Preparation of amorphous Fe-based magnetic powder by water atomization

Abstract

Fe–Si–B–C–P amorphous powders were fabricated successfully by water atomization. The morphology, chemical composition, phase structure and soft magnetic properties were then studied in this work. The results indicate that the powders are completely amorphous in nature. DSC analyses show that the powders have a glass transition temperature T_g about 768 K and a crystallization temperature T_x about 816 K, which are almost the same to those of the bulk form. The morphology of the powders can be modified by adjusting the water atomization parameters, and both dendritic particles and spherical particles can be made. Compared with the gas-atomized powders and the bulk form made by casting, the water-atomized powders contain a higher amount of oxygen (about 1800 ppm), and also a higher content of Fe. It suggests that the high content of oxygen impurity may further increase the glass forming ability and a higher content of Fe may increase the saturation magnetization of the material. The water-atomized powders can be easily densified into bulk forms. The soft magnetic properties depend on the densification and heat treatment temperatures. The bulk Fe-based metallic glass, made by using the water-atomized powders, has a saturation magnetization as high as 1.64 T and a value of coercivity as low as 5.9 A/m. Therefore, the water-atomized powders are very cost-effective and suitable for industrial applications.

Introduction

Fe-based metallic glass has a very low coercivity and a high saturation magnetization, thus is considered as promising soft magnetic materials [1], [2]. In 1982, Inoue et al made the first Fe₇₅Si₁₀B₁₅ metallic glass with a diameter of 0.27 mm [3], and in 1995 developed a Fe₇₃Al₅Ga₂P₁₁C₅B₄ metallic glass in a diameter of 1 mm [4]. Since then, many Fe-based bulk metallic glass compositions have been found, for example, Fe₇₀B₂₀Zr₈Nb₂ and Fe₅₆Co₇Ni₇Hf₈M₂ (M = Nb or Ta) [5], [6], Fe₃₀Co₃₀Ni₁₅Si₈B₁₇ [7]. In 2003, V. Ponnambalam and S. Joseph Poon developed a nonferromagnetic amorphous steel with a critical diameter of 4 mm [8]. In 2004, Lu and Liu developed a Fe-based metallic glass composition with a critical diameter of 14 mm [9], and in 2005, Shen also added rare earth elements in a Fe-Co-Cr-Mo-C-B alloy and obtained a critical diameter of 16 mm [10].

For magnetic applications, the Fe–Si–B system is usually studied, because of their superior magnetic properties, such as the relatively high saturation magnetization of 1.5–1.7 T, good magnetic softness due to lack of intrinsic magneto-crystalline anisotropy, and also low material cost and excellent productivity [11]. However, the glass forming ability of this system is very low, and usually ribbons as thin as 25 μm, made by melt-spinning, were used. Recently, bulk metallic glasses based on Fe–Si–B–C–P have reportedly a good glass forming ability (a critical size of 3 mm) and high saturation magnetization (1.5 T) [12]. However, this critical size still limits the outer shape and the application of Fe-based metallic glass in a bulk form. Thus, Fe–Si–B metallic glasses are usually used as powder cores. The fabrication process includes the disintegration of the ribbons, mixing with polymers and consolidation.

Bulk metallic glasses can also be made by powder metallurgy, which is a process much more flexible in controlling the size, shape and microstructures of the components. Metallic glass powders can be made by atomization methods and mechanical alloying. In atomization, usually high purity raw materials and Ar are used, in order to decrease the impurity level and increase the glass forming ability. This is especially essential for making Zr-based and Al-based metallic glass powders due to their high affinity to oxygen. Recently, Lu et al reported to develop a Fe–Si–B–C–P bulk metallic glass by using industrially raw materials [13]. This implies that the Fe-based metallic glass may not be as sensitive to impurities as other metallic glass systems. So far, there are some researches on making Fe-based metallic glass powder by the atomization method [14], [15]. In this work, the authors tried to fabricate Fe–Si–B–C–P metallic glass powders by using water-atomization, rather than the costly gas-atomization, and investigate the chemical and physical properties of the powders.