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

• Published in: Materials science & engineering. A, Structural materials : properties, microstructure and processing Structural materials : properties, microstructure and processing, 1998-11, Vol.256 (1-2), p.197-207
• Main Authors: CHEN, S.-F, WEI, R. P
• Format: Article
• Language: English
• Subjects: Applied sciences; Corrosion; Corrosion mechanisms; Exact sciences and technology; Fractures; Mechanical properties and methods of testing. Rheology. Fracture mechanics. Tribology; Metals. Metallurgy
• ISSN: 0921-5093; 1873-4936
• DOI: 10.1016/s0921-5093(98)00791-6

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 973K, 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, and 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 ~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 plus/minus 10 kJ mol sup -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 italic et al..