High Resolution Transmission Electron Microscope Study of Amorphous and Partially Crystalline Fe 78 B 13 Si 9 Alloy
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HIGH RESOLUTION TRANSMISSION ELECTRON MICROSCOPE STUDY OF AMORPHOUS AND PARTIALLY CRYSTALLINE Fe,,B 1 3Si, ALLOY A.R. BHATTI, J.C. BARRY* and B CANTOR Department of Metallurgy & Science of Materials, University of Oxford, Parks Road, Oxford OXI 3PH, UK *Present Address: Centre for Solid State Science, Arizona State University, Tempe, Arizona
ABSTRACT Amorphous and partially crystalline Fe,,B, 3 Si 9 alloys have been examined by high resolution transmission electron microscopy at 200kV. In lattice imaging mode, the as-quenched alloy shows small regions, with a mean size of - 20A, consisting of well defined patterns of fringes. Three different types of crystals can be identified at the very earliest stages of crystallisation of the alloy.
INTRODUCTION Numerous models [1-6] have been developed by different workers to define the structure of amorphous alloys. Until recently, structural models of amorphous alloys which assume purely non-crystalline arrangements of atoms have been favoured [2-5]. Recent transmission electron microscopy, Mbssbauer spectroscopy and Neutron diffraction experiments [7-13], however, suggest that amorphous alloys might consist of small domains of regular arrangements of atoms. In the present work, high resolution transmission electron microscopy (HRTEM) has been used to investigate (a) the initial as-quenched structure of an Fe 7 ,B,3Si the earliest 9 amorphous alloy and (b) stages of crystallisation of the same alloy during heat treatment.
EXPERIMENTAL TECHNIQUE The as-quenched Fe,,B, 3 Si 9 alloy was in the form of ribbon, with a 300m 0 x 2.5cm cross-section. Ribbon samples were heat-treated at 510 C under an Ar atmosphere, at a series of annealing times chosen from previous calorimetric work [14], in order to observe isothermal crystallisation of the amorphous alloy. Thins foils of as-quenched and heat treated samples were prepared by Ar ion milling in a Gatan ion thinner. Electron microscopy was performed in a JEOL 200CX with a structural resolution of 2.5A and an information limit of 1.7A. Bright field phase contrast images were obtained by using a large objective aperture such that all spacings greater than 1.9A could contribute to the image. Dark field diffraction contrast images were obtained using an objective aperture which allowed spacings greater than 8A to contribute to the image, and by tilting the beam so as to align the
segment of the diffused ring of interest with the microscope optic axis. The JEOL 200CX objective lens has a c = 1.2mm and the images were taken at near to the optimum defocus value of -960A, and with a beam divergence angle of 0.9mRad. RESULTS AND DISCUSSION Figs.1-3
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transmission electron micrographs of as-quenched amorphous Fe7 sB,,Si, and a corresponding
diffraction
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Mat. Res. Soc. Symp. Proc. Voa. 58. ' 1986 Materials Research Society
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Fig.1. High resolution bright field electron micrograph of the as-quenched amorphous alloy showi
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