Determination of driving forces for nucleation at a plane Ni/NiAl interface with jumps in the chemical potential
Nucleation of new phases and phase selection at an off-equilibrium interface is influenced by a variety of thermodynamic and kinetic entities. The driving force for the formation of a new phase depends on the local concentrations at the interface and is thus a function of time owing to diffusion in the adjacent phases and across the interface. During diffusion, local supersaturations on the length scale of a few tens of nanometers are built up, whereas the overall diffusion lengths are in the range of micrometers. Thus, determination of local concentrations and the resulting driving forces demand methods with high resolution down to 1nm utilizing macroscopic polycrystalline diffusion couples. The current study combines the advanced method and the special samples and permits a direct observation of the evolution of long range concentration profiles and supersaturations beyond the current state in the literature.
The characterization of the early stages of diffusive phase transformations is carried out on Ni/NiAl diffusion couples by aberration-corrected scanning transmission electron microscopy (STEM) combined with energy-dispersive x-ray spectroscopy (EDS) providing concentration measurements with spatial resolutions distinctly below 1nm. Phase selection and nucleation of the competing phases Ni3Al and Ni5Al3 at plane Ni/NiAl interfaces are characterized considering concentration distributions and crystallographic analysis of the nuclei and their interfaces. Local driving forces at the interface are determined by a comprehensive analysis of concentration profiles across the interface using a CALPHAD consistent description of Gibbs free energy curves from the literature. The influence of thermodynamic driving forces on the order of phase selection is evaluated.