Fabrication and Characterization of Melt-Extracted Co-Based Amorphous Wires
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INTRODUCTION
BECAUSE of the excellent giant magnetoimpedance (GMI) effect and great potential application of highly sensitive magnetic sensors, Co-based amorphous wires have been focused on for many years.[1–3] To now, Cobased amorphous wires were prepared by several methods such as glass coating, in-water quenching, and melt extraction technology (MET).[3–5] Compared with others, the solidification rate of wires prepared by MET is the highest, which leads the wires to express an excellent GMI effect. Thus, MET has been increasingly used to produce Co-based amorphous wires in recent years. The concept of melt extraction was first introduced by Maringer[6] in 1974 to produce metallic fibers. The basic principle is to use a high-speed wheel with a sharp edge to contact the molten alloy surface and then to rapidly extract and cool a molten layer to be wires. Therefore, the solidification rate of metallic wires can reach 105 to 106 K/s, and MET has been used to fabricate amorphous wires and a few ceramic fibers, e.g., CaOÆAl2O3[7] and Al2O3ÆY2O3.[8] Because it does not require any liquid cooling mediums, MET can also be used to fabricate some high reactivity alloys such as aluminum,[9] titanium,[10] zirconium,[11] and magnesium[12] wires. Surprisingly, contrary to related methods such as melt spinning and planar flow casting, very few reports throw light on the influence of process parameters on the wires’ quality, and the formation of wires during melt extraction is far from understood. Inoue[13] studied the changes in diameter and roundness for Fe75Si10B15 HUAN WANG and XIAODONG WANG, Doctoral Candidates, DAWEI XING, Associate Professor, and JIANFEI SUN, Professor, are with the School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, People’s Republic of China. Contact e-mail: [email protected] Manuscript submitted April 5, 2010. Article published online November 20, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
wires as a function of wheel velocity and molten rising velocity and produced high circular amorphous wires in the high wheel speed region. Lotze[14] believed that heat transfer controlled the fiber formation and described a simple solidification model to investigate the fiber formation of eutectic Al-Cu alloy, calculating the heat transfer coefficient and the cooling rates during melt extraction. Anthony[15] supposed that the melt-extracted thermal layer must be thicker than the viscous shear layer in the molten and described the vorticity and temperature distribution adjacent to the rotating wheel, which were determined to be unsuitable later by Takeshita.[16] Allahverdi et al.[17] investigated the extraction of ceramics and produced ZAT fibers with diameters ranging from 15 to 30 lm at the low wheel velocity of 1.5 m/s. They believed that the molten layer was extracted by the momentum of the wheel, and the fiber thickness was controlled by the viscosity of molten, i.e., momentum transfer. Carefully considering the work mentioned previously, it can be seen that detailed extraction expe
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