Environmentally assisted crack growth in a nickel(18)-chromium(18)-iron ternary alloy at elevated temperatures

The primary objective of this study is to assess the viability of the pressure mechanism proposed by Bricknell and Woodford (1981) to account for the environmental enhancement of crack growth in nickel-base superalloys at elevated temperatures. This study also served to provide complementary evidence in support of a postulated deleterious role of niobium in the enhancement of crack growth by oxygen in these alloys (Gao et al., 1994). A low carbon, Ni-18Cr-18Fe ternary alloy is used. This simple, solution strengthened alloy, with major alloying elements identical to those of Inconel 718, contains sufficient carbon (0.02 percent by weight) for assessing the role of grain boundary carbides as a source of internal CO/COThe assessments were carried out by using fatigue crack growth experiments under trapezoidal loading to circumvent problems of large scale plastic deformation and crack branching that prevailed in sustained load tests. The experiments were performed as a function of stress intensity factor range (i.e., A new superposition model was developed to decompose from the fatigue crack growth rates the separate contributions of time-dependent and cycle-dependent processes, as well as the effects of environment. The model was found to work well over the ranges of experimental conditions used, and facilitated data interpretation. It is recognized, however, that there would be an upper limit in hold-time because of the onset of large scale plastic deformation.The experimental results showed that oxygen had a small effect on the cycle-dependent component of fatigue crack growth (increasing the growth rate by about a factor of two), but its effects were negligible on the time-dependent (sustained-load) component of growth rates. Fractographic examinations confirmed that grain boundary cavitation was the basic micromechanism for crack growth. Taken together, these results indicate that CO/CO...