Time: 10:45 a.m.
Place: TEMPMEKO 2013, Madeira, Portugal
In this paper we present the simulation of the eutectic phase transitions in the Pt-C system, in terms of both freezing and melting, using the multi-phase field multi-component model. The simulation is performed, using the open source library “Open Phase”. By appropriate scaling, heat extraction and -injection rates associated with the induction of freezing and melting in the experiments referred to, i.e. on a macroscopic scale, are converted into the corresponding rates as simulated on a microscopic scale.
Due to the extreme differences in the volume fractions of the FCC-Pt rich phase on the one hand and graphite (C) on the other, the stability of the system is not always given and computing times are excessive. Nevertheless satisfactory results for the kinetics of solidification and melting have been obtained, involving reasonable offsets in temperature -inducing freezing and melting - with respect to the equilibrium eutectic temperature.
For freezing in the simulations the needle/rod-like morphology, as experimentally observed, was reproduced for differing heat-extraction rates. The seemingly anomalous peak characterizing the simulated freezing curves is ascribed to the speed up of the solidification process due to the curvature effect. Similarly a peak is observed in the experimental freezing curves, also showing up more clearly with increasing freezing rates. It appeared that the nucleation mechanism, triggered at the start of and during freezing, should be further studied to explain the discontinuous needle like morphology requiring a high number of nucleation events.
Melting was simulated starting from a frozen structure produced by a freezing simulation. The simulations reproduce the experimental melting curves and help -together with the simulated freezing curves- to understand the phase transition of the Pt-C eutectic.
Finally the effect of metallic impurities was studied. As shown for Au, impurities can affect the morphology of the eutectic structure, their impact increasing with the impurity content, i.e. they can act as modifiers of the structure, as earlier reported for irregular eutectics. By this example we showed that the proposed modeling technique is also suitable for quantifying the effect of impurities on the solidification kinetics and thus justifies more detailed studies, involving other impurities, as detected in real samples.
Measurements of the eutectic melting and freezing temperatures of Pt-C eutectics are important for application as reference points for high temperature thermometers. To fully understand the kinetics of the involved phase transition a phase-field simulation was carried out successfully replicating the needle-like solidification as well as showing realistic cooling-/heating-temperature-curves. Further investigations include the morphology-dependence of the applied cooling-rate and the inclusion of impurities and their effect on the temperature-curves and resulting microstructure.