Rapid solidification mechanism and magnetic property of ternary equiatomic Fe_33.3Cu_33.3Sn_33.3 alloy

Rapid solidification is a typical non-equilibrium phase transition process, and the crystallization rate of liquid metal is larger than 1 cm·s^-1. If the alloy is solidified in this case, the solute segregation is reduced or even eliminated and the solid solubility can be improved significantly. Rapid solidification technique can be used to refine the microstructures of alloys, which provides an effective method to prepare the novel metastable materials and improve their strengths, plasticities magnetic properties, etc. In this work, the rapid solidification mechanism and magnetic property of ternary equiatomic Fe_33.3Cu_33.3Sn_33.3 alloy are investigated by drop tube and melt spinning techniques. It is known that Fe-Cu-Sn ternary alloy forms a typical immiscible system. However, the experimental results reveal that the liquid phase separation does not take place during the rapid solidification of ternary equiatomic Fe_33.3Cu_33.3Sn_33.3 alloy. The solidification microstructures are all composed of primary αFe dendrites together with Cu₃Sn and Cu₆Sn₅ phases. Under the free fall condition, as the drop tube technique provides microgravity and containerless states, the maximum surface cooling rate and maximum undercooling of alloy droplets are 1.3×10⁵ K·s^-1and 283 K (0.19T_L), respectively. When the surface cooling rate reaches 1.9×10³ K·s^-1, the primary αFe phase appears as coarse dendrites, and its maximum dendrite length is 41 μm. Meanwhile, the Cu₃Sn and Cu₆Sn₅ phases are distributed in the αFe interdendritic spacings. Once the surface cooling rate increases up to 3.3×10³ K·s^-1, the morphology of the primary αFe phase transforms from coarse dendrites into broken dendrites. It is found that the cooling rate and undercooling greatly affect the solidification microstructure of alloy droplets. During the melt spinning experiments, since the large temperature gradient exists between the wheel surface and free surface, the solidification microstructure is subdivided into two crystal zones according to the different microstructure morphologies of αFe phase: fine grain (zone I) and coarse grain (zone II), where zone I is characterized by granular grains while zone II has some dendrites with secondary branch. Under the rapid cooling condition, the microstructures of ternary equiatomic Fe_33.3Cu_33.3Sn_33.3 alloy ribbons are refined significantly and show soft magnetic characteristics. As the surface cooling rate increases from 8.9×10⁶ to 2.7×10⁷ K·s^-1, the lattice constant of αFe solid solution rises rapidly and the coercivity increases from 93.7 to 255.6 Oe. Furthermore, the results indicate that the grain size of αFe phase is the main factor influencing the coercivity of alloy ribbons.