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MM: Fachverband Metall- und Materialphysik

MM 22: Computational Materials Modelling - Phase Stability II

MM 22.2: Vortrag

Dienstag, 12. März 2013, 12:00–12:15, H24

Structural stability of topologically close-packed phases in the Fe-Nb system — •Alvin Noe Ladines, Thomas Hammerschmidt, and Ralf Drautz — ICAMS, Ruhr Universität Bochum, Germany

The amount of refractory metals in powder metal steels is limited by carbide formation during atomization of the powder. Diffusion alloying makes it possible to push this limit by starting from a carbon-free melt. This route leads to the formation of topologically close-packed (TCP) phases which are transformed into carbides by subsequent addition of graphitic carbon during powder consolidation. In order to optimize this new processing method, one has to determine the properties of the observed TCP phases and to understand the mechanism of TCP-carbide transformation. In this study, we employ density functional theory (DFT) to determine the stability of TCP phases in the Fe-Nb system. The TCP phases that we considered include the A15, Laves(C14,C15 and C36), χ, µ and σ. Each of the Wyckoff positions were filled with either Fe or Nb allowing us to change the composition of the binary system. According to our calculated values of the heat of formation, the Laves phases and the µ phase are stable with stoichiometric compositions of Fe₂Nb and Fe₇Nb₆, respectively. For the Laves phases we observe a very close competition with the heats of formation differing only by a few meV. Our calculations also suggest a broad stability range for the µ phase extending up to approximately 54 at % Nb. Our findings are consistent with the experimental phase diagram of the Fe-Nb system. In addition, we also determined the influence of C defects on the stability of the Laves and the µ phases.