Effect of free surface, oxide and coating layers on rafting in $\gamma-\gamma'$ superalloys

TL;DR

This study investigates how free surfaces, oxide layers, and coatings influence microstructure evolution, specifically rafting and gamma prime depletion, in Ni-based superalloys using experimental analysis and a phase field model.

Contribution

It introduces a coupled mechanical-diffusion phase field model implemented in finite element code to analyze microstructure evolution in superalloys.

Findings

Rafting and gamma prime depletion are strongly coupled with mechanical behavior.

Local Al flux significantly influences microstructure evolution.

Surface and coating layers affect microstructure degradation.

Abstract

Complex microstructure evolution has been observed \rev{both bare and coated } Ni-based single crystal superalloys. Rafting and $\gamma'$ depletion are investigated in this study through a brief experimental analysis and a detailed phase field model to account for mechanical-diffusion coupling. The proposed model has been implemented in a finite element code. As a main result, it is shown that rafting, $\gamma'$ depletion close to free surface/oxide layer or $\gamma'$ coalescence close to coating layer, and mechanical behavior are strongly coupled. The local additional flux of Al explains this coupling to a large extent. Finally, a discussion of strain localization and local flux of Al paves the way for clarification of these cases that degrade the performance of superalloys.