Effect of Cr Addition on the Multiphase Equilibria in the Nb-rich Nb-Si-Ti System - Thermodynamic Modeling and Designed
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Effect of Cr Addition on the Multiphase Equilibria in the Nb-rich Nb-Si-Ti System - Thermodynamic Modeling and Designed Experiments Y. Yang1, B.P. Bewlay2, S.-L. Chen1, M.R. Jackson2 and Y.A. 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 are considered as candidates of next-generation high temperature materials (i.e. >1200°C). Ti and Cr have been shown to have beneficial effects on the oxidation resistance and mechanical properties of Nb-Si alloys. Phase equilibria in the Nb-Si-Ti system have been studied in detail. The present study has investigated multiphase equilibria in the Nb-Si-Ti alloys with Cr additions via an approach of integrating thermodynamic modeling with designed experiments. The alloying effects of Cr on the microstructure of the Nb-Si-Ti alloys are described using both phase equilibria and solidification paths that were calculated from the thermodynamic description of the Nb-Cr-Si-Ti system developed in the present study.
INTRODUCTION In searching for higher temperature materials (i.e. > 1200°C) for future propulsion systems, Refractory Metal-Intermetallic Composites (RMICs) [1] based on the Nb-Si-X system have received considerable interest [2]. The typical alloying elements (X’s) are Ti, Cr, Hf, and Al. Ti and Cr have been shown to have beneficial effects on the oxidation resistance and mechanical properties of Nb-Si alloys [3-6]. Phase diagrams of the Nb-Si-Ti-Cr system, are a prerequisite for the successful development of this family of materials. In the present study, a thermodynamic database that includes the Gibbs energy functions of all of the phases in the Nb-Si-Ti-Cr system was developed using the Calphad approach. The thermodynamic description was then coupled with Pandat software for the calculation of phase equilibria and solidification paths of multicomponent Nb-Si-Ti-Cr alloys. The calculated results were then compared with the results of designed experiments, and the subsequent analyses were used to refine the thermodynamic description.
THERMODYNAMIC MODELING AND EXPERIMENTAL PROCEDURES The strategy for building the thermodynamic database for the Nb-Si-Ti-Cr quaternary system began with deriving Gibbs energy of each phase in each of the constituent binaries. There are six constituent binaries: Nb-Si, Nb-Cr, Nb-Ti, Cr-Si, Cr-Ti, and Ti-Si. After thermodynamic descriptions for all the constituent binary phases were established, the Gibbs energy of a phase in a
ternary system was obtained by the weighted average of those of the same phase in the constituent binaries using geometric models, such as the Muggianu model [7] in the present study. This extrapolation method for obtaining the thermodynamic description of each ternary phase works quite well in many cases when describing thermodynamic properties and phase equilibria. However, in the pre
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