Coarsening of γ′-precipitates in single-crystal superalloy SRR 99

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Abstract
The effects of isothermal annealing between 800 and 1200°C on the microstructure of single–crystal SRR 99 have been investigated. During aging at either 800 or 900°C, the precipitates adopt an irregular, rounded, and highly interconnected array which suggests that they have undergone coalescence. Aging at higher temperatures was found to produce a regular cuboidal precipitate morphology, although long–term aging promoted the formation of γ′-‘rafts’. The precipitate size distributions were found to be broader than those predicted by the Lifshitz-Slyosov and Wagner theory, and the coarsening kinetics also showed significant deviations from the power law prediction of this theory. The reasons for these deviations are discussed in the light of the available models, and it is suggested that a progressive transition in the dominant coarsening mechanism takes place. MST/371