Effect of post-solutionizing cooling rate on microstructure and low cycle fatigue behavior of a cast nickel based superalloy

The influence of post-solutionizing cooling rate on low cycle fatigue (LCF) and cyclic stress-strain behavior at 650 °C has been investigated for a double solution-treated and aged nickel-based superalloy (Ni-8.09Cr-9.35Co-9.41W-0.52Mo-3.24Ta-0.82Ti-5.51Al-1.53Hf-0.075C). The LCF behavior of this alloy has been characterized by dual-slope Coffin-Manson relationship. The bilinear Coffin-Manson plot for the samples subjected to the slowest post-solutionizing cooling rate has shown relatively lower transition plastic strain for the slope change, and this observation is attributed to their higher cyclic work hardening exponent and more uniform plastic deformation. The increase in amount of cyclic hardening with increasing total strain range and decreasing cooling rate is considered to be due to the increase in dislocation density. The presence of higher volume fraction of fine spherical secondary γ′ between the cuboidal primary γ′ precipitates, absence of γ′-depleted region near the grain boundaries, and uniform distribution of discrete secondary carbides (such as Cr23C6 and HfC) along the grain boundaries contribute to higher fatigue life and stable cyclic stress-strain behavior for the samples subjected to the slowest cooling rate.