**1. Introduction**

The broad solidification temperature range is the temperature range of a solid–liquid two-phase mixture which is larger than 0.2 *TL*, in which the alloy ingot is in a semisolid state for this liquid/solid range during the solidification; this range is called the broad mushy zone [1,2]. For a binary single-phase alloy, a solidification nucleates from the liquid to solid phase over a broad temperature interval, and then grows rapidly and forms a single-phase microstructure for a considerable time in this mushy zone [2]. Therefore, the phenomenon of solidification shrinkage and hot tearing is caused by thermal contraction, as the higher density of the solid driving the cracking process and hindered liquid flow prevents any void from being filled with liquid. Several defects, such as low fluidity, shrinkage porosity, heat cracking, segregation, and so on, take place inside this interval [1–5]. The rapid solidification of high undercooling is an effective method to avoid or eliminate these defects.

The rapid solidification of metallic alloys under high undercooling has the advantages of extended solubility, refined microstructures and the formation of a metastable phase [6–11]; thus, it can reduce defects, effectively improve microstructure and achieve excellent physical properties. Some binary single-phase alloys with a broad solidification temperature range show special phenomena during the rapid solidification of high undercooling; for example, dendritic growth velocity decreases when the undercooling exceeds a critical value, and the final microstructure is vermicular dendrite instead of equiaxed grains. Moreover, the undercooling and cooling rates have significant influence on the microstructure, microhardness and electrical resistivity properties during the rapid solidification [12,13].

**Citation:** Wang, W.; Li, W.; Wang, A. Dendritic Solidification and Physical Properties of Co-4.54%Sn Alloy with Broad Mushy Zone. *Metals* **2023**, *13*, 1046. https://doi.org/10.3390/ met13061046

Academic Editor: Maciej Motyka

Received: 17 February 2023 Revised: 17 April 2023 Accepted: 28 April 2023 Published: 30 May 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Co-Sn alloys have been widely studied in recent years owing to their application properties; for example, the production of Co-Sn-based metallic glasses and anode materials in lithium ion batteries [14–16]. In addition, the investigation of Co-Sn alloys have also focused on the structural transformation of different Sn content or eutectic alloys [17], physical properties of the viscous flow, surface tension and enthalpy of mixing near the eutectic concentration [18,19]. However, there are scarce studies on the structural evolution, dendritic growth and physical properties of binary Co-Sn alloys with a broad solidification temperature range and rapid solidification at high undercooling.

In this work, we selected a special composition of Co-4.54%Sn alloy with a broad solidification temperature range of 375 K (0.21 *TL*) to explore the microstructural evolution and dendritic growth characteristics under substantial conditions of undercooling. Moreover, the physical properties of the microhardness, electrical resistivity and magnetic characteristics are investigated at different undercoolings.
