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S5.30.1

Experimental Investigation and Thermodynamic Modeling of Phase Equilibria in the Hf-Ti-Si System Y. Yang1, B.P. Bewlay2, M.R. Jackson2 and Y.A. Chang1 1 2

University of Wisconsin-Madison,Wisconsin 53706. USA. General Electric Global Research,Schenectady, New York 12301. USA

ABSTRACT Phase equilibria in ternary Hf-Ti-Si alloys were studied in the as-solidified and heat treated conditions using scanning electron microscopy, x-ray diffraction, and electron beam microprobe analysis. Selected solid-solid phase equilibria at 1350°C and a partial liquidus projection of the Hf-Ti-Si system at the metal rich end of the phase diagram were established. These data were then used to develop a thermodynamic description of the Hf-Ti-Si system using the CALPHAD (CALculation of PHAse Diagram) approach. The calculated isothermal section at 1350°C and the liquidus projection can satisfactorily account for the available experimental phase equilibria data and solidification paths. Both the calculations and the experimental data suggested that the metal-rich end of the ternary phase diagram possesses one transition reaction: L + (Hf,Ti)5Si3 → Hf(Ti)2Si + β(Hf,Ti,Si).

INTRODUCTION Directionally solidified in-situ composites based on niobium and niobium-based silicides are presently being developed for high-temperature structural applications. There has been extensive work on composites generated from binary Nb-Si alloys [1-5], as well as those with additions such as Ti, Hf, Cr and Al. Hf and Ti are important alloying additions because they can improve oxidation resistance and strength [1, 2, 5, 6]. However, there is little previous knowledge of phase equilibria in the Hf-Ti-Si system; this is required for determination of higher order Nb based systems that contain Hf and Ti. This paper first presents the experimental details for the solid-solid phase equilibria at 1350°C and the partial liquidius projection at the metal rich end of the Hf-Ti-Si system, and then describes thermodynamic modeling for the Hf-Ti-Si system using the CALPHAD approach. Finally, comparisons between the calculated results and experimental observations will be presented.

EXPERIMENTAL DETAILS The alloys for this study were directionally solidified using cold crucible directional solidification [1, 4] after triple melting the starting charges from high purity elements (>99.99%). The directional solidification procedure has been described in more detail previously [1, 6]. Mass losses were measured after preparation of each sample and they were found to be less that 0.1 wt

S5.30.2

%. The interstitial levels of the C, H, N and O in the Hf and Ti used were less than 250 weight ppm, respectively. All of the samples were examined using scanning electron microscopy (back scatter electron (BSE) imaging) and energy dispersive spectrometry (EDS). Electron beam microprobe analysis (EMPA), x-ray diffraction (XRD), and automated electron back scattering diffraction analysis in the SEM (EBSD) were also performed on selected samples. High purity Ti, Hf, and Si were u