Environment-dependent fatigue crack growth in alloy 718.

摘要

At frequencies below that of the transitional frequency, the elevated temperature fatigue crack growth process in Alloy 718 is viewed to be fully environment-dependent. Of all the crack growth stages, this stage, while is the most critical in high temperature applications due to its highly accelerated crack growth rate, is the least studied or understood. The objective of this study is to focus on the understanding of the controlling mechanisms of this stage in Alloy 718 in order to develop the ability to predict the crack growth performance under different loading and environment conditions. For this purpose, two major studies have been carried out. The first was to provide evidence of the existence of the fully environment-dependent stage in which the crack growth rate is viewed to be equal to the oxygen penetration rate at the crack tip. As a result, the relationship between the intergranular oxygen diffusion rate and the value of the stress intensity factor has been established. This relationship, when integrated over the cycle effective oxidation time, results in a closed form solution describing the environment-dependent fatigue crack growth rate. A good agreement was obtained by comparing the results of these solutions and the experimental data.The second study was to establish a micromechanical based quantitative model to predict the environmentally-dependent crack growth performance. This model was constructed on the concept of the intergranular two-stage crack-tip oxidation mechanism. Thus, the reduction of the grain boundary ductility along the grain boundary fracture path depends on the rate of formation of the chromia layer in relation to the build-up of other oxide types at the crack trip. The determination of the amount of Cr