Phase selection and nucleation at a plane phase interface with jumps in the chemical potential

在远离热力学平衡的平面界面上，确定扩散相变的相选择和成核标准。热力学驱动力、动力学系数、界面能和弹性应变能的影响和相互作用将被评估。利用吉布斯自由能曲线对透射电子显微镜（TEM）纳米范围内的浓度和应变进行了综合分析，确定了相边界处的局部驱动力。内部实验设置允许系统地改变温度和压力，但也调整短和长退火时间与明确定义的边界条件。通过选择在Cu-Zn和Al-Ni体系中具有明显溶解度范围的相作为连接伙伴，浓度梯度和过饱和可以以一种确定的方式独立变化。此外，通过原位扩散实验，首次通过实验确定了成核前的最大过饱和度和成核所需的临界浓度梯度。迄今为止，对扩散相变早期阶段的表征大多是使用层状薄膜进行的，层厚远低于扩散长度。在本项目中利用宏观多晶扩散偶，可以直接观察到文献中当前状态以外的远程浓度分布和过饱和度的演变。用透射电镜对核及其界面的浓度分布进行了表征和晶体学分析。将测量并考虑弹性晶格位移。宏观样品尺寸有助于通过对众多成核位置的比较评估对每个影响因素进行统计评估，特别是关于能量有利的界面配置。除了宏观样品的短期扩散实验外，还设想了微观样品的原位实验，采用最先进的基于芯片的TEM加热装置。围绕成核事件的临界时间范围内的过程将以几秒的时间分辨率进行分析。由于实验分离了影响参数，将实现对热力学驱动力、动力学过程、晶体学和弹性晶格位移及其相互作用的更详细和全面的理解，并将其结合在远离热力学平衡的平面界面上的相选择和成核的更一般的模型中，明显超出了目前文献中的最新水平。

The criteria for phase selection and nucleation during diffusive phase transformations at plane interfaces far from thermodynamic equilibrium will be determined. The influence and the interaction of thermodynamic driving forces, kinetic coefficients, interface energies and elastic strain energy will be evaluated. Local driving forces at the phase boundary are determined by a comprehensive analysis of concentrations and strain with resolution in the nanometer range in the transmission electron microscope (TEM) using the Gibbs free energy curves. An in-house experimental set-up allows for varying temperature and pressure systematically, but also to adjust both short and long annealing times with well-defined boundary conditions. By selecting phases as joining partners that have a pronounced solubility range in the systems Cu-Zn and Al-Ni, concentration gradients and supersaturations can be varied independently in a defined manner. Furthermore by applying in-situ diffusion experiments in the TEM both the maximum supersaturation directly before nucleation and the critical concentration gradient necessary for nucleation will be determined experimentally for the first time.The characterization of the early stages of diffusive phase transformations has so far mostly been carried out using layered thin films with layer thicknesses far below the diffusion length. Utilizing macroscopic polycrystalline diffusion couples in the present project permits a direct observation of the evolution of long range concentration profiles and supersaturations beyond the current state in the literature. The characterization of concentration distributions and crystallographic analysis of the nuclei and their interfaces is carried out by TEM. Elastic lattice displacement will be measured and taken into account. The macroscopic sample dimensions facilitate a statistical evaluation of each influence factor by comparative assessment of numerous nucleation sites, particularly with regard to energetically favorable interface configurations. Besides the short-term diffusion experiments on macroscopic samples, in-situ experiments on microscopic samples are envisaged, employing a state-of-the-art chip-based TEM heating device. The processes in the critical time range around a nucleation event will be analyzed with a time resolution of a few seconds.Due to the experimental separation of the influencing parameters, a more detailed and comprehensive understanding of thermodynamic driving forces, kinetic processes, crystallography and elastic lattice displacement and their interaction will be achieved and combined in a more general model of phase selection and nucleation at plane interfaces far from thermodynamic equilibrium, distinctly beyond the current state-of-the-art in the literature.