Scripta METALLURGICA Vol. 29, pp. 1271-1274, 1993 Pergamon Press Ltd. et MATERIALIA Printed in the U.S.A. All rights reserved OBSERVATION OF A METASTABLE B2 PHASE IN RAPIDLY SOLIDIFIED RIBBONS OF Nb-AI ALLOYS H. Kohmoto*, J. Shyue, M. Aindow** and H.L. Fraser Dept. of Materials Science and Engineering, The Ohio State University, Columbus, OH 43210, USA. * Now at; Mitsubishi Materials Company, Omiya, Japan. ** Now at; School of Metallurgy and Materials, The University of Birmingham, Edgbaston, B 15 2TT, England. (Received July 19, 1993) Introduction Alloys based on the Nb-A1 system are of considerable interest since the A15 compound Nb3A1 is superconducting with high Jc and Hc. Moreover, alloys based on this compound have received considerable attention recently for potential high temperature structural use. Unfortunately, the compound is extremely brittle and impossible to shape using conventional methods. Thus the full potential of alloys based on this compound is unlikely to be realized unless a more fundamental understanding of the physical metallurgy of the system is achieved, upon which basis improvements in properties of the compound may be made. The A15 phase in the Nb-AI system is formed by a peritectic reaction between Al-rich liquid and an A2 (bcc) solid solution of A1 in Nb at ~-1950°C. Whilst the A2 and A 15 are the only equilibrium phases for alloys with <25 at.% A1, several authors have noted the presence of B2 ordering in solid solutions. Most of these observations concern the composition Nb - 18at.% A! which corresponds to the lower bound of the A15 phase field on the phase diagram under ambient conditions. The first observation of B2 order was by Pan et al [1] who quenched Nb-18AI from above the A15 transus. A mixture of B2 and A15 phases has been observed in as-cast Nb-18AI produced by pouring into a water cooled copper hearth [2] or by melt-overflow spinning [3]. In contrast, previous studies of splat-quenched droplets of binary Nb-A1 alloys by Schulze, Miiller and Petzow [4] have shown that supersaturated solid solutions with over 20 at.% AI can be formed in room temperature microstructures. In this latter work, the identities of phases were established using x-ray diffraction. It has been shown [5] that it is extremely difficult using x-ray diffraction to observe superlattice reflections originating from the B2 structure; it is tempting to speculate that the A2 phase in the work of Schulze, et al [4] may have been incorrectly identified. It is interesting to note that in other work Sudareva et. al. observed a mixture of A15 and A2 phases in slowly-cooled as-cast Nb-20A1, but a mixture of A15 and B2 phases in more rapidly cooled ingots [6]. It is not clear why a B2 phase should occur at these compositions when such phases are usually associated with approximately equal amounts of each element, but it has been suggested that its occurrence may be related to the presence of interstitial oxygen or nitrogen [2] in a similar manner to that proposed previously for tantalum [7]. In this paper we describe a TEM study of the phases present in melt-spun ribbons of binary Nb-AI alloys. In part, this study has been carried out to determine whether upon rapid solidification the A2 or B2 version forms. The effects of heat treatment are considered and the significance of these observations for the origins of the B2 phase are discussed. Exoerimental Method Ribbons of binary Nb-AI alloys were produced by melt-spinning onto an iron wheel. The charge was arc- melted under an atmosphere consisting of a mixture of He and Ar on a water-cooled copper hearth, and the molten charge was forced through a small nozzle by increasing the pressure of the Ar/He gas mixture. Alloys with 9.1, 13.5 and 16.9 at.% A1 were produced by this method. Portions of each ribbon were then heat treated at 800°C for 10 hours. Specimens for TEM were prepared from each of the ribbons in the as-cast and heat treated conditions by trepanning discs, 3mm in diameter, and grinding to reduce the thickness to <1501.tinwhere appropriate. Thin foils were produced from the discs using the twin-jet electropolishing technique with 10% H2SO4 in methanol at ~-25°C 1271 0956-716X/93 $6.00 + .00 Copyright (c) 1993 Pergamon Press Ltd.