In-Situ Time-Resolved X-Ray Studies of Eutectic Crystallization in Amorphous Fe 1-x B x and Co 1-x Zr x
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IN-SITU TIME-RESOLVED X-RAY STUDIES OF EUTECTIC CRYSTALLIZATION IN AMORPHOUS Fel_,B, AND Co-LZr. HENRY E. FISCHER', S. BRAUER', M. SUTTON-, J. STROM-OLSEN', A. ZALUSKA', G. B. STEPHENSONt AND UWE KOSTERt * Centre for the Physics of Materials and the Department of Physics, McGill University, 3600 University Street, Montreal, PQ, H3A 2T8, Canada t IBM Research Division, T. J. Watson Research Center, Yorktown Heights, NY 10598 t Department of Chemical Engineering, Universitit Dortmund, D-4600, F.R.Germany ABSTRACT Using a millisecond time-resolved x-ray scattering technique at the National Synchrotron Light Source, we have simultaneously obtained in situ both large-angle (LAXS) and smallangle (SAXS) diffraction patterns for isothermal crystallization of amorphous Fel-.B. (0.16 < x < 0.21) and Col_.Zr. (0.08 < x < 0.11). The large-angle x-ray data are analyzed to determine which of the two crystalline products nucleate first in each compound, and also to assess the evidence for eutectic growth as shown by a time-independent ratio of volume fractions of these product phases. The small-angle data give the length scales of the eutectic structure as well as information about its time evolution. Combining our x-ray results with TEM studies, we examine the effects of changes in temperature and composition on the structural evolution of both the amorphous and crystalline phases. Introduction Eutectic crystallization of binary amorphous alloys is qualitatively different from polymorphic or primary crystallization. Beginning either from the amorphous or liquid state, eutectic crystallization involves the simultaneous production of two different crystal phases of distinct chemical composition, which generally form a granular or lamellar structure, and which cause no overall composition change in the surrounding matrix during crystallization. The average composition of the starting mixture is usually near, but not necessarily equal to, that of the eutectic point on the equilibrium phase diagram. We chose to study eutectic crystallization in binary amorphous alloys in order to understand the kinetics which lead to the unique pattern formation in these systems. We began because it is wellour investigations [1] with the metal-metalloid eutectic system Fe 1jB, characterized and has a considerable history of experimental study [2]. We then proceeded to the metal-metal system Co_1 .Zr. for comparison. Although eutectic crystallization has long been observed in several systems through the sunburst-like microstructure (i.e. radial lamellae) of the polycrystalline product, previous time-resolved crystallization studies have not employed in situ methods, and therefore the kinetics of this crystallization process remain poorly studied. Experimental procedure The samples used in our experiments were prepared by melting together the starting elements (99.999% Fe, 99.9% B, 99.9% Co, 99.95% Zr) in a tungsten arc furnace under a titanium gettered Argon atmosphere of about 1/3 atm. The samples were then melt-spun in - 0.4 atm helium against a Cu
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