Phase and Structure of Bulk LaFe11.6Si1.4 Alloy and Studies of Magnetocaloric Effect of Optimization Alloy
In this paper,the phases and microstructures of the kilogram-class LaFe11.6Si1.4 as-cast discs with 3％and 9％excessive La element as melting loss were studied firstly.X-ray diffraction(XRD)patterns showed that the main phases in different regions of the two discs were Fe and LaFeSi,and La5Si3 phase existed in the disc with 9％excessive La.In cast center sample,Fe was in a parallel columnar shape,while the white rich La phase was located between columnar dendrites,indicating that during the solidification pro-cess,nucleation occurred first due to the lowest solidification point of Fe.The maximum subcooling occurred along the vertical direc-tion of the mold bottom,and Fe nuclei grew along this direction to form columnar crystals perpendicular to the mold bottom.Due to the difference in cooling speed between the lower and upper parts of the casting disc,the fast cooling zone in the lower part led to fine grain,while in the upper part,coarse grain was formed in the opposite direction.1∶13 phase could be observed in the edge region of two discs,however it could not be observed in the samples at 1/2 radius and center regions.In the edge and center regions of the four symmetrical fan-shaped bulks of 9％La annealed at 1503 K for 3,5,8 and 10 h,the contents of Fe and LaFeSi phases were smaller than those of the corresponding region samples in 3％La.The phases and structures of the center and the edge region samples in the an-nealed 9％La were basically similar.And more,the microsegregation caused by the difference of cooling rate in the as-cast disc could not be observed.La5Si3 phase caused by excessive La could still be observed in the center region sample of 9％La annealed for 10 h.This indicated that once La5Si3 phase was present in the cast LaFe13-xSix alloy,it was difficult to eliminate it through later heat treat-ment.At the same time,energy spectrum analysis showed that the atomic percentages of La,Fe and Si in 1∶13 phase were 8.32％,80.14％and 11.55％,respectively.This was similar to Fe atomic percentages in 1∶13 phase(7.77％,80.15％,12.09％)during 10 h of heat treatment with the addition of 9％La edge sample,which was beneficial for the consistency of the alloy's magnetic and magneto-caloric 1 properties.Curie temperatures of the center region samples annealed for 5,8 and 10 h were near 191.5 K.The corresponding maximum magnetic entropy changes and relative refrigeration capacities were 20.0,24.4,24.4 J·kg-1·K-1 and 158,183 and 188 J·kg-1 under 2 T magnetic field,respectively.It could be seen that the cooling capacity of the alloy was equivalent after 8 and 10 h of heat treatment,so 8 h heat treatment time was appropriate.In addition,after 8 and 10 h of heat treatment,the maximum magnetic entropy changed and relative refrigeration capacities of the center and edge samples were basically the same.The above research indicated that when preparing kilogram-class LaFe11.6Si1.4 discs using an intermediate frequency induction furnace,adding 9％La as melting loss would introduce a certain amount of brittle and poor corrosion resistance La5Si3 phase into the cast alloy,and a certain amount of this phase would still be retained in the alloy after long-term high-temperature heat treatment.However,compared to adding 3％La as melt-ing loss corresponding disc alloy,the content of impurity Fe and LaFeSi phase was lower.Therefore,when preparing kilogram-class LaFe13-xSix alloy,it could be used as a reference to further adjust the amount of La added as melting loss,and obtain the expected mate-rial with lower La5Si3 phase content.Although there were significant structural differences caused by cooling rate during the casting of 9％La cast discs,the approximate XRD and scanning electron microscope(SEM)images of the edge and center alloy samples after heat treatment were not different,which fully proved that large LaFe13-xSix alloys with the same related system and uniform microstruc-ture could be obtained through appropriate heat treatment.After the same heat treatment,Curie temperature,maximum magnetic en-tropy change,and relative cooling capacity of the center and edge samples of the bulk LaFe11.6Si1.4 alloy were basically consistent,fur-ther confirming that the magnetic and thermal properties of the bulk LaFe11.6Si1.4 alloy studied in this paper had good stability.
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