Article

# Vibrational properties of alpha- and sigma-phase Fe-Cr alloy.

Faculty of Physics and Applied Computer Science, AGH University of Science and Technology, aleja Mickiewicza 30, 30-059 Krakow, Poland.

Physical Review Letters (Impact Factor: 7.73). 04/2010; 104(15):155503. DOI: 10.1103/PhysRevLett.104.155503 Source: PubMed

- Journal of Materials Science 11/2012; 47(21):7385-7398. · 2.31 Impact Factor
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**ABSTRACT:**In steels and single-crystal superalloys the control of the formation of topologically close-packed (TCP) phases is critical for the performance of the material. The structural stability of TCP phases in multi-component transition-metal alloys may be rationalized in terms of the average valence-electron count N and the composition-dependent relative volume-difference dV/V. We elucidate the interplay of these factors by comparing density-functional theory calculations to an empirical structure map based on experimental data. In particular, we calculate the heat of formation for the TCP phases A15, C14, C15, C36, χ, μ and σ for all possible binary occupations of the Wyckoff positions. We discuss the isovalent systems V/Nb–Ta to highlight the role of atomic-size difference and observe the expected stabilization of C14/C15/C36/μ by dV/V at N=0 in V–Ta. In the systems V/Nb–Re, we focus on the well-known trend of A15→σ→χ stability with increasing N and show that the influence of dV/V is too weak to stabilize C14/C15/C36/μ in Nb–Re. As an example for a significant influence of both N and dV/V, we also consider the systems Cr/Mo–Co. Here the sequence A15→σ→χ is observed in both systems but in Mo–Co the large size-mismatch stabilizes C14/C15/C36/μ. We also include V/Nb–Co that cover the entire valence range of TCP stability and also show the stabilization of C14/C15/C36/μ. Moreover, the combination of a large volume-difference with a large mismatch in valence-electron count reduces the stability of the A15/σ/χ phases in Nb–Co as compared to V–Co. By comparison to non-magnetic calculations we also find that magnetism is of minor importance for the structural stability of TCP phases in Cr/Mo–Co and in V/Nb–Co.New Journal of Physics 11/2013; 15:115016. · 4.06 Impact Factor - [Show abstract] [Hide abstract]

**ABSTRACT:**Using density-functional theory in combination with the direct force method and molecular dynamics we investigate the vibrational properties of a binary Cr-Re σ-phase. In the harmonic approximation, we have computed phonon dispersion curves and density of states, evidencing structural and chemical effects. We found that the σ-phase is mechanically unstable in some configurations, for example, when all crystallographic sites are occupied by Re atoms. By using a molecular-dynamics-based method, we have analysed the anharmonicity in the system and found negligible effects (∼0.5 kJ/mol) on the Helmholtz energy of the binary Cr-Re σ-phase up to 2000 K (∼0.8Tm). Finally, we show that the vibrational contribution has significant consequences on the disordering of the σ-phase at high temperature.The Journal of Chemical Physics 04/2014; 140(14):144502. · 3.12 Impact Factor

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