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0980-II05-34

Effect of Cr Addition on the Phase Equilibria of the Nb-Si System B. P. Bewlay1, Y. Yang2, R. L. Casey1, M. R. Jackson1, and Y. A. Chang3 1 GE Global Research Center, Schenectady, NY, 12301 2 CompuTherm LLC, 437 S. Yellowstone Dr., Madison, WI, 53719 3 University of Wisconsin-Madison, Madison, WI, 53706 ABSTRACT Nb-silicide based in-situ composites are promising materials for future high-temperature structural applications. Nb-silicide composites are typically alloyed with Hf, Ti, Cr, and Al to provide a balance of mechanical and environmental properties. A thermodynamic description of the Nb-Cr-Si system has been developed previously in literature based on reported isothermal sections. According to the previously calculated phase diagrams, selected alloys were directionally solidified. The as-solidified microstructures could not be interpreted using the liquidus projection calculated from the existing thermodynamic descriptions. Therefore, an improved thermodynamic description was developed by incorporating the new experimental data. 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 [1-4], as well as those with additions such as Ti, Hf, Cr and Al. Cr is an important alloying addition because it can improve oxidation resistance [1, 2]. In order to understand and improve the microstructure and properties of such composites, phase diagram knowledge is a prerequisite. Phase equilibria of the Nb-Si system have been extensively studied in literature [5, 6]. However, the effect of Cr addition on the phase equilibria of Nb-Si is not well understood. This paper describes Nb-Cr-Si phase equilibria using thermodynamic modeling coupled with experimental data. Thermodynamic modeling and experimental details regarding the Nb-Cr-Si system will be described first, and then comparisons between the calculated results and experimental data will be described. There have been a few prior experimental studies on the solid-state phase equilibria of the Nb-Cr-Si system [7-12]. The accepted phase equilibria of the Nb-Cr-Si system at 1000 and 1150°C are based on Zhao’s work [8]. Shao [9] and Yang [10] independently modeled this system thermodynamically by considering these experimental data. Both calculations yielded almost identical isothermal sections at 1000 and 1150°C, which are in general agreement with Zhao’s work [8] except for the calculated phase equilibria of (Nb)+αNb5Si3+C14_Cr2Nb. Zhao et al. [8] identified (Nb) and αNb5Si3 are in equilibrium with the CrNbSi phase. The calculated phase equilibria of (Nb)+ αNb5Si3+C14_Cr2Nb were later experimentally identified by Geng et al.[11]. Recently, David et al. [12] studied the phase equilibria of Nb-Cr-Si at 1200°C using a diffusion couple and thermodynamic modeling. Their studies established that Nb3Cr2Si3 (same structure as βNb5Si3) is stable at this temperature. There