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Thermodynamic Calculations of Phase Equilibria and Phase Fractions of a β-Solidifying TiAl Alloy using the CALPHAD Approach

Published online by Cambridge University Press:  21 December 2012

Robert Werner ,
Martin Schloffer ,
Emanuel Schwaighofer ,
Helmut Clemens  and
Svea Mayer
Robert Werner
Affiliation:
Department of Physical Metallurgy and Materials Testing, Montanuniversität Leoben, A-8700Leoben, Austria
Martin Schloffer
Affiliation:
Department of Physical Metallurgy and Materials Testing, Montanuniversität Leoben, A-8700Leoben, Austria
Emanuel Schwaighofer
Affiliation:
Department of Physical Metallurgy and Materials Testing, Montanuniversität Leoben, A-8700Leoben, Austria
Helmut Clemens
Affiliation:
Department of Physical Metallurgy and Materials Testing, Montanuniversität Leoben, A-8700Leoben, Austria
Svea Mayer
Affiliation:
Department of Physical Metallurgy and Materials Testing, Montanuniversität Leoben, A-8700Leoben, Austria
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Abstract

The CALPHAD (CALculation of PHAse Diagrams) method is widely recognized as a powerful tool in both scientific and industrial development of new materials and processes. For the implementation of consistent databases, where each phase is described separately, models are used which are based on physical principles and parameters assessed from experimental data. Such a database makes it possible to perform realistic calculations of thermodynamic properties of multi-component systems. However, a commercial available TiAl database can be applied for thermodynamic calculations to both conventional Ti-base alloys and complex intermetallic TiAl alloys to describe experimentally evaluated phase fractions as a function of temperature. In the present study calculations were done for a β-solidifying TiAl alloy with a nominal composition of Ti-43.5Al-4Nb-1Mo-0.1B (in at. %), termed TNMTM alloy. At room temperature this alloy consists of ordered γ-TiAl, α2-Ti3Al and β0-TiAl phases. At a certain temperature α2 and β0 disorder to α and β, respectively. Using the commercial database the thermodynamic calculations reflect only qualitative trends of phase fractions as a function of temperature. For more exact quantitative calculations the commercial available thermodynamic database had to be improved for TiAl alloys with high Nb (and Mo) contents, as recently reported for Nb-rich γ-TiAl alloys. Therefore, the database was modified by experimentally evaluated phase fractions obtained from quantitative microstructure analysis of light-optical and scanning electron micrographs as well as conventional X-ray diffraction after long-term heat treatments and by means of in-situ highenergy X-ray diffraction experiments. Based on the CALPHAD-conform thermodynamic assessment, the optimized database can now be used to correctly predict the phase equilibria of this multi-component alloying system, which is of interest for applications in automotive and aircraft engine industry.

Keywords

phase equilibriaphase transformationx-ray diffraction (XRD)
Type
Articles
Information
MRS Online Proceedings Library (OPL) , Volume 1516: Symposium JJ – Intermetallic-Based Alloys—Science, Technology and Applications , 2013 , pp. 59 - 64
Copyright
Copyright © Materials Research Society 2012 

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References

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