Effect of Nb Concentration on Thermal Stability and Glass-Forming Ability of Soft Magnetic (Fe,Co)-Gd-Nb-B Glassy Alloys
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INTRODUCTION
SINCE the syntheses of bulk glassy alloys (BGAs) in multicomponent Ln-[1] (Ln: lanthanide metal) and Mg-[2] based alloys by the copper mold casting method in 1989, much effort has been devoted to develop new BGAs exhibiting high glass-forming ability (GFA) and functional properties. As a result, a large number of BGA systems were developed, and they exhibited unique properties such as high strength, large elastic strain, high hardness, good soft magnetic properties, excellent corrosion resistance, and Newtonian viscosity in the supercooled liquid temperature region.[3–5] For ferromagnetic BGAs, since the synthesis of the Fe-(Al,Ga)(P,C,B,Si),[6] the Fe- and Co-based BGAs have gained considerable interest because they exhibit excellent soft magnetic properties, high GFA, distinct supercooled liquid region DTx (defined by the difference between glass transition temperature Tg and crystallization temperature Tx), and high fracture strength.[7] The W. ZHANG, Associate Professor, Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan, is Guest Professor, School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024, P.R. China. Contact e-mail: wzhang@ imr. tohoku.ac.jp F. JIA, Associate Professor, and X.G. ZHANG, Professor, are with the School of Materials Science and Engineering, Dalian University of Technology. G.Q. XIE, Assistant Professor, and A. INOUE, Professor and President, are with the Institute for Materials Research, Tohoku University. This article is based on a presentation given in the symposium ‘‘Bulk Metallic Glasses VI,’’ which occurred during the TMS Annual Meeting, February 15–19, 2009, in San Francisco, CA, under the auspices of TMS, the TMS Structural Materials Division, TMS/ASM: Mechanical Behavior of Materials Committee. Article published online July 22, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
ferromagnetic BGAs can be generally classified into five systems, i.e., (Fe, Co)-(Al, Ga)-(P, C, B, Si),[6,8] (Fe, Co)-TM(= Zr, Hf, Mo, Hf, Ta, W)-B,[9–11] (Fe, Co)-LnB,[12,13] (Fe, Co)-TM-(B, Si),[7,14,15] and Fe-(P, B, Si).[16] The Fe62Co9.5Ln3.5B25 glassy alloys exhibited larger DTx above 50 K and good soft magnetic properties with high saturation magnetization, and the alloy with Ln = Gd showed the largest DTx of 63 K and best soft magnetic properties.[17] We have noticed that the TM elements have the intermediate atomic radius between Fe(Co) and Ln.[18] In addition, the TM-Fe, TM-Co, and TM-B pairs have negative heats of mixing.[19] Thus, the (Fe,Co)-LnTM-B alloy system with higher GFA and higher stabilization of supercooled liquid can be expected.[5] We also confirmed that the addition of a small amount (2 at. pct) TM to Fe62Co9.5Nd3Dy0.5B25 alloy increased DTx and GFA. The largest DTx was 87 K for the alloy with TM = Nb addition.[20] However, the GFA was not so high, and the critical diameter (dc) for formation of a single glassy phase was below 1.5 mm.[21] In this study, with the aim of developing a new ferromagnetic BGA with high
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