Influence of microstructure on fatigue properties of alloy 718

The influence of grain size and grain boundary delta phase on the smooth and notch high cycle fatigue property at room temperature and 650 degrees C was investigated. This paper attempts to correlate the microstructure and fatigue property obtained during the initial development phase and certification of bar stock for compressor blades and other applications. A rotating beam fatigue machine was used to evaluate the fatigue life. All specimens were tested in the solution treated and standard aged condition. The microstructure was examined at one end of the grip portion of the specimens. The observations included grain size ranging from coarse to fine and amount of grain boundary spheroidised delta phase from moderate to nil. Grain size appears to be a significant factor influencing both the smooth and notch fatigue at room temperature and at 650 degrees C i.e finer the grain size better the fatigue life. The delta phase did not seem to have any influence in the fatigue property in the test conditions studied. It was expected that the delta phase which has a significant influence on the notch sensitivity of this alloy at 650 degrees C in stress rupture tests did not have the same influence on the fatigue test. This may be explained as the difference in the deformation mechanism during the stress rupture and high cycle fatigue test. In case of stress rupture test, the failure is predominantly intergranular and hence the presence of the grain boundary delta phase retards crack propagation rate, whereas under fatigue test conditions of the present study, the fractures were transgranular and as such delta phase had no effect on fatigue crack.