Effect of environment on fatigue crack propagation behavior of alloy 718 at elevated temperatures
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Effect of Environment on Fatigue Crack Propagation Behavior of Alloy 718 at Elevated Temperatures H.H. SMITH and D. J. MICHEL The influence of temperature, loading mode, and environment on the deformation mechanisms and failure processes in high temperature materials depends on the interaction of these factors in a complex manner. With the increased emphasis on reliability and efficiency of high performance, high temperature materials, a knowledge of these complex interactions, and their impact on materials performance must be developed. One example of a material which is sensitive to these interactions is alloy 718. Previous studies have shown that the fatigue crack propagation performance of alloy 718 was influenced by the interaction of cyclic and static loading, temperature, and alloy microstructure.14 The results show that an appreciable increase in the crack propagation rate in air was observed as static loading periods were included in the fatigue cycle.l'2 The results from vacuum tests demonstrated that the effect of air was due primarily to oxygen3'4 and that the time dependence of the high temperature results in air and in vacuum were composed of both thermal and/or environmental components.3 Other studies have suggested that the mechanism by which the environment influenced the crack propagation rate was to alter the crack propagation mode through interaction with the slip processes. 5 The intent of the present study was to examine further the influence of environment on the fatigue crack propagation behavior of alloy 718 at elevated temperatures. In particular, the effect of hold time and temperature on the crack propagation rate and mode were investigated to develop further insights concerning the mechanisms through which the environment influences the crack propagation process. The alloy 718 used in this study was in the form of wrought alloy plate. The details of the chemical composition
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