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A combined DFT and compound energy formalism study of phase stability in the Co-W system

Date: 27.05.2013
Time: 9:00 a.m.
Place: CALPHAD XLII, San Sebastian, Spain

Jörg Koßmann
Mauro Palumbo
Abed Breidi
Suzana Fries
Thomas Hammerschmidt
Ralf Drautz

Co-based alloys, similarly to Ni-based alloys, can show L12 (γ’) coherent ordering in an A1 (γ) matrix what makes mechanical properties of both classes of materials, comparable. They are, therefore, interesting for high temperature applications as aircraft turbine blades. There is, however no Co3Al stable alloy with L12 crystal structure and the well known γ/ γ’ morphology is only obtained with the addition of tungsten. There are still open questions about the stability of the ternary Co3(Al,W) γ’phase as some experimental results indicate that it can be metastable [1,2]. A further similarity of Co-based and Ni-based superalloys is the tendency to form topologically close packed (TCP) phases [3] which are detrimental to the mechanical properties.


Here we use a combination of density functional theory (DFT) calculations of configuration enthalpies of formation and Bragg-Williams-Gorsky approximation as model for configuration entropy in order to obtain the Gibbs energies of competing solid phases [4] . We study not only the FCC based structures like γ and γ’ but also the other competitive phases like B2, D19, μ, σ, C14, C15, C36, A15, etc. In this work we present the results for the Co-W binary system. The figure shows at the top the formation enthalpies for several phases calculated in the GGA_PBE approximation, in the middle the calculated phase diagram obtained by the use of the Compound Energy Formalism CEF, and at the bottom, the experimental and calculated phase diagram accessible in the literature [5]. We will discuss the metastability of FCC ordering and preliminary results for the binary systems Al-Co and Al-W.

Acknowledgement:
The authors acknowledge funding by the Deutsche Forschungsgemeinschaft (DFG) through
projects C1 and C6 of SFB/Transregio 103 as well as ICAMS sponsors,ThyssenKrupp AG, Salzgitter Mannesmann Forschung GmbH, Robert Bosch GmbH, Bayer Materials Science AG,
Bayer Technology Services GmbH, Benteler Stahl/Rohr AG, state of North Rhine-Westphalia and
European Commission in the framework of the European Regional Development Fund (ERDF).

References
[1] K. Shinagawa, T. Omori, S. K. Oikawa, I. Ohnuma, R. Kainuma and K Ishida, Materials
Transactions, 49 (6) 1474-1479 (2008)
[2] Q. Yao, H. Xing and J Sun, Applied Physics Letters 89, 161906 (2006)
[3] Seiser, T. Hammesrschmidt, A. Komolgorov, et al., Physical Review B 83, 224116 (2011)
[4] M. Palumbo, T. Abe, S. G. Fries, and A. Pasturel, Physical Review B 84, 144109 (2011).B.
[5] J. Sato, K. Oikawa, R. Kainuma and K Ishida. Materials Transactions, 46 (6) 1199-1207 (2005)

Supporting information:

Kossman_Palumbo_etall_CALPHAD2013.pdf
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