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

在控制实验中，在低或中等冷却速率下，单晶合金的液-液相分离除了在靠近临界溶点的狭窄成分范围内外，表现为一级相变。因此，从均质熔体中形成两种化学性质不同的液相的双节点形成动力学涉及过冷、成核和生长阶段，类似于结晶转变。然而，在液相中具有混相间隙的体系中，防止形成粗尺度结构的唯一有效方法是应用快速凝固处理技术。这种行为也可以通过考虑通常发生在混相间隙临界点附近的临界点润湿来解释。临界点润湿的一个后果是，在混相间隙一侧的有限成分范围内，相分离转变没有过冷。此外，相平衡的微妙平衡延伸到一个有限的组成范围，在这个范围内，界面过量自由能密度控制成核条件和微观结构的演变，这表明有机会利用外部场，例如高教授小组所做的磁场，来改变平衡，从而调整相变和微观结构的发展。为了定量地解决成核动力学，我们建议使用一种基于描述成核为非均匀泊松过程的方法。利用这种方法和我们之前在纯金属和合金上开发的相应的实验计划，可以在大范围的过冷值范围内定量地获得成核速率和成核势垒高度。这些测量将分别在Fe-Cu和Co-Cu合金的亚稳混溶间隙的组成范围内进行，并将与D. Herlach教授小组使用悬浮熔化的测量结果进行比较。这些测量的目的是双重的：确定相分离转变的成核动力学，而不应用外部磁场应使用作为参考，与高教授的小组一起进行的实验，在存在外部磁场的情况下对相同的合金成分进行类似的实验。此外，应将详细的成核动力学分析应用于组成区域，在该区域发生从完全润湿到正常润湿的转变，因为这种转变会导致潜在相变机制的剧烈变化，而迄今为止从未直接分析过。单晶合金在混相间隙区凝固之前，通常会发生较大的成分偏析，形成两相，从而导致固态中粗大的尺度相分布。此外，两种液相中的一种在较低的温度下结晶，从而阻碍了对as相分离组织的微观结构分析。在这方面，微芯片量热法在低熔点单晶合金上的应用提供了一个独特的机会，因为通过这种方法可以实现的快速加热和冷却速度可以超过每秒几千度，允许对相变进行多次扫描，以获得具有统计意义的数据集，同时有助于有效地冻结微观结构形成的早期阶段。对于这些测量，二元单晶Bi-Ga合金应在稳定混相间隙的成分范围内进行分析。这些测量的目的是分析上述相分离转变的成核动力学，并且-对于选定的成分-在相分离后尽可能快地冷却材料，以便在相分离温度的知识下分析早期阶段的微观结构演变。

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.