Effect of Microsegregation on High‐Temperature Microstructure Evolution in Rapid Solidification Processed Nb‐Rich Ni Superalloys
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abstract
Rapid solidification processes are used for coating, repairing, and manufacturing of metal alloy components in numerous industries. However, many alloys experience microsegregation of elements at subgrain boundaries at the termination of solidification, which have potential consequences for phase transformations at elevated temperatures if used without homogenization heat treatments. Herein, the effect of this microsegregation and solidification microstructure on the coarsening kinetics of the δ phase in a Nb‐rich Ni superalloy is evaluated. Microsegregation in rapid solidification processed (RSP) alloy 718 results in a greater availability of nucleation sites for the precipitation of the δ phase, resulting in smaller precipitate sizes. Coarsening of the δ phase is found to follow a Lifshitz–Slyozov–Wagner model, which is used to determine that Nb diffusion is the limiting factor in δ phase coarsening. With the formation of the δ phase, and the concurrent coarsening and dissolution of the γ″ strengthening phase, a decrease in both tensile strength and microhardness is observed. An improved understanding of the influence of microsegregation on phase transformations allows for a more informed application of non‐homogenized, RSP alloys in industry.
