Elevated temperature fatigue crack growth model for DS-GTD-111

Abstract

The removal of grain boundaries normal to the principal loading direction with the introduction of directionally solidified (DS) grains has significantly improved creep strength, resistance to thermal fatigue, crack growth and oxidation. In this study, a model is developed for representing and predicting the high temperature fatigue crack growth behavior of directionally solidified Ni base alloy, DS GTD-111. A new physically-based model is proposed that accurately represents the influence of temperature on the fatigue crack growth behavior. The test material was cast in the form of slabs that were approximately 254 mm long, 197 mm wide and 32 mm thick. Fatigue crack growth tests were carried out using 50.8 mm wide compact type specimens at 24, 649, 760 and 871°C in LT and TL orientations. The results showed that the dependence of the fatigue crack growth exponent, m, on the test temperature as well as the specimen orientation was weak. Hence, m was considered as constant in the temperature ranges considered in this study for both specimen orientations. The fatigue crack growth coefficient, c, was seen to increase with increasing temperature. A model based on thermal activation of dislocations is developed and shown to represent all the data.