Effects of low-temperature aging on the microstructure and soft magnetic properties of rapidly quenched Fe-Si-B alloys
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I.
INTRODUCTION
THE addition of Si to the Fe-B binary alloy remarkably extends the formation range of the amorphous phase and results in a considerable rise in both the crystallization temperature (Tx) and Curie temperature (To) without a drastic decrease in the saturation magnetization (B~).t],2,3] Due to the excellent soft magnetic properties, such as a large B~ and low core loss (Wr), the amorphous Fe-Si-B alloys have attracted particular attention as core elements for power transformers. A number of research studies[4-71have already been performed on the amorphous Fe-Si-B alloys in order to investigate their applicability as magnetic core materials. However, less attention has been paid to the aging effect at lower temperatures. The soft magnetic properties, such as coercivity (Hc) and effective permeability (/xe), appear to be significantly affected by a trivial relaxation of the amorphous structure; therefore, they are thought to change even through the prolonged aging below the Tx. Thus, an investigation of their behavior is indispensable from the viewpoint of their practical use as soft magnetic materials. Moreover, it is an important subject for a complete understanding of the physical properties in the amorphous phase. In the present work, an attempt has been made to clarify the aging behavior of the microstructure and soft magnetic properties in the amorphous Fe-Si-B alloys, paying attention to temperatures below 573 K. Additionally, the present data have been interpreted with the aid of the structural model originally proposed by Dubois and co-workers. E8,9,'~ II.
EXPERIMENTAL PROCEDURE
Figure 1 shows the compositional region in which an amorphous phase is formed by rapid quenching of the FeSi-B alloysY] The specimens used in the present work were Fe-based alloys containing 12 at. pct Si and 4 to 20 at. pct B. These alloy compositions are marked with open circles in the figure. Mixtures of pure metals and metalloids were
T. NAOHARA, Lecturer, is with the Department of Materials Science and Engineering Ehime University, Matsuyama 790, Japan. Manuscript submitted November 22, 1995. 2454--VOLUME 27A, SEPTEMBER 1996
melted under an argon atmosphere in an induction fumace. The melts were sucked into a quartz tube with an inner diameter of 5 mm and then allowed to solidify in air. From these master alloys, continuous-ribbon specimens approximately 20-/xm thick and 2-mm wide were prepared using a melt-spinning apparatus having a steel roller with a diameter of 200 mm. The rapidly quenched specimens were subjected to the aging for 0.6 Ms at 473, 523, and 573 K. The microstructure of the as-quenched and aged specimens was investigated using X-ray diffraction (XRD) analysis with filtered Co, K s radiation and transmission electron microscopy (TEM). The//~ value was estimated from a d.c. B-H loop under an applied field of 80 Am -~, 800 Am -~, or 8.0 kAm-L The/x e was measured with a vector impedance analyzer in the frequency range from 1 kHz to 10 MHz in a driving field of 0.8 Am-L
IlL
RESULTS
A. Microstr
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