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处扩散相变过程中的相选择和成核准则将被确定。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进行评估。在相边界的局部驱动力是通过在透射电子显微镜（TEM）中使用吉布斯自由能曲线的纳米范围内的分辨率的浓度和应变的综合分析确定的。内部实验装置允许系统地改变温度和压力，而且还可以通过明确定义的边界条件来调整短退火时间和长退火时间。通过选择在Cu-Zn和Al-Ni系统中具有显著溶解度范围的相作为连接伙伴，可以以限定的方式独立地改变浓度梯度和过饱和度。此外，通过在TEM中应用原位扩散实验，直接在成核之前的最大过饱和度和成核所需的临界浓度梯度将首次通过实验确定。扩散相变的早期阶段的表征到目前为止大多是使用层厚度远低于扩散长度的层状薄膜进行的。在本项目中利用宏观多晶扩散偶，可以直接观察文献中超出当前状态的长程浓度分布和过饱和度的演变。用透射电子显微镜对晶核及其界面的浓度分布和晶体学分析进行了表征。将测量并考虑弹性晶格位移。宏观样品尺寸有利于统计评估的每个影响因素的比较评估的许多成核位点，特别是关于积极有利的界面配置。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。将以几秒的时间分辨率分析一个成核事件周围临界时间范围内的过程，由于实验分离了影响参数，对热力学驱动力、动力学过程、晶体学和弹性晶格位移以及它们的相互作用将被实现并结合在平面上的相选择和成核的更一般的模型中。远离热力学平衡的界面，明显超出了文献中的当前最先进水平。

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.