, and then it decreases once the undercooling exceeds 175 K. Otherwise, the microhardness and resistivity of Co-4.54%Sn alloy increase with undercooling owing to the refinement of the microstructure, and the increase of solute content in the primary αCo phase is caused by the high undercooling and rapid solidification. In addition, the saturation magnetization reduces with the increase of undercooling, which is also due to the crystal particle and boundary increasing significantly and solute content being enhanced in the primary αCo phase, which results in the undercooling and dendritic growth velocity under rapid solidification. Moreover, the remanent magnetization and coercivity enhance shows a "increase first and then decrease" trend.

**Author Contributions:** Methodology, A.W.; software, W.W. and W.L.; formal analysis, W.W. and W.L.; data curation, W.W.; writing—original draft preparation, W.W. and W.L.; writing—review and editing, W.W. and W.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** Financial support was provided by the National Natural Science Foundation of China [grant numbers 51931005, 52171048, 51571163] and the Key Industry Innovation Chain Project of Shaanxi Province (2020ZDLGY12-02).

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.
