Formation of NaZn 13 -type phase in LaFe 11.5 S 1.5 alloy during solidification process
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Formation of NaZn13-type phase in LaFe11.5Si1.5 alloy during solidification process Xueling HOU 1,2,*, Chunyu LIU 1,2, Yun Xue1,2, Ning Han3, Hui Xu1,2, Chunwei Ma3 , M.H. Phan4 1 Laboratory for Microstructures of Shanghai University, 99 Shangda Road, BaoShan District, Shanghai, 200444, China. 2 School of Materials Science and Engineering, Shanghai University, 149 Yangchang Road, Zhabei District, Shanghai, 200072, China. 3 School of Materials Science and Engineering,Shanghai University Of Engineering Science, 333 Longteng Road, Songjiang District, Shanghai, 201620, China. 4 Department of Physics, University of South Florida, Tampa, FL 33620, USA.
ABSTRACT Low-cost La(FexSi1-x)13 alloys exhibiting the large magnetocaloric effect (MCE) are one of the most promising magnetic refrigerant candidates for room temperature magnetic refrigeration. The NaZn13-type phase (hereinafter 1:13 phase) is believed to play a key role in the MCE of these alloys. While the formation of the 1:13 phase directly from the melt upon cooling was challenging, in this paper we demonstrate that the 1:13 phase can be formed directly during solidification. We found that three kinds of solidification microstructure were formed because a competitive nucleation occurred between the 1:13 and α-(Fe,Si) phase during the solidification of LaFe11.5Si1.5 alloy. In case of a high cooling speed, a large amount of NaZn13–type phase with equiaxed grains and a small amount of α-(Fe,Si) phase were formed because of a dominant nucleation rate of 1:13 phase. When the cooling rate was small, a large number of α-(Fe,Si) phase with dendrites were formed because the nucleation rate of α-(Fe,Si) phase is larger than that of the 1:13 phase. These results revealed that nucleation rates of phases is very important to the composition formation and microstructure of LaFe11.5Si1.5 alloys. KEYWORD: La-Fe-Si; NaZn13-type phase; Microstructure; nucleation rate; Solidification
INTRODUCTION Magnetic refrigeration technology based on the magnetocaloric effect (MCE) has generated growing attention in the scientific community due to its high efficiency and environmental friendliness. The discoveries of various magnetic refrigerants, such as Gd5(SixGe1-x)4 compounds [1], MnAs-based compounds [2,3], La(FexSi1-x)13 compounds [4,5] have brought a dramatic breakthrough for room temperature magnetic refrigeration. However, there are still many difficulties for the practical use
of those materials. For example, the high-purity raw materials are necessary for preparing Gd5(SixGe1-x)4 compounds with giant MCE. Also, Arsenic of MnAs-based compounds is a poisonous element, which should be avoided [6]. Compared with these two compounds, the La(FexSi1-x)13 compounds exhibit large MCE without using expensive raw materials or deleterious elements. Such distinctive advantages make this type of material one of the most promising room temperature magnetic refrigerants [3]. In this material, the cubic NaZn13-type structure phase (hereinafter 1:13 phase) plays a key role in the large MCE of La(FexS
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