Evaluation and Comparison of Damage Accumulation Mechanisms During Non-isothermal Creep of Cast Ni-Based Superalloys

The creep behavior under non-isothermal and isothermal creep conditions was studied for a single crystal, a directionally solidified, and a polycrystalline superalloy. Regardless of the alloy type, the non-isothermal creep life of the alloy was much lower than that under isothermal conditions, yet the samples ruptured at higher plastic strains. Irregular interfacial network formation and early (but homogeneously distributed) shearing of the γ′ phase was identified as the main source of damage for all three alloys. Additionally, secondary precipitates with thermal expansion coefficients different from the surrounding matrix led to the formation of creep voids, especially under thermal cycling. In that respect, surface carbides were seen as particularly damaging due to their selective oxidation, which resulted in the extension of secondary cracks. The higher plastic strains exhibited by the non-isothermal creep samples were correlated to the more homogeneously distributed damage accumulation, in comparison with the more localized deformation during isothermal creep. This study provides an overview of the damage accumulation sources in the three types of available superalloys and highlights areas which need to be further considered when designing future alloys for the non-isothermal creep regime.