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. (C) 1998 Elsevier Science S.A. All rights reserved.