Phase Equilibria in the Nb-rich Nb-Ti-Si-Cr-Hf alloys
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Phase Equilibria in the Nb-rich Nb-Ti-Si-Cr-Hf alloys Ying Yang1, Bernard P. Bewlay2, and Austin Chang3 1
CompuTherm LLC, Madison, Wisconsin 53719, USA General Electric Global Research, Schenectady, New York 12301, USA 3 University of Wisconsin-Madison, Madison, Wisconsin 53706, USA 2
ABSTRACT Refractory Metal-Intermetallic Composites (RMICs) based on the Nb-Si system have been considered as candidates for the next-generation high temperature materials (i.e. !1200°C). Ti, Cr and Hf have been shown to have beneficial effects on the oxidation resistance and mechanical properties of Nb-Si alloys. The present study has determined phase equilibria in the Nb-rich region of the Nb-Si-Ti-Cr-Hf system via the Calphad approach. The alloying effects of Cr and Hf on the microstructure of Nb-Si-Ti alloys are understood based on isothermal sections, liquidus projections, and solidification curves that were calculated from the thermodynamic models of the Nb-Ti-Si-Cr-Hf system developed in the present study. This work provides important guidelines on the development of new Nb-Si-Ti-Cr-Hf alloys.
INTRODUCTION In searching for higher temperature materials (i.e. ! 1200°C) for future propulsion systems, Refractory Metal-Intermetallic Composites (RMICs) based on the Nb-Si system have received considerable interest [1-2]. The Nb-rich Nb-Si phase diagram is shown in Figure 1 [3]. (Nb) and Nb3Si form eutectic at ~1920qC between 3.5at% and 25 at% Si. Nb3Si is decomposed into (Nb)+Nb5Si3 at 1920qC T0.5 at% Si) through an eutectoid reaction 1770qC However, the kinetics of this eutectoid decomposition are very sluggish. Therefore, the microstructure of Nb-rich Nb-Si alloys often consists of a three-phase mixture: Nb+Nb3Si+Nb5Si3. Nb5Si3 has better mechanical properties and oxidation resistance than the Nb3Si phase. It is desired to form the (Nb)+Nb5Si3 eutectic microstructure directly from liquid. This cannot be realized in the Nb-Si binary, Figure 1 Phase diagram in however, adding other elements may offer some the Nb-rich Nb-Si system [3]. opportunities. The typical alloying elements are Ti, Cr, Hf, and Al. They have been shown to have beneficial effects on the oxidation resistance and mechanical properties of Nb-Si alloys [4-7]. Phase equilibria in the Nb-Ti-Si system have been well studied [8-9], and the effect of Ti on the microstructure of Nb-Si alloys has been well understood. Ti can stabilize Nb3Si to lower temperatures, and promote the formation of (Ti,Nb)5Si3 phase (hP16), which is detrimental to material mechanical properties. It
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also reduces the temperature of the eutectic reaction Liquid o (Nb) + Nb3Si. There is no direct eutectic between (Nb) and Nb5Si3 in the Nb-Ti-Si system. The goal of the present work is to map the phase equilibria in the Nb-rich region of the Nb-Ti-Si-Cr-Hf system and to understand the effect of Cr and Hf on the phase stability of Nb-Ti-Si alloys. First of all, thermodynamic models of phases in the Nb-Ti-Si-Cr-Hf system were developed using the CALPHAD (CALculation of PHAse Diagram) approach. The t
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