Phase Stability in the Mo-Ti-Zr-C System via Thermodynamic Modeling and Diffusion Multiple Validation
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INTRODUCTION
MO-RICH alloys in the Mo-Ti-Zr-C system have for decades been employed in high temperature applications requiring superb structural stability.[1] Despite the broad use of molybdenum alloys in high temperature furnaces, lamps, isothermal forging dies, X-ray tubes, glassmelting electrodes, radiation shields, and rocket nozzles, to name just a few, there is a dearth of published phase equilibrium assessments beyond the binary systems. A majority of studies in this system are now well over a quarter of a century old.[2–5] In fact, despite the increasing reliance on CALPHAD-generated models in the steel and specialty alloy industries, the authors are aware of only one attempt to calculate the phase equilibrium in a carbide-forming molybdenum alloy system.[6] The goals of the present work were to develop and critically assess the first complete CALPHAD description in the Mo-rich corner of the quaternary system Mo-Ti-Zr-C, to experimentally validate the thermodynamic database thus developed using diffusion multiples, and to elucidate key aspects of thermodynamic phase stability in this system via targeted thermodynamic calculations using the validated database. Depending on its composition and thermal history, a carbide-forming molybdenum alloy can have a variety of second phases dispersed within the BCC-molybdenum matrix. Table I summarizes all the documented phases in
SUJOY KUMAR KAR, formerly Materials Scientist with GE Global Research, Bangalore, India, is now Assistant Professor with Metallurgical & Materials Engineering Department, Indian Institute of Technology, Kharagpur, Kharagpur 721302, West Bengal, India. Contact e-mail: [email protected] VORAMON S. DHEERADHADA and DON M. LIPKIN, Materials Scientists, are with GE Global Research, Niskayuna, NY. Manuscript submitted October 15, 2012. Article published online April 20, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
the Mo-Ti-Zr-C system along with their corresponding space groups[7] and also the corresponding designations in the present model. Mo-rich alloys typically have FCC(Ti,Zr)C and HCP-Mo2C carbides of which the FCC MC phase has been claimed to provide superior strengthening to the HCP Mo2C phase.[2] It is therefore imperative to have a good understanding of the thermodynamic phase stability (as well as the kinetics) of the system to optimally design the alloy composition and processing route for specific applications. CALPHAD-based thermodynamic modeling is an essential tool in accelerating this process. Assessment of the quaternary Mo-Ti-Zr-C system requires consistent thermodynamic descriptions for four elements, six binary subsystems, and four ternary subsystems. In the following, we summarize the state of the art in the thermodynamic assessment of the binary and ternary constituents of the Mo-Ti-Zr-C quaternary system. Gaps in the reported literature are identified and addressed, enabling a complete thermodynamic database to be developed. The quaternary model is developed using a CALPHAD-based extrapolation from the constit
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