Experimental Determination of the Ti-Nb-Al Phase Diagram At 1200°C.

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EXPERIMENTAL DETERMINATION OF THE Ti-Nb-Al PHASE DIAGRAM AT 1200°C. T. J. JEWETT, J. C. LIN, N. R. BONDA, L. E. SEITZMAN, K. HSIEH, Y. A. CHANG, J. H. PEREPEZKO, University Wisconsin-Madison, 1509 University Ave., Madison, WI 53706.

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ABSTRACT Studies consisting of both bulk sample and diffusion couple experiments have been conducted in order to establish an isothermal section of the Ti-Nb-Al system. Bulk samples were prepared by arc-melting and then annealed in a gettered, flowing, argon atmosphere for extended equilibration periods. The results obtained so far have established the existence of two ternary phases with the following approximate compositions, TI (Ti-18 at% Nb-34 at% Al) and T2 (Ti-ll at% Nb-44 at% Al). Moreover, several of the binary phases were found to have extensive solubilities, including the TiAl phase. The phases TiAI 3 and NbAI 3 form a continuous solid solution. Several heterogeneous equilibria were also determined. Determination of these phase fields was accomplished through the use of electron microprobe analysis (EMPA), x-ray diffraction, microstructural observations and backscatter electron imaging (BEI). INTRODUCTION In the search for materials for use in high temperature, aggressive environments, such as those encountered in many aerospace applications, alloys based on the intermetallic TiAl (Li 0 ) are becoming increasingly important. The benifits of using TiAl based alloys in hostile environments are their relatively high melting points, high strength at elevated temperatures, high stiffness and low densities compared to other high temperature, high strength alloys. Thus, Ti-Al alloys offer the possibility of improved or comparable structural properties at a considerable savings in weight. These alloys, however, are not without certain disadvantages. Chief among these is a lack of low temperature ductility. Several attempts have been made to improve the ductility of these alloys through the addition of other alloying elements such as Ta [1], Mn [2] and Nb [3-7]. Due to the enhancement of the ductility by Nb additions to TiAl [8], an interest has developed in Ti-Nb-Al alloys. At present, limited data is available in the literature on the phase equilibria of this system. This lack of phase equilibrium data hampers an efficient method for alloy developement and hinders the modeling of the ternary phase diagram. In order to assess the system thermodynamically, extension of binary phases into the ternary, the existence of any ternary phases and the composition of the existing phases must be determined. It would be highly desirable to have thermodynamic data of the phases. However, this data is not availible. The approach of this study is to determine the phase relationships at 1200°C and to employ thermodynamic models to estimate the phase relations at other temperatures. The phase diagram is determined using equilibrated bulk alloy samples and diffusion couple experiments. Mat. Res. Soc. Symp. Proc. Vol. 133. "1989 Materials Research Society

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EXPERIMENTAL PROCEDURES Bulk alloy