Environmentally assisted crack growth in a Ni–18Cr–18Fe ternary alloy at elevated temperatures

Fatigue crack growth experiments were conducted on a Ni–18Cr–18Fe ternary alloy under trapezoidal loading, in high-purity argon and oxygen at 873, 923 and 973 K, as a part of a broader study to elucidate the mechanism and rate controlling process for crack growth in nickel-base superalloys. The data were analyzed in terms of a new superposition model which facilitates the decomposition of data into the time-dependent and cycle-dependent contributions, as well as the respective mechanical and environmental components. The results demonstrated the effectiveness of the model, and showed that oxygen only affected cycle-dependent (or fatigue) crack growth, increasing the growth rates by about 2X. The time-dependent (or sustained-load) crack growth was principally associated with grain boundary cavitation and controlled by creep at these temperatures, with an apparent activation energy of 210±10 kJ mol−1. These results suggest that the pressure mechanism proposed by Bricknell and Woodford for oxygen enhanced crack growth in nickel-base superalloys did not operate at these temperatures. The near absence of environmental sensitivity in this Nb-free, Ni–Cr–Fe ternary alloy suggested the internal oxidation of Ni, Cr and Fe was not a significant contributor to the enhancement of crack growth. The results, instead, provided complementary support for the embrittling role of Nb that had been proposed by Gao et al.