Role of Precipitates Statistics on Intergranular Crack Growth in Ni-Based Superalloys

This paper examines the role of microstructure, in particular, the size and volume fraction of secondary and tertiary γ’ precipitates, on intergranular crack growth in P/M IN100. Heat treatments have been carried out on the material in order to control the γ’ statistics. Dwell fatigue crack growth experiments were performed on different microstructures at 650°C, resulting in variations of crack growth rate (CGR) by one order of magnitude. CGR is shown to decrease with an increase in yield strength. This is discussed in terms of the hardening behavior of the continuum surrounding the grain boundary (GB) and its effect on the opening displacement rate during the intergraular cracking process. This trend was further investigated by considering the individual contributions of solid solution, γ’_s and γ’_t to the total yield strength. For this, the hardening contribution of the γ’ to the yield considers dislocation/precipitate interactions involving particle shearing and/or Orowan by-passing mechanisms. Results of the CG tests indicated that both the γ’_s and γ’_t yield components are inversely proportional to CGR. Additionally, there was a higher dependency of the γ’_t yield component in relation to CGR and yield, indicating that γ’_t would indirectly play a role in intergranular cracking. Thus, conditions where the γ’_t are unstable, such as dissolution, could lead to an increase in CGR. This has been studied in ME3 alloy at 760°C, where the γ’_t particles near the GB crack path have shown evidence of dissolution resulting in a transient microstructure and variations in CGR.