Talks and Poster Presentations (without Proceedings-Entry):
E. Povoden-Karadeniz, E. Eidenberger, H. Leitner, E. Kozeschnik:
"Thermodynamics of the Fe-Co-Mo system and modeling of early precipitation in Fe-25 at.% Co-9 at.% Mo alloy";
Talk: Calphad XL,
Rio de Janeiro;
Solution-treated and quenched Fe-25 at.% Co-9 at.% Mo alloys show a remarkable increase in hardness during ageing at elevated temperatures . The tremendous role of semicoherent -phase, (Co,Fe)7Mo6 for precipitation hardening has been demonstrated in several experimental studies. Systematic investigations of the microstructure and composition of these precipitates suggest a close relation of -phase formation to initial decomposition processes . However, up to now the mechanisms, which govern these early decomposition stages in the Fe-Co-Mo system are not well understood, and explained controversially in the literature. Chain-like regions along elastically soft <100> Matrix-directions have been interpreted either as spinodal decomposition , or as metastable coherent, nanoscale bcc-structured nuclei with compositions that differed significantly from the composition of macroscopic precipitates predicted by the phase diagram .
We apply classical nucleation theory in combination with a CALPHAD assessment of thermodynamic model parameters of the Fe-Co-Mo system, in order to verify the existence of potential precursor phases of the -phase in Fe-25 at.% Co-9 at.% Mo alloy. The theoretical nucleation and growth regime is distinguished from the composition range of spinodal decomposition in the Fe-Co-Mo system by calculating the coherent spinodal, with the Gibbs energies of the phases in the Fe-Co-Mo system being assessed over a wide range of temperatures. For compositions outside the spinodal, the most probable nucleus composition of nano-scaled particles can then be predicted by minimization of the critical energy of the nucleation event. Recently, this strategy has been successfully applied to the simulation of bcc-Cu precipitation in Fe-Cu . Since the molar Gibbs energy is included in the minimum energy equation for the nucleation event, the thermodynamic assessment of the Fe-Co-Mo system is an essential part for a correct prediction of the nucleus composition. The determined nucleus composition of potential pre- precipitates in Fe-25 at.% Co-9 at.% Mo alloy is used as input parameter for the thermo-kinetic simulation of the precipitation sequence during continuous heating of solution-treated and quenched Fe-25 at.% Co-9 at.% Mo alloy. We predict nucleation and growth of bcc-type particles before the precipitation of -phase. The calculated composition of the pre- phase is in good agreement with 3D atom probe measurements.
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Created from the Publication Database of the Vienna University of Technology.