Thermodynamic Modeling and Solidification Simulation of Ti-Al-Cr alloys
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Thermodynamic Modeling and Solidification Simulation of Ti-Al-Cr alloys Y. Yang1, B.P. Bewlay2, 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 Titanium aluminide based alloys are candidate materials for high temperature structural applications. They are typically alloyed with elements such as Nb, Ta, Mo, Cr and B for property enhancement. To understand the relationship between microstructure and alloy composition/processing condition, detailed models of phase equilibria in multicomponent Ti-Al based alloys are needed. In this work, we developed thermodynamic models for the phases in the Ti-Al-Cr system based on critically assessed binary models and ternary experimental data in literature, using the CALPHAD approach. Isothermal sections at 1200, 1150, 1000 and 800qC, and the liquidus projection, were calculated from the currently developed thermodynamic models; these are in satisfactory agreement with experimental data. Isopleths were calculated at specified Cr concentrations, and solidification paths were simulated under the Scheil conditions for a range of Ti-Al-Cr alloys. From the calculated phase diagrams and solidification paths, the effect of Cr on the microstructure of Ti-Al alloys can be understood.
INTRODUCTION TiAl-based alloys are considered as a promising class of high temperature materials due to many attractive properties, such as low density, good corrosion resistance, and high strength at high temperatures [1]. However, two major limitations for their applications are low room-temperature ductility and processing difficulties [1]. Alloying with Cr and Nb raises their ductility, resistance to high temperature oxidation, and high temperature strength [1]. The microstructures of Ti-Al-Cr-Nb alloys are sensitive to the alloy composition, solidification sequence, and heat treatment conditions. Therefore, an accurate understanding of the phase diagram is a prerequisite for successful design and fabrication of TiAl-based alloys. As an important step to establish a model for phase equilibria in the Ti-Al-Cr-Nb system, thermodynamic modeling of the Ti-Al-Cr system has been performed in the present study using the CALPHAD (CALculation of PHAse Diagram) approach [2,3]. Extensive experimental studies on the phase equilibria in the Ti-Al-Cr system have been performed and they were reviewed by Hayes [4], Raghavan [5], and Bochvar [6]. The Hayes’ review was based on the work up to 1990. He presented partial isothermal sections in the Ti-rich corner and in the range of 600~1200qC mainly based on the results of Ence et al.[7]. Raghavan [5] and Bochvar [6] reviewed more recent experimental studies by Nakayama [8], Brady [9], Hao [10], Jewett [11,12,13,14], Schwanold [15], Park [16] and Kainuma [17]. The proposed isothermal sections by both reviewers agree reasonably with each other. Raghavan and Bochvar also
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postulated liquidus projections based on bin
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