Abstract
The forming and evolving mechanism of γ′ precipitates during continuous cooling after super-solvus heat-treatment (SSHT) for a nickel-based powder metallurgy (PM) superalloy FGH97 was systematically investigated. A series of cooling rates ranging from 0.1 °C/s to 500 °C/s were carried out using the DIL 805A dynamic dilatation machine. The results show that the size of the secondary γ′ precipitates was linearly related to the logarithm of the cooling rate, and the serrated grain boundaries became straight with the increase of cooling rate. A bimodal size distribution of γ′ precipitates, including secondary γ′ precipitates and tertiary γ′ precipitates, was observed at a relatively lower cooling rate, in contrast to a monomodal size distribution of γ′ precipitates at higher cooling rates. The morphology of the secondary γ′ precipitates changed from spherical to cuboidal, butterfly-like and octet shapes as the cooling rate decreased, at the same time the γ/γ' lattice mismatch increased, and the microhardness decreased. The Al elements were found to be enriched in the secondary γ′ precipitates according to the TEM results, while Cr and Co elements were found to have a higher concentration in the γ matrix.
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