Direct formation of La(Fe,Si)₁₃ phase with enhanced mechanical property of off-stoichiometric La_1.7Fe_11.6Si_1.4 alloys by directional solidification

The off-stoichiometric La_1.7Fe_11.6Si_1.4 alloys fabricated by directionally solidification using zone melting liquid metal cooling is systematically studied in terms of morphology, microstructure, magnetocaloric effect, phase transition and mechanical property. Different from the conventional casting in which the La(Fe,Si)₁₃ phase can only be obtained after a long-time heat treatment, a large volume fraction (over ∼80%) of La(Fe,Si)₁₃ phase is formed directly during solidification. The Curie temperature is increased and the magnetic entropy change under applied fields of 2T attains 9.1 J kg⁻¹ K⁻¹. The activation energy of the magnetic phase transition is calculated using the Johnson-Mehl-Avarmi-Kolmogorov equation and the Arrhenius relation for the first time, showing a higher value than that of NiMn-based alloys. Compared with the Fe-rich alloys, the directionally solidified off-stoichiometric La_1.7Fe_11.6Si_1.4 alloys show higher bending strength, which is due to the existence of the preferential growth α(Fe,Si) phase. Furthermore, the α(Fe,Si) phase plane interface and periodic morphology are detected along the crystal growth direction, and the corresponding formation mechanism is discussed.