PSI - Issue 57

Rando Tungga Dewa et al. / Procedia Structural Integrity 57 (2024) 762–771 Dewa et al./ Structural Integrity Procedia 00 (2019) 000 – 000

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largest source of nickel ore. Superallloys are critical resources that has evolved into a key material in the modern defense industry, particularly utilised in gas turbine for military aircraft, marine propulsion, military electric motor, submarines, nuclear reactor, and even drone turbojet. It is expected the demand for superalloys will continue to rise in the future and must confirm the various possibilities of applications in the military industry. Superalloy/INCONEL 617 is a solid solution strengthened alloy by chromium, cobalt and molybdenum. The Alloy 617 provides good properties for components of power generating plants, gas turbine, and high temperature superior required applications. The alloy is now being widely used for high-temperature applications by Dewa et al. (2016, 2018), Ekaputra et al. (2016), and Rao et al. (1988). Resistance to low-cycle fatigue (LCF) and creep-fatigue (CF) interaction is an important requirement for the successful design in the high temperature environments due to the complex interactions between metals, high temperature, and air. LCF loadings are expected to result from thermally induced strain cycles and fluctuations during operations. The time-dependent mechanisms due to environmental factors may influence fatigue life either synergistically or independently. Hence, the essential prerequisite for accurate prediction of LCF life for extreme temperature condition can be characterised by rate controlling damage process that influences cyclic deformation under the appropriate combination of experimental stress/strain, temperature, strain rate, environment, and prior metallurgical condition of the material. The effect of these time-dependent processes may reduce an alloy's cyclic life resistance by orders of magnitude as compared to the room temperature (RT) behavior. Therefore, in this study, the phenomenon is being initially tested at RT in the laboratorium scale to study the damage mechanism, which is closely depended with strain rate and their structures. To the best of author’s knowledge, literatures on the slow strain rate deformation of alloy 617 is still rare, especially at initial RT set up. Up to now, recent studies reported the behavior of slow strain rate test (SSRT) on the type of stainless steel and structural steel are essentially the same as under creep by Luo et al. (2013), Kim et al. (1988), and Calmunger et al. (2013). Numerous studies have been conducted to modify and improve the relationship to cover materials model through conventional or adapted methods by Chen et al. (2014), Nakai et al. (2012), Chou et al. 2016, Ekaputra et al. (2020). However, SSRT is found to be promising as a low cost methodology for evaluation of materials capability to response with such environmental interference. Variance of strain rates in order of magnitudes can be employed in LCF test at RT. The difference in fractography appearance and proper method of life prediction under the circumstances are some of the most valuable information for continuing rapid assessment of the elevated temperature test behavior that can be performed for Alloy 617. The objective of the present work is to determine the influence of individual and interactive time-dependent processes by performing the SSRT under LCF loadings for Alloy 617 at RT. Furthermore, this work focuses on the fatigue life interpolation methods and their comparison in the predictions of fatigue life is performed in order to realize whether the power-law relationship is a suitable expression. The influence of change in strain rates on the damage mechanisms is investigated; thorough understanding of the LCF mechanism and the conclusion related to the time-dependent damage will be established. 2. Methods and Procedures The composition (wt%) of the commercial grade Alloys used for material chosen in this study are 53.11Ni, 22.2Cr, 12.3Co, 9.5Mo, 1.06Al, 0.08C, 0.949Fe, 0.4Ti, 0.084Si, 0.029Mn, 0.027Cu, 0.003P, <0.002S, and <0.002B. The as-received microstructure of the alloy 617 is shown in Fig.1. Alloy 617 has a fully austenitic face centered cubic (FCC) structure which maintains superior mechanical properties at high temperature by Dewa et al. (2018). Fig. 2 shows a monotonic stress-strain behaviour of Alloy 617. The FCC matrix, mainly consists of nickel, cobalt, iron, chromium, and molybdenum. However, the microstructure appearance in Alloy 617 is well-uniformed equiaxed grains. Small grain size ranging from 10 to 30 μm and large grain size is approximately 40 – 100 μm in diameter. The polished cylindrical specimens with 6.0 mm of diameter in the reduced section and a gauge length of 12.5 mm were used for the LCF test specimens. The specimens design was set according to the standards; this is to prevent the premature buckling or deformation under the highest tension stress anticipated during the LCF test. Fully-reversed LCF tests were performed in ambient air at RT with different strain rates, i.e. 5E-4, 1E-3, 6E-3, and 1E-2 s -1 , respectively under 1.2% total strain range. The high precision extensometer was attached to the specimen to record and collect the real time data of the stress-strain response. Triangular waveform was used for all LCF tests.