Scripta METALLURGICA Pergamon Press, Inc. Vol. 8, pp. 1239-1244, 1974 Printed in the United States GROWTH OF Pb-Bi PERITECTIC ALLOYS AT MODERATE AND HIGH VALUES OF G/R G. I. Shcherbakov~* S. A. David,** and H. D. Brody** *Physics Department University of Dnepropetrovsk U.S .S.R. **Department of Metallurgical and Materials Engineering University of Pittsburgh Pittsburgh, Pennsylvania 15261 (Received August 9, 1974) Although considerable attention has been paid to unidirectional freezing of alloys with eutectic reactions, the unidirectional freezing of alloys with peritectic reactions has not been the subject of detailed analytical and experimental studies until recently. Uhlmann and Chadwick (i), Chalmers (2), and Flemings (3) have proposed conflicting hypotheses as to the structures expected in such alloys. Uhlmann and Chadwick predict that a steady state coupled growth of (~ and ~ phases at a planar interface is not possible for alloy compositions between N and L (Figure i). But Chalmers proposes a coupled growth of the two phases for compositions between N and L except at P. According to Flemings such a coupled growth is possible only for compositions between N and P. Experimental work discussed at recent meetings (4-6) indicates that the structures actually produced by unidirectional freezing of peritectic alloys at high and low values of the ratio G/R (where G is the thermal gradient at the interface and R is the rate of interface advance) are not in complete agreement with the predictions of any of the above authors. Indeed the structures produced at high values of G/R are not predicted by any of the authors. Observations of microstructures in unidirectionally frozen Pb-26, 28, and 32Yo Bi alloys are reported herein. Hansen (7) reports a peritectic reaction for Pb-Bi (Figure i) as (23.3% Bi) + 5(36% Bi)~(33% Bi) The equilibrium solidus for Pb-Bi alloys between 23.3 and 33% Bi would be the peritectic isotherm, and the structure at the end of equilibrium freezing would contain C~ +~ • From 33% Bi - 42% Bi the equilibrium solidus bounds the single phase ~ region, and would be the product of equilibrium freezing. EXPERIMENTS A zone melting and freezing technique~ originally developed by Reggiardo (8) was used for unidirectional freezing. By this technique G and R may be independently controlled. Lead- Bismuth alloys were prepared from high purity lead (99.99%) and bismuth (99.99%). The alloys were melted and then drawn into 7ram - O.D. and Smm - I.D. Pyrex glass tubes. A 4 rail chromel- alumel thermocouple sheathed in a imm diameter alumina protection tube was inserted along the axis of the glass tube prior to filling and, thus, the thermocouple was imbedded into the sample.