Berlin 2018 – scientific programme

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MM: Fachverband Metall- und Materialphysik

MM 47: Poster Session II

MM 47.2: Poster

Wednesday, March 14, 2018, 18:30–19:45, Poster C

Towards an ab initio phase diagram of Fe-Nb: Importance of Magnon-Phonon coupling — •Ali Zendegani, Fritz Körmann, Tilmann Hickel, and Jörg Neugebauer — Max-Planck-Institut für Eisenforschung GmbH, Max-Planck-Straße 1, 40237 Düsseldorf, Germany

Ferritic steels containing Fe-Nb TCP phases form a promising materials class for high-temperature applications. To tailor this high-temperature strength, e.g., via heat treatments, a precise knowledge of the thermodynamic stability of the TCP, i.e. the hexagonal and Laves (C14, C36) phases, and the cubic Laves (C15) phase, is essential.

Combining density functional theory (DFT) and thermodynamic concepts, we evaluate the ground state energy as well as finite temperature entropy contributions of these structures. All relevant magnetic configurations, ranging from non-magnetic simulations over selected ferro/ferri-magnetic ones to fully paramagnetic are considered. In particular, the spin-space averaging (SSA) method is utilized to average forces over many magnetic configurations characterizing the disordered magnetic states. Only with these methods, paramagnetic phonons, which are of central importance for temperature dependent phase stabilities, can be calculated. We demonstrate that in contrast to non-magnetic (non-spin-polarized) calculations, not only quantitative changes are obtained, but even dynamical instabilities, e.g. in C36, will disappear. Only when employing the paramagnetic state explicitly, we obtain agreement with experiment. The results provide direct insight into the mechanisms that stabilize these phases.