PSI - Issue 60

Md Rakim et al. / Procedia Structural Integrity 60 (2024) 298–310

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Md Rakim et al. / Structural Integrity Procedia 00 (2023) 000 – 000

Long-time ageing conditions at a temperature of 710 °C for up to 20000 hours happen to form various precipitations like intermetallic gamma prime (γ´) precipitate, M 6 C (Mo-rich) and M 23 C 6 (Cr-rich) carbides, which greatly impacts on the properties of hardness and tensile strength for the alloy after ageing for several hours [A.N. Singh et al. (2018), Y. Guo et al. (2013), X. Liu et al. (2016)]. The above carbides consist of both intergranular as well as intragranular atoms. Prolonged duration of aged decreases J IC in material still observations suggest that the alloy has a higher J IC with aged conditions at 710 °C for 20000 hours. Crack blunting and stretch zone formation that depends on the behaviour of strain hardening contribute significantly to the values of fracture toughness ( J IC ). At intermediate temperature ranges (500-900 °C), the alloy inhibits embrittlement behaviour that shows decreased fracture ductility in comparison with low and high-temperature regions [L. Zheng et al. (2012)]. Ch. V. Rao et al. (2019) suggest that the ductility value of alloy 617M is much lower at temperature ranges of 600 °C and 700 °C that contributing to a great increase in the value of hardness. The increase in hardness and decrement of fracture ductility with ageing time and temperature may be the reason for the sudden decrease in J IC value. For the explanation regarding J IC variations, the effects related to Mo and Cr on the deformation criterion play a crucial role. Hence, the parameters on strain-hardening happen to be influenced strongly by the alloy contents of solid solution. As alloy 617M consists of a higher composition constituting Al and Ti so γ´ eventually becomes principal precipitation. The γ´ precipitation can be observed in alloy 617M when aged at 710 °C for 1000 to 20000 hours. At 2000 hours, precipitation is primarily γ´ -phase that exhibits in limited quantities with smaller particle sizes. Besides γ´ phase precipitation, carbides like Mo-rich (M 6 C) and Cr-rich (Cr 23 C 6 ) can be observed. The particles of Mo-enriched carbides are larger and the precipitation happens in the grain boundaries along with the grains. Whereas, for Cr-enriched carbides, some of the precipitation occurs at the grain boundaries that are comparatively larger than the nano needled type formed on grain which are smaller along with tensed [A.N. Singh et al. (2018), Y. Guo et al. (2013), X. Liu et al. (2016)]. Observation reveals that the particle dimension of γ´ -phase ageing at 710 °C for 20000 hours shows a relatively enlarged dimension compared to the ones aged for 1000 hours. Z. Wang et al. (2020) also reveal that the Ti(C, N) phase has better stability compared to M 23 C 6 at this aged temperature. Moreover, the phases of Ti(C, N) presented are in limited amounts.