High Cycle Fatigue Damage Mechanisms of MAR-M 247 Superalloy at High Temperatures

High cycle fatigue tests of the MAR-M 247 superalloy under constant load were conducted with the aim to determine S-N curves at temperatures 650, 800, 900 and 950 A degrees C. It has been found that the slope of the S-N curves reflects changes of the cyclic deformation mechanism with temperature. At temperatures 650 and partly at 800 A degrees C, fatigue crack propagation exhibits crystallographic character (propagation along the {111} system) in early stages of the fatigue life. The crystallographic mode of the fatigue crack propagation was found to disappear at temperatures 900 and 950 A degrees C.