Low cycle fatigue, deformation and fracture behaviour of Inconel 617 alloy

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Inconel 617 alloy is one of the potential materials for high temperature applications such as steam generator super heater, re-heater parts and intermediate heat exchangers in gas cooled reactors. In the present study, low cycle fatigue tests were performed on solution annealed Inconel 617 alloy at room temperature and 750 °C in air, under fully reversed axial strain controlled mode at constant strain rate of 5x10−3s−1 to determine the influence of temperature on low cycle fatigue (LCF), deformation and fracture behaviour. Symmetrical triangular wave form and total strain amplitude ranging from ±0.20% to ±0.50% were used at the two temperatures to study fatigue behaviour. The cyclic stress responses and strain life relations were analysed at these temperatures. While there was continuous cyclic hardening at room temperature at lower strain amplitudes (±0.20% and ±0.25%), cyclic softening was observed after initial cyclic hardening for 100 cycles at higher strain amplitudes (±0.42% and ±0.50%) with a transition at ±0.375% strain amplitude. Continuous cyclic hardening was observed up to peak hardening at 750 °C irrespective of the strain amplitude. The number of cycles to fracture (fatigue life) decreased with increase in temperature. Number of cycles to peak hardening increased with decrease in strain amplitude at both the temperatures. The degree of cyclic hardening increased with increase in strain amplitude. Non-Masing behaviour is observed at both the temperatures. TEM studies revealed change in dislocation substructure and precipitation of carbides with increase in temperature and strain amplitude. SEM examination of the failed samples revealed increase in inter-striation spacing and also extensive branching of cracks at higher strain amplitudes with increase in temperature.