PSI - Issue 60

Md Rakim et al. / Procedia Structural Integrity 60 (2024) 136–148 Md Rakim et al. / Structural Integrity Procedia 00 (2023) 000 – 000 Generally, increase in hardness decreases the threshold stress intensity factor ( ). The alloy 617M undergoes thermal ageing along with ′ -phase (Ni 3 (Al,Ti)) precipitation, which has a significant impact on hardness as well as tensile parameters after various ageing times. The possible reason behind the increase in the hardness value is in-situ M 23 C 6 carbides or intermetallic ′ -phase (Ni 3 (Al,Ti)) precipitation at elevated temperature regions. At intermediate temperatures (500-900 °C), the alloy shows embrittlement behaviour resulting in lower fracture ductility compared to lower and elevated temperature ranges [L. Zheng et al. (2012)]. Ch.V. Rao et al. (2019) revealed that alloy 617M exhibits a much lower ductility value at 600 °C and 700 °C that shows a remarkable increase in hardness value. An increase in hardness with ageing time may be the reason for sudden decrease in value. However, for 20000 hours of ageing time, the decrement of value has been less because the hardness value decreased for further increase in ageing time. On the other hand, dynamic strain ageing (DSA) accelerates the FCG rate and decreases the values. At high temperatures (580-800 °C), DSA presence in the material enhances material strength but the elongation decreases due to the solute atom’s interaction with the moving dislocations that happen at th e time of plastic deformation [V. Shankar et al. (2017)]. Moreover, at that temperature, strong DSA is present which causes considerable material hardening. Consequently, at test temperature of 650- 750 ℃ a further decrease in value has been observed. The prolonged ageing (up to 20000 hours) at elevated temperature (710 °C), metallic alloys induce microstructural changes and degrade mechanical properties. Reports suggest ageing duration and temperature effect plays a significant role in the precipitation formation namely M 23 C 6 , M 6 C carbides and γʹ -phase. Carbide precipitation happens at grain boundary as well as on grains whereas γʹ -phase precipitation is located in the intra granular areas during the ageing procedure [A.N. Singh et al. (2018), Y. Guo et al. (2013)]. At the grain boundaries, there happens a discontinuous dispersion of the carbide particles after being aged for 1000 hours but after 5000 hours aged duration, along the grain boundaries the carbide particles start to pile up and distribute themselves continuously and there is a significant increase in M 6 C particles. At 10000 hours or more aged duration, there is a considerable unchanged effect that happens within the grains of carbi des and γʹ -phase particles [A.N. Singh et al. (2018), Y. Guo et al. (2013)]. For as-received, fracture surface nature is ductile due to the formation of large dimples. Ageing duration of 1000 hours or more, a significant change happens in fracture criterion from ductile to brittle due to carbide formation within grain boundaries. However, grain boundary weakening happens due to carbides at that location and because of the decohesion effect at the interface of the carbide and matrix, separation occurs, which becomes the initiation site for fracture. Therefore, cracks at grain boundaries tend to form during the fatigue test. These resulting intergranular crack decreases the value of for the aged alloys. 143 8