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

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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.