Crystallization of amorphous Fe 82 P 18
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I. INTRODUCTION
AMORPHOUS alloys have received considerable attention in the last few decades[1–4] owing to their interesting physical behavior and their potential applications in many fields. Amorphous solids, however, are in a thermodynamically metastable state and will transform into more stable states after specific treatment.[5] Usually, amorphous solids crystallize into one or more metastable or stable polycrystalline phases when they are subjected to heat treatments, irradiation, or mechanical attrition. The crystallization process itself is thus of high scientific and practical interest. Crystallization from the amorphous state can furthermore be used for the synthesis of nanocrystalline materials. Crystallization of amorphous metal-metalloid (e.g., Ni80P20)[6] and metalmetal (e.g., Ni34Zr66)[7] alloys have been investigated systematically. Fe-based amorphous alloys are a specifically interesting material owing to their high saturation of magnetism in addition to their soft magnetism related to their noncrystalline structure. Among them, amorphous Fe-P and Fe-B are the most widely studied systems.[8] Recently, magnetic properties of partially crystallized, amorphous, Fe-based alloys prepared by rapid solidification have received extensive investigations since the pioneering work of Yoshizawa et al.[9] This new class of materials, with a characteristic nanometer-sized grain structure of bcc a-Fe solid solution embedded in an amorphous matrix, exhibits excellent magnetic properties with a good combination of low coercivity, high permeability, and almost zero magnetostriction.[5] Several studies on the production,[10,11] structure,[12] and magnetic properties of amorphous Fe-P have been reported.[1,13] However, a systematic study of the phase transformation and the microstructure formation during the nanocrystallization of amorphous Fe-P using magnetothermal analysis combined with high-resolution transmission electron microscopy BINGQING WEI, Associate Professor, and YUBAO LI, Postdoctoral, are with the Department of Mechanical Engineering, Tsinghua University, Beijing 100084, P.R. China. PHILIPP KOHLER-REDLICH, Research ¨ ¨ Fellow, REINHARD LUCK, Reseach Fellow, and MANFRED RUHLE, ¨ Prof. Director, are with the Max-Planck-Institut fur Metallforschung, D-70174 Stuttgart, Germany. Manuscript submitted September 8, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
(HRTEM) has not been reported to the knowledge of the authors. In the present article, the crystallization of amorphous Fe-P and the resulting microstructure of the material after isothermal and isochronal heat treatment have been studied using magnetothermal analysis (MTA), HRTEM, and Xray diffraction (XRD). The kinetics of the crystallization process, the subsequent phase transformation, and the influence of oxygen contamination are discussed. II. EXPERIMENTAL PROCEDURE AND DETAILS The raw materials, 55 wt pct Fe-P alloy powder with 20 wt pct P and 45 wt pct Fe powder (purity, better than 99.9 wt pct), were compressed uniaxially into cylinders at 800
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