Time: 10:00 a.m.
Place: ICAMS, UHW 12/1222
Evgeniya Kabliman, Department of Applied Physics, Vienna University of Technology, Vienna, Austria
The talk concerns the simulation of the atomic site distribution in binary sigma-phases, which are found in many industrial alloys and usually lead to a destructive effect on the mechanical properties of these alloys. Among them the Fe-Cr σ-phase is the most investigated one due to relevance to different types of steels, where its formation should be avoided due to extreme brittleness. This phase has a low symmetry structure (P42/mnm) with a tetragonal unit cell of 30 atoms divided into 5 nonequivalent sites (2a,4f,8i,8i',8j) denoted as (A,B,C,D,E). One of the most difficult issues for theory as well as for experiment is the description of the σ-phase stability, which first of all implies the accurate determination of the atomic distribution among its five nonequivalent sites. In present work [1,2] we have proposed a simple and powerful approach to calculate the site occupation numbers in binary σ-phases as function of temperature and composition. It is based on the single-site mean-field description of the free energy, where the total energy is expanded in terms of on-site effective cluster interactions. We have shown that the Fe-Cr system exhibits a non-trivial magnetic behavior at high temperatures, which affects the site occupations. The calculated site occupations in the Fe-Cr σ-phase are in very good agreement with available experimental data [3,4]. The structural variation (volume and c/a), that might be present due to irradiation and thermal expansion, can lead to an additional atomic redistribution.
The suggested method has been further applied to the Fe-V system and some tests were also performed for the Co-Cr and Re-W sigma-phases. The calculated atomic site occupancies show some general trends, but also some individual features in agreement with empirical scheme of ordering proposed earlier by Kasper and Waterstrat .
The present work has been performed in collaboration with Prof. A. V. Ruban from Royal Institute of Technology, Stockholm.
 E. Kabliman, P. Blaha, K. Schwarz, A. V. Ruban, B. Johansson, Phys. Rev. B 83, 092201 (2011)
 E. Kabliman, P. Blaha, K. Schwarz, O. Peil, A. V. Ruban, B. Johansson, submitted to Phys. Rev. B (2011).
 H. L. Yakel, Acta Crystall. B, 20 (1983)
 J. Cieslak, M. Reissner, S. Dubiel, J. Wernisch, and W. Steiner, J. All. Comp. 460, 20 (2008)
 J. S. Kasper and R. Waterstrat, Acta Crystall. 9, 289 (1956)