The microstructural stability of Alloy 617 mod. during aging for up to 3000 h at temperatures of 700°C and 750°C was investigated. The precipitates of the aged alloy included M23C6 carbides located both inside grains (intragranular) and at grain boundaries (intergranular) and γ′ phase dispersed within grains. During aging, the intergranular precipitates showed a good stability. Intragranular particles increased substantially after the aging for 3000h at 750°C. Inter and intragranular carbide particles resulted in the precipitation hardening of the aged alloys. The precipitation of γ′ phase particles during aging at 700°C and 750°C is also an important factor for an enhanced hardness and an obvious decrease of the impact absorbed energy. Additionally, the intergranular cracks apparently lead to a decrease in the impact absorbed energy for the aged alloys due to carbide particles at grain boundaries. The impact absorbed energies of the aged alloys were fairly stable within the dwelling time from 300 h to 3000 h and were in the range of 63∼65J and 75∼83J for the 700°C and 750°C aging, respectively.

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