Microstructure evolution and stress-rupture properties of Nimonic 80A after various heat treatments

Four heat treatments have been designed to produce multi-modal size distributions of γ′ phase in γ matrix, and stress-rupture properties of Nimonic 80A have been investigated. The γ′ phase exhibits two typical coherent orientation relationships with γ matrix. The fine γ′ particles are in spherical shape when the average size is about 20 nm. However, the coarse γ′ particles present a cuboidal shape with round corners when they are larger than 75 nm. The Al and Ti elements diffuse from γ matrix to γ′ phase, and about 80% Al and Ti are used to form γ′ phase after various heat treatments. The increased volume fraction of the fine γ′ particles with average size of 15.8–20.3 nm increases the stress-rupture life of Nimonic 80A at 750 °C/310 MPa. The rod-like Cr23C6 carbide at grain boundary can suppress grain boundary sliding at high temperature and benefit the stress-rupture life. The coarse γ′ particles facilitate the movement of dislocations and contribute to the ductility. The precipitate of multi-modal size distributions of γ′ phase is beneficial to the stress-rupture ductility.

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► Stress-rupture life increases with the increased vol.% of the fine γ′.
► About 80% Al and Ti are used to form the strengthening γ′ after heat treatment.
► Rod-like Cr23C6 at GBs can suppress GB sliding and benefit the stress-rupture life.
► Coarse γ′ facilitates the dislocation movement and contributes to the ductility.
► Multi-modal size distribution of γ′ benefits the stress-rupture ductility.