Phase Transformations in Nb-Al-Ti alloys
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I.
INTRODUCTION
RECENTLY, niobium-based alloys have attracted much attention as potential high-temperature materials.V,21 Nb-A1 " alloys have been examined, especially with a view to producing intermetallic matrix composite materials with a distribution of ductile bcc precipitates in the Nb3A1 matrix. [3,4] The Nb3A1 phase exhibits the A15 crystal structure and is extremely brittle at temperatures below 1475 K. Attempts at alloy development are being carried out by several researchers, and studies on Ti and other alloying additions to Nb-A1 alloys are being performedY~ At the same time, several studies on Nb additions to Ti-AI alloys are available, [6,7,81 and phase stability in this ternary system is still being critically examined, tg,l~ However, most of the phase boundaries in the Nb-rich comer of the ternary system involving the bcc, Nb3A1, and Nb2A1 phases, shown in reported isothermal sections of the Ti-A1-Nb phase diagram, t9,12,13,141are only speculative, based on judicious extrapolations of the binary Nb-A1 phase diagram proposed by Jorda et al.tts] a number of years ago. Apart from the ordered Nb3A1 phase with the A15 crystal structure, an ordered phase with the B2 structure has been reported to form in a Nb-18 at. pct A1 alloy which has been deformedI~6]and also in melt-spun ribbons IlTl with the same composition. Actually, all phase determination studies conducted by Xray diffractionVS.~s,~91failed to notice the superlattice reflections associated with the B2 structure and inferred the structure to be bcc, while all electron microscopy studies [16,17,2~ observed the extra reflections. It may be pointed out here that the B2 structure has indeed been observed in Ti-AI-Nb alloys in a wide range of compositions.t9.10,11,12,21,22,23,24]
E.S.K. MENON, Research Assistant Professor, is with the Materials Science Section, Department of Mechanical Engineering, US Naval Postgraduate School, Monterey, CA 93943-5100. P.R. SUBRAMANIAN, Scientist, is with UES Inc., Dayton, OH 45432-1894. D.M. DIMIDUK, Group Leader, is with MLLM, USAF Wright Laboratory, Materials Directorate, Wright-Patterson AFB, OH 45433-7817. This article is based on a presentation made during TMS/ASM Materials Week in the symposium entitled "Atomistic Mechanisms of Nucleation and Growth in Solids," organized in honor of HJ. Aaronson's 70th Anniversary and given October 3-5, 1994, in Rosemont, Illinois. METALLURGICALAND MATERIALSTRANSACTIONS A
The present study was initiated to systematically investigate the phase transformation behavior in Nb-rich Nb-A1Ti alloys and specifically to provide information in several areas where little or only ambiguous results are available in literature. Thus, experimental work in the following areas was carried out. (a) Initially, phase boundaries involving the bcc and the Nb3AI phases were experimentally determined and isothermal sections of the Nb-rich comer of the Ti-A1-Nb system established at 1923, 1473, and 1273 K. (b) The presence or absence of the B2 phase was investigated and the regions of stability or
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