Comparison of the Crystallization Behavior of Fe-Si-B-Cu and Fe-Si-B-Cu-Nb-Based Amorphous Soft Magnetic Alloys
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OVER the past 20 years, there has been great interest in partially de-vitrified amorphous materials, such as the FINEMET alloy with the composition Fe73.5Si13.5B9Nb3 Cu1. This alloy exhibits excellent soft magnetic properties combining high saturation magnetization with excellent permeability, and low coercivity values.[1,2] These excellent properties result from a random distribution of nanocrystals embedded within the amorphous matrix.[3,4] Previous investigations have reported that in case of FINEMET, Cu clusters act as heterogeneous nucleation sites for a-Fe3Si nanocrystals and substantially reduce the crystallization temperature.[5–7] Using atom probe tomography (APT), Hono et al.[7,8] revealed that after short periods of annealing at 823 K (550 C), Cu clusters form and act as heterogeneous nucleation sites for the a-Fe nanocrystals. Hono et al.[5] also proposed a model wherein the Cu clusters resided at the interface between the a-Fe3Si nanocrystal and the amorphous matrix. Independently, Ayers et al.[9,10] CASEY SMITH, Masters Student, SHRAVANA KATAKAM, Doctoral Student, SOUMYA NAG, Research Assistant Professor, NARENDRA B. DAHOTRE and RAJARSHI BANERJEE, Professors, are with the Center for Advanced Research and Technology, Department of Materials Science and Engineering, University of North Texas, Denton, TX. Contact e-mail: [email protected] Y.R. ZHANG, Ph.D. Student, J.Y. LAW, Research Fellow, and RAJU V. RAMANUJAN, Professor, are with the School of Materials Science and Engineering, Nanyang Technological University, Singapore, Singapore. Manuscript submitted September 16, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
proposed that the similarity of the face-centered cubic (fcc) structure of the Cu clusters and the DO3 structure of a-Fe3Si played an important role in the former acting as a nucleation site for the latter. However, in contrast to the model proposed by Hono et al.[5], Ayer et al.[9] proposed a model suggesting that the a-Fe nanocrystals form a shell, enveloping the heterogeneous nucleant Cu cluster at its core. Nb is known to restrict the grain growth rate but different mechanisms have been proposed for how this is achieved. Yavari and Negri[11] suggested that a concentration gradient of Nb is developed at the crystal/ amorphous interface, restricting the diffusion of Fe and Si. However, Mossbauer spectrometry studies have indicated that Nb diffuses out of the crystal and homogeneously diffuses into the amorphous matrix, which leads to the Nb concentration gradient.[12] Herman et al.[13] suggested that the combination of B and Nb act as diffusion barriers for nanocrystal growth. While there have been extensive research efforts, including APT studies, focused on the FINEMET alloy composition containing both Cu and Nb,[5,9] there have been rather limited efforts on investigating FINEMETlike compositions that do not contain all of the alloying elements present in the commercial composition. A previous attempt was made to distinguish the roles of Cu and Nb in the FINEMET alloy, prima
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