The Mechanism of Grain Boundary Serration and Fan-Type Structure Formation in Ni-Based Superalloys

A model of discontinuous precipitation of gamma ' phase at grain boundaries (GBs) in polycrystalline Ni-based superalloys during continuous cooling from supersolvus temperature was developed. The model calculates the size and gamma ' phase fraction in GB precipitates as a function of cooling rate and GB mobility. The model is based on the classical mechanism of diffusion-controlled precipitation. It considers fast diffusion of gamma '-forming elements along the GB and the motion of the GBs under driving force induced by alloy decomposition. The model predicts either discrete GB gamma-prime particles and GB serration or the growth of fan-type structures depending on the cooling rate and GB mobility. We conclude from the model predictions that the GB mobility is the key factor controlling the type and morphology of GB precipitates. High GB mobility results in formation of fan-type structures, while the lower GB mobility leads to GB serration. The predicted dependence of the size of GB precipitates on the cooling rate is in good agreement with the experimental observations.