Executive Summary : | The study aims to investigate the impact of grain size variation on diffusion and kinetically driven transformations in alloys, particularly in medium entropy alloys (MEAs). The research will involve preparing and annealing diffusion couples of Al or CoCrNi and NiAl or CoCrNi at various temperatures for different times. The study will use thermo-mechanical processing routes to produce various grain sizes of CoCrNi alloy, and will use EPMA and correlative microscopy approaches to capture phase formation at the nanoscale. A CALPHAD-based optimization procedure will be developed to predict effective interdiffusion coefficients of diffusing species, particularly Al, as a function of temperature, composition, and average grain size. The study will also use a numerical optimization procedure to deconvolute effective diffusivities of species into lattice and GB contributions, obtaining the effective GB diffusivity of Al as a function of temperature and grain size for a given composition. The CoCrNi MEA is chosen for its exceptional mechanical properties at low and high temperatures, thermal stability, and high deformability. The success of this work will lead to the development of a robust framework for analyzing and optimizing IDZ design in superalloys and nuclear power plant welds and joints. |