Mathematische Modellierung der Ausscheidung ... - OPUS-Datenbank

Zusammenfassung XI
Zusammenfassung
A new model has been developed in this work which is capable of simulating the precipita-
tion kinetics of brittle phases, especially TCP-phases (topologically close packed phases)
in ruthenium containing superalloys. The model simultaneously simulates the nucleation
and the growth stage of precipitation for any number of precipitating phases. The CAL-
PHAD method (Calculation of Phase Diagrams) is employed to calculate thermodynamic
properties, such as the driving force or phase compositions in equilibrium. For calculation
of diffusion coefficients, kinetic mobility databases which are also based on the CALPHADmethod
are used. The model is fully capable of handling multicomponent effects, which
are common in complex superalloys. Metastable phases can be treated and will automatically
be dissolved if they get unstable. As the model is based on the general CALPHAD
method, it can be applied to a broad range of precipitation processes in different alloys as
long as the relevant thermodynamic and kinetic databases are available.
The developed model proves that the TCP-phases precipitate in a sequence of phases.
The first phase that is often formed is the metastable σ-phase because it has the lowest
interface energy due to low-energy planes at the interface between matrix and precipitate.
After several hundred hours the stable µ- and P-phases start to precipitate by nucleating at
the σ-phase which is energetically favourable. During the growth of these stable phases
the σ-phase is continuously dissolved. It can be shown by thermodynamic CALPHAD cal-
culations that the σ-phase has a lower Gibbs free enthalpy than the µ- and P-phase. All
required parameters of the model, such as interface energy and nucleate densities, have
been estimated.
The mechanisms of suppression of TCP-phase precipitation in the presence of ruthenium
in superalloys were investigated with the newly developed model. It is shown by the simulations
that ruthenium mostly affects the nucleation stage, while the growth stage of precipitation
is nearly unchanged and the equilibrium phase fraction of the TCP-phases is
hardly changed by ruthenium. Nucleation is partly slowed down by a reduction of the driving
force through so called “reverse partitioning”, which means that rhenium is pushed into
the γ’-phase if adding ruthenium to the alloy. According to the results additionally the driv-
ing force is reduced by a lower γ’-phase fraction, which can be caused by ruthenium in
some alloys depending on other elements. Thermodynamic calculations prove that “reverse
partitioning”, which is a disputed concept, exists in some alloys. However, this is not