Nucleation kinetics of the liquid-liquid phase separation under extreme external conditions

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Liquid-liquid phase separation of monotectic alloys presents a first order phase transformation except in a narrow composition range near the critical consolute point, where the chemical spinodal is directly accessible also in controlled experiments at low or moderate cooling rates. Therefore, the kinetics of the binodal formation of two chemically distinct liquid phases from the homogeneous melt involves undercooling, nucleation and growth stages, similar to crystallization transformations. Yet, the only effective method to prevent the formation of coarse-scaled structures in systems with a miscibility gap in the liquid state is by application of rapid solidification processing techniques. This behaviour can also be accounted for by considering critical-point wetting which occurs in general in the vicinity of the critical point of a miscibility gap. One consequence of critical point wetting is the absence of undercooling of the phase separation transformation within a finite composition range on one side of the miscibility gap. Additionally, the delicate balance of phase equilibria that extends over a finite composition range over which the interface excess free energy densities control the nucleation conditions and also the microstructure evolution indicate the opportunity to utilize external fields, e.g. magnetic fields as performed in the group of Prof. Gao, for altering the balance and thus tuning the phase transformation and the microstructure development.In order to address the nucleation kinetics quantitatively, we propose using an approach that is based on describing nucleation as an inhomogeneous Poisson process. With this approach and the corresponding experimental schedule developed in our previous work on pure metals and alloys, nucleation rates and nucleation barrier heights can be obtained quantitatively over a large range of undercooling values. These measurements shall be conducted on Fe-Cu and Co-Cu alloys in the composition range of their metastable miscibility gaps, respectively and shall be compared to measurements using levitation melting in the group of Prof. D. Herlach. The goal of these measurements is two-fold: the determination of the nucleation kinetics of the phase separation transformation without application of an external magnetic field shall be used as reference for the experiments performed together with Prof. Gao’s group, performing similar experiments on identical alloy compositions with external magnetic field present. Additionally, the detailed nucleation kinetics analyses shall be applied to the composition region, where a transition from perfect – to normal wetting conditions occur, since this transformation should result in a drastic change of the mechanism of the underlying phase transformation, which so far has never been analyzed directly.Solidification of monotectic alloys within the miscibility gap region is often preceded by large compositional segregation into two liquid phases which results in coarse scaled phase distributions in the solid state. Additionally, one of the two liquid phases crystallizes at a significantly lower temperature, thus impeding microstructure analyses of the as-phase separated microstructure. In this respect, the application of microchip calorimetry on low-melting monotectic alloys presents a unique opportunity, since the fast heating and cooling rates achievable by this method that can exceed several thousands degrees per second allow multiple scanning of the phase transformation to obtain statistically significant datasets and at the same time facilitate effective freezing-in of the early stages of microstructure formation. For these measurements, binary monotectic Bi-Ga alloys shall be analyzed within the composition range of the stable miscibility gap. Goal of these measurements is to analyze the nucleation kinetics of the phase separation transformation as described above and – for selected compositions – to cool the material after the phase separation as quickly as possible to analyze the microstructure evolution in the early stages with knowledge of the temperature of phase separation.