Scripta METALLURGICA Vol. 23, pp. 1697-1702, 1989 Pergamon Press plc Printed in the U.S.A. All rights reserved MICROSTRUCTURES AND PHASE RELATIONSHIPS IN THE Ti3A! + Nb SYSTEM H.T. Kestner-Weykamp*, C.H. Ward +, T.F. Broderick + and M.J. Kaufman ° *Department of Materials Science and Engineering, Univ. of Washington, Seattle, WA 98195 +Wright Research and Development Center, Wright-Patterson AFB, OH 45433 °Department of Materials Science and Engineering, Univ. of Florida, Gainesville, F1 32611 (Received March 31, 1989) (Revised July 17, 1989) Introduction Alloys based on the tx2-Ti3Al compound (hexagonal DOt9 structure) are currently experiencing limited use as advanced aerospace materials. To date, the alloys with the most desirable properties contain additions of I~ stabilizers, such as Nb, Mo and V, which promote the formation of a two-phase mixture of ot2+[5 or t~2+B2 (where B2 refers to the ordered CsC1 structure). Unfortunately, the phase relationships in these systems have not been established in sufficient detail to allow their more widespread application. Recently, there has been a series of investigations aimed at alleviating this deficiency in the ternary Ti-A1-Nb system [ 1-9]. These studies have clearly indicated the existence of the ordered B2 phase, which, in the higher Nb alloys, can be retained at room temperature by rapid cooling from the liquid or solid state. In addition, a number of previously unreported phases have been identified: an orthorhombic phase, Cmcm space group, derived from the hexagonal DO19 phase [5], an "omega-like" phase with the B82 structure [3] and two as-yet-unidentified phases designated T1 and T2 by Jewett, et al [9]. However, there remain serious gaps in the understanding of the phase transformation sequences and equilibria in these alloys. In the present study, (TiNb)3A1 alloys (from 0 to 30 at. pct. Nb), were studied after conventional and nonequilibrium (i.e., rapid solidification) processing with an emphasis on providing further insight into the transformation sequences and phase equilibria in these alloys. Exoedmental Procedure Melt-spun ribbons of Ti-25% A1 containing 0, 5, 8, 10, 12, 20, 25 and 30% Nb substituted for Ti were prepared and then examined in their as-prepared condition and after heat treating for 0.25 - 5 h in the range 700 to 800°C using optical microscopy, X-ray diffraction (XRD), scanning and analytical-transmission electron microscopy (SEM and AEM). Bulk samples of Ti-24AI-11Nb, Ti-25AI-20Nb, and Ti-25A1-30Nb (referred to as 24-11, 20Nb and 30Nb in the following) were also prepared using conventional methods, heat treated at temperatures from 800-1250°C and times ranging from 4 to 100 h, and then examined using the methods de- scribed above. All heat treatments were performed in cleaned, evacuated and Ar back-filled quartz ampules. Samples were wrapped in Ta foil and Ti sponge was included in the quartz tube to act as a getter for oxygen. Occasional interstitial analyses were performed to insure cleanliness of the sample preparation and heat treating procedures. Results and Discussion Ordering of the fl Phase vs. Temperature for 25% Al As pointed out in [7], the microstructures and selected-area diffraction patterns (SADPs) obtained after holding the bulk samples at 1200°C for 4 h indicated that all three alloys consisted of the B2 phase after quenching to room temperature. However, the sizes of the thermal antiphase domains (APDs) and the intensities of the superlattice reflections varied considerably. For example, Fig. 1 reveals that the APDs in the 24-11 alloy (similarly in the 20Nb alloy) were considerably smaller that those in the 30Nb alloy while the supedattice intensities were much stronger. This indicates that, at 1200°C, the ~-phase was disordered in 24-11 and ordered in 30Nb. As noted in [7], the lower superlattice intensities in the higher Nb alloy can be understood by considering that the scattering power from the two sublattices are nearly equal; this implies that Nb and A1 preferentially occupy the same sublat- 1697 0036-9748/89 $3.00 + .00 Copyright (c) 1989 Pergamon Press plc