Phase Relations And Dendrite Growth Kinetics During Solidification Of La-Fe-Si Alloys

The La(Fe,Si)13compound is a potential material for room-temperature magnetic refrigeration. A single-phase material is usually obtained by annealing as-cast bulk ingots at elevated temperatures for a long time. Recently, it has been found that rapid solidification processing promotes direct crystallization of the La(Fe,Si)13compound from the liquid phase, allowing for a great reduction of the annealing time. In order to understand the mechanism of phase selection in the rapidly solidified mateiral, phase relations and dendritic growth kinetics during solidification of La-Fe-Si alloys were investigated in the present thesis.Alloys of composition LaFe13-xSix (x=0.5-7.0) alloys were prepared using the arc-melting technique and the induction melting technique. Phase constituents and microstructure of the alloys were investigated by x-ray diffraction (XRD) analysis and scanning electron microscopy (SEM). Dendrite growth velocities of primary phases in undercooled La-Fe-Si alloy melts were determined by high-speed potography. The phase selection mechanism operative during rapid solidification of La-Fe-Si alloys was discussed in terms of the measured growth velocities. Major findings and conclusions were summarized as follows.1. Under near-equilibrium solidification conditions, the La(Fe,Si)13compound cannot be formed in any La-Fe-Si composition under study. The α-(Fe,Si) phase, the LaFe2Si2phase, and the FeSi phase was solidified as the primary phase in sequence of increasing x. Besides these phases, ternary phases such as LaFeSi, LaFe2Si3, LaFeSi2and binary phases such as La5Si3, Fe3Si, FesSi3were crystallized from the liquid phase. The LaFe2Si3phase was identified as a novel compound, which was formed in the alloys with x=5,5.5and6.2. Under non-equilibrium solidification condition, the La(Fe,Si)13compound was crystallized into two structures, cubic and tetragonal. The cubic phase was formed in alloys with low Si content (x=1.5) as a peritectic solidification product, whereas the tetragonal phase was crystallized primarily in alloys with high Si content (x=2.8). It was suggested that cooling rate played a subsidiary role in the preferred crystallization of the cubic La(Fe,Si)13-type phases during rapid solidification processing of La-Fe-Si alloys, whereas liquid undercooling played a decisive role.3. The measurements of dendrite growth velocities demonstrated that the addition of the rare earth element La reduced the growth velocity of α-(Fe,Si) phase. For a given undercooling, the cubic La(Fe,Si)13phase reached a lower growth velocity than that of the a-(Fe,Si) phase. It was thus concluded that phase selection in the rapidly solidified La-Fe-Si alloys was not controlled by competitive growth kinetics.