First principles calculations of the σ and χ phases in Binary and Ternary Rhenium-based systems

Date & Time
28 April 2010 (Wed), 13:30 - 15:00
6F small seminar room, Central Bldg., Sengen site
Dr. Jean-Claude Crivello
CNRS – ICMPE – CMTR (Thiais, France)
First principles calculations of the σ and χ phases in Binary and Ternary Rhenium-based systems
The topologically close packed phases appear in many important multi-components systems, like superalloys, in which refractory elements are added. The precipitation of these intermetallic compounds may cause problems due to their brittleness which affects the mechanical properties of the alloy, motivating the present study about their stability.
Systematic DFT calculations have been performed for the complete set of ordered configurations generated by distributing A or B elements in the σ and χ phases among the different sites for the four binary A–B systems, with A = {Nb, Ta, Mo, W} and B = Re. Moreover, the ternary -Cr-Mo-Re system has been investigated since the two binary Cr-Re and Mo-Re -phase are known for showing inverse Re sites preference: high coordination number (CN) sites in Cr-Re [1] and low CN in Mo-Re [2].
Total energies have been used in order to compute the atomic distribution at finite temperatures in the Bragg–Williams (BW) approximation. It is shown that the configurational entropy stabilizes the σ and χ phases in MoRe and WRe, while there are stable in NbRe and TaRe. Computed sites occupancies are in very good agreement with the experimental measurements. About the ternary Cr-Mo-Re -phase, this system presents a miscibility gap at low temperature and its Gibbs energy surface is convex above ~750K [3]. Re site preference is shown to change progressively in the ternary field when passing from Mo-Re to Cr-Re binary borders.
[1] M. Palumbo, T. Abe, C. Kocer, H. Murakami, H. Onodera, in preparation.
[2] J.-C. Crivello and J.-M. Joubert, J. Phys.: Condens. Matter 22 (2010) 035402.
[3] J.-C Crivello, M. Palumbo, T. Abe and J.-M. Joubert, in preparation.
Dr. Taichi Abe(阿部太一)