Nucleation Kinetics Of Crystalline Phases In Undercooled La-Fe-Si Melts

La(Fe,Si)13 alloys have attracted much attention in recent years because they exhibit a large magnetocaloric effect near room temperature. Usually, they are prepared by arc melting, which is followed by a time-consuming annealing treatment under an inert gas atmosphere. Recently, it has been found that rapid quenching can help shorten the annealing time in an efficient way. However, the phase selection mechanism involved in rapid quenching is not clear. For this reason, the present thesis carried out an investigation of nucleation kinetics of two competitive phases, a(Fe,Si) phase and La(Fe,Si)13 phase, in the undercooled liquid.A model proposed by Spaepen was used to construct the solid-liquid interface of La(Fe,Si)13 phase and to calculate theα-factor of it. First, the densest plane of the cubic cell of the La(Fe,Si)13 phase was determined. With it as the base, the solid-liquid interface was constructed following construction rules of the model. In each step of the construction, the packing density of the interface and the a-factor were calculated, until the packing density became converged. Then, a model proposed by Miedema was used to calculate the melting entropy△ASf, of La(Fe,Si)13 phase. Furthermore, the solid/liquid interfacial energyσls, ofα(Fe,Si) phase and La(Fe,Si)13 phase was calculated. Finally, the steady-state nucleation rates of the two phases as a function of liquid undercooling were calculated in the light of the classical nucleation theory.Results showed that:(1) theα-factor of the liquid/solid interface of La(Fe,Si)13 phase is 0.417; (2) the melting entropy of phase La(Fe,Si)13 is 21.27 J/(mol·K); (3) the liquid/solid interfacial energy of La(Fe,Si)13 is 0.430 J/m2; and that (4) La(Fe,Si)13 phase will reach a higher nucleation rate than that ofα(Fe,Si) phase, when the liquid undercooling exceeds 308K. The present data predicted that direct crystallization of the peritectic La(Fe,Si)13 phase from the liquid will be difficult under small undercooling condition because of an insufficient nucleation rate, whereas it is enabled for undercoolings of larger than 308 K because of the achievement of a considerable nucleation rate. Such predictions are in agreement with the experimental data available so far.