Dr. J.-C. Crivello
Chimie Métallurgique des Terres Rares
Institut de Chimie et des Matériaux Paris-Est
CNRS, Université de Paris

H First principles calculations of the σ and χ phases in binary and ternary rhenium-based systems

Abstract: 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, all the ordered configurations in a ternary sigma phase, i. e. 53=243 configurations, have been investigated. The Cr-Mo-Re system has been judiciously chosen to be investigated since the two binary Cr-Re and Mo-Re sigma phase are well known for showing different sites preference for Re.
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 Mo-Re and W-Re, while these two phases are stable in Nb-Re and Ta-Re. Computed sites occupancies are in very good agreement with the experimental measurements.

References:
[1] J.-C. Crivello and J.-M. Joubert, J. Phys.: Condens. Matter 22 (2010) 035402.
[2] J.-C. Crivello, M. Palumbo, T. Abe and J.-M. Joubert, in preparation

The seminar takes place at 10 a.m. in the ICAMS seminar room UHW 11/1102, Stiepeler Straße 129, 44801 Bochum.

For further information contact Hildegard Wawrzik, hildegard.wawrzik@rub.de