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

极端外部条件下液-液相分离的成核动力学

基本信息

批准号：
224751288
负责人：
Professor Dr.-Ing. Gerhard Wilde
金额：
--
依托单位：
Institut für Materialphysik
依托单位国家：
德国
项目类别：
Research Grants
财政年份：
2012
资助国家：
德国
起止时间：
2011-12-31 至 2016-12-31
项目状态：
已结题

来源：
https://gepris.dfg.de/gepris/projekt/224751288?language=en
关键词：
Nucleation kinetics liquid phase separation

项目摘要

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.

偏晶合金的液-液相分离呈现一级相变，除了在临界会凝点附近的狭窄组成范围内，其中在低或中等冷却速率的受控实验中也可以直接获得化学亚稳线。因此，从均质熔体的两个化学上不同的液相的双节点形成的动力学涉及过冷、成核和生长阶段，类似于结晶转变。然而，唯一有效的方法，以防止在系统中形成粗尺度的结构，在液体状态下的结晶间隙是通过应用快速凝固处理技术。这种行为也可以通过考虑临界点润湿来解释，临界点润湿通常发生在可渗透性间隙的临界点附近。临界点润湿的一个后果是在可渗透性间隙一侧的有限组成范围内不存在相分离转变的过冷。此外，在有限组成范围内延伸的相平衡的微妙平衡，在该范围内界面过量自由能密度控制成核条件，并且微观结构演变也指示利用外部场的机会，例如在Gao教授的小组中进行的磁场，用于改变平衡，从而调节相变和微结构发展。为了定量地解决成核动力学，我们建议使用的方法是基于描述成核作为一个非均匀的泊松过程。用这种方法和相应的实验计划，在我们以前的工作中开发的纯金属和合金，形核速率和形核势垒高度可以定量地获得在很大范围内的过冷值。这些测量应分别在Fe-Cu和Co-Cu合金的亚稳态可熔性间隙的组成范围内进行，并应与D.赫拉赫这些测量的目标是双重的：在不施加外部磁场的情况下，相分离转变的成核动力学的确定将被用作与Gao教授的小组一起进行的实验的参考，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发生从理想润湿条件到正常润湿条件的转变的组成区域，因为这种转变应导致潜在相变机制的急剧变化，到目前为止，还没有直接分析过。在凝固间隙区域内，一元系合金的凝固通常是在成分偏析之后，液相导致固态中的粗尺度相分布。另外，两种液相中的一种在显著较低的温度下结晶，从而阻碍了相分离的微观结构的微观结构分析。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率可以超过每秒数千度，允许对相变进行多次扫描，以获得统计上显著的数据集，同时促进微观结构形成早期阶段的有效冻结。对于这些测量，应在稳定可混性间隙的组成范围内分析二元单金属Bi-Ga合金。这些测量的目的是分析如上所述的相分离转变的成核动力学，并且对于选定的组成，在相分离之后尽可能快地冷却材料，以在了解相分离温度的情况下分析早期阶段的微结构演变。