Eunan J. McEniry
Tilmann Hickel, Jörg Neugebauer
Max-Planck-Institut für Eisenforschung, Düsseldorf, Germany
Thermodynamics and kinetics of precipitation in compositionally complex metallic alloys
Precipitation hardenable Ni-base superalloys offer exceptional mechanical performance at elevated temperatures. The characteristic microstructural feature of such alloys, is the addition of coherent γ´´ (D022) intermetallic precipitates to the typical γ/ γ´ microstructure of conventional superalloys, where γ’’ precipitates are of the form Ni3X, where X is a refractory element such as Nb and Mo. In this work, we have employed direct atomistic simulations within the Metropolis and kinetic Monte Carlo frameworks to study the thermodynamics and kinetics of precipitation in a model Ni-Nb-Al alloy. Key challenges include the fact that the Nb and Al concentrations are far from the dilute limit, and that the effects of elastic strain have a huge impact on the precipitation kinetics. In the thermodynamic limit below the critical temperature, one
finds the formation of mixed Γ-Ni6NbAl precipitates. However, kinetic simulations reveal co-precipitation of the Al-rich γ’ L12 phase and the Nb-rich γ’’ phase, with the mixed Γ phase being kinetically hindered. The results of such simulations can be used to produce TTT diagrams and to parameterize continuum-scale precipitation kinetics models. Preliminary results from a similar approach applied to the Al-Sc alloy system will also be shown, where the complex relationship between diffusion kinetics and precipitation growth determines the microstructure of the material.