**3. Results and Discussion**

### *3.1. Rapid Solidification Structures*

The phase constitution of Co-4.54%Sn alloy was selected with the largest solid solubility point of Sn solute in Co solvent; as shown in the binary Co-Sn phase diagram of Figure 1a,b, the liquidus and solidus temperatures were 1760 and 1385 K, respectively, and the solidification temperature range attained 375 K (0.21 *TL*). From the equilibrium phase diagram, it can be seen that the primary face-centered cubic αCo solid solute phase formed in the liquid phase when the temperature reduced to 1760 K; firstly, during the solidification, the solute content was about 0.56 wt.%Sn in the αCo phase, and then the βCo3Sn2 intermetallic phase was generated as the temperature lowered to about 1443 K. Subsequently, the close-packed hexagonal εCo phase had the potential to form once the temperature decreased to about 695 K. Furthermore, the X-ray diffraction patterns showed three phases of αCo, βCo3Sn2 and εCo displayed at the different undercoolings under the rapid solidification, which corresponds with the equilibrium phase diagram. The microstructures of binary Co-4.54%Sn alloy relate to the undercooling at rapid solidification, which are shown in Figure 2a–d. At the small undercooling of 11 K, the microstructure of the primary αCo phase was characterized by the coarse and well-developed dendrites, the

βCo3Sn2 phase distributed into the interdendrites, shown as the white phase and the εCo phase displayed the dark structures, as seen in Figure 2a. Subsequently, the microstructure was remarkably refined and the dendritic feature disappeared gradually; the volume fraction of βCo3Sn2 phase decreased at the same time, but the volume fraction of the εCo phase increased with the increase of undercooling. Ultimately, the microstructure of the primary αCo phase grew with the disorder at the largest undercooling of 189 K, and a small number of βCo3Sn2 particles and a large number of εCo phase distributed randomly into the matrix of the primary αCo phase. Clearly, the phase constitution has not changed at the rapidly solidified condition. It is worth noting that the eutectic-like microstructure exists in the whole undercooling of the binary Co-4.54%Sn alloy, as seen in Figure 3a,b, which grew around the βCo3Sn2 phase. Meanwhile, the eutectic-like structure appears coarsening and the volume fraction increased obviously with increase of undercooling and decrease of the βCo3Sn2 volume fraction, and formed eutectic cell characteristics at the largest undercooling of 189 K.

**Figure 1.** Selection of alloy composition and X-ray diffraction of rapidly solidified alloy droplets. (**a**) Co-4.54%Sn alloy location in phase diagram and (**b**) XRD pattern.

**Figure 2.** Microstructures at different undercoolings, (**a**) 11 K, (**b**) 94 K, (**c**) 122 K, and (**d**) 189 K.

**Figure 3.** Eutectic structures within interdendritic spacings, (**a**) Δ*T* = 11 K, and (**b**) Δ*T* = 189 K.
