| La(Fe,Si)13 pseudobinary compounds with the NaZn13 structure are enjoying more and more interest in recent years due to the large magnetocaloric effect. In common ways, the NaZn13 structure alloys are produced by arc-melting and followed by a long time (1273 K/15 days) annealing in the Ar atmosphere. Rapid quenching and subsequent short time annealing (1273 K/20 minutes) is an effective production route for La-Fe based alloys with the NaZn13 structure. From the commercial point of view, however, such a time-consuming process is unfavorable. In a word, it is of great technical interest to control the solidification behavior of La-Fe based alloys. However, the mechanism of the formation phases is not clear in La-Fe based alloys. Hence, the present work is designed to investigate the solidification behavior and the mechanism of phase election in La-Fe based compounds through experiments and theoretical calculation.First in the paper, solidification behavior of La-Fe based alloys was investigated using arc-melting. The results show that the microstructure of the LaFe13-xSix alloys is made of a-(Fe,Si) phase, the LaFeSi phase and La(Fe,Si)13 phase. With x>1.5, the La(Fe,Si)13 phase occur in the microstructure. It help to fomation La(Fe,Si)13 phase with the x value increasing. The microstructure of the LaFe13-x-ySixCOy alloys is made of a-(Fe,Si,Co) phase, the La(Fe,Co)Si phase and La(Fe,Si,Co)13 phase. With x=1.5 and y=0.2-0.6, the La(Fe,Si,Co)13 phase occur in the microstructure. It help to fomation La(Fe,Si,Co)13 phase with the y value increasing.Phase formation and structure in induction melted La-Fe based alloys has been investigated in order to research on solidification behavior of alloys under near equilibrium solidification condition. The results show that the 1:13 phase is difficult to formation under the small cooling rate or low undercooling. The phase relations of LaFei3-xSix alloys are displayed as follow. The primaryα-(Fe,Si) phase first is formed form liquid phase at high temperature, then the LaFeSi eutectic phase is crystallized at low temperature with values (x=0.5-1.5). The primary a-(Fe,Si) phase first separate out from the liquid then the LaFe2Si2 phase and the LaFeSi phase with values (x=2.0-2.5). At x=3.0, first the a-(Fe,Si) phase is formed then the lamellar eutectic phase is occurred which are LaFe2Si2 phase plus Fe3Si phase. The a-(Fe,Si) phase and the LaFe2Si2 phase is mainly phase in the microstructure of alloy with low and high Si contents, respectively. The phase relations of LaFe13-x-ySixCoy alloys are displayed as follow. The primary a-(Fe,Si,Co) phase first is formed form liquid phase at high temperature, then the La(Fe,Co)Si eutectic phase is crystallized at low temperature with values (x=0.5-1.5). The liquid temperature of LaFe13-xSix and LaFe13-x-.SixCoy alloys was measuremented by using infrared radiation thermometers. The results show that the liquid temperature of La-Fe based alloys decrease with Si content increasing; and TN(1:1:1)< TN(1:13)< TN(1:2:2) in LaFe13-xSix alloys.Solidification behavior of La-Fe based alloys was investigated under rapid solidification condition by using rapid quenching and levitating. Phase formation and structure of La-Fe based alloys has been investigated with the undercooling and the cooling rate increasing, respectively. The results show that under higher cooling rate, composition with x=1.5, the La(Fe,Si)13 phase is crystallization in localized thin area near contact surface of ribbon sample. When x=2.5, the La(Fe,Si)13 phase is directly crystallization without via peritectic reaction at contact surface of ribbon sample of LaFe13-xSix alloys. It implies that the solidification route change and La(Fe,Si)13 phase priority the primary a-(Fe,Si) phase in competition during rapid quenching. The solidification behavior of LaFe13-x-ySixCoy alloys depend on Si and Co contents x and y. The microstructure of compositions with x=0.5-1.0 and y=0-0.4 is made ofα-(Fe,Si,Co) phase and La(Fe,Co)Si. The microstructure of compositions with x=1.0 and y=0.6 occur La(Fe,Co,Si)13 phase. Under large undercooling, composition with x=1.5 and△T≥40K, the La(Fe,Si)13 phase is crystallization in localized thin area bottom of the sample LaFe13-xSix alloys. When x≥2.5 and△T≥55K, the La(Fe,Si)13 phase is directly crystallization without via peritectic reaction. It implies that the solidification route change and La(Fe,Si)13 phase priority the primaryα-(Fe,Si) phase in competition during levitating. The solidification behavior of LaFe10.9Co0.6,Sii.5 alloys depend on undercooling. When△T≤10K, the microstructure of the sample is made of a-(Fe,Si,Co) dendrites and in interdendritic La(Fe,Co)Si phase.α-(Fe,Si,Co) phase is primary phase. When△T≥40K, the microstructure of the sample is made ofα-(Fe,Si,Co) dendrites and in interdendritic La(Fe,Si,Co)B13B phase and La(Fe,Co)Si phase. The La(Fe,Si,Co)B13B phase increase with undercooling increasing. The solidification behaviors and microstructure of LaFe13-xSix alloys strong depend on Si contents, cooling rate and undercooling.The phase selection mechanisms were discussed using classical nucleation theory and transient nucleation theory. According to the calculation results of competitive nucleation, the a-(Fe,Si) phase has higher nucleation rate than La(Fe,Si)13 phase above a critical undercooling, and the primary phase isα-(Fe,Si) phase. The primary a-Fe is replaced by La(Fe,Si)13 below a critical undercooling. Based on transient nucleation theory, the calculation results of incubation time of a-(Fe,Si) phase and La(Fe,Si)13 phase in rapidly solidified alloys, it indicates that the incubation periods of a-(Fe,Si) phase are less than that of La(Fe,Si)13 phase under small undercooling. So a-(Fe,Si) phase is formed from the liquid phase at first. Under large undercooling, the incubation time of La(Fe,Si)13 phase are less than that of a-(Fe,Si) phase, and La(Fe,Si)13 phase is formed from the liquid phase at first. |
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