Temperature Dependent Deformation Mechanisms of Alloy 718 in Low Cycle Fatigue

Recent developments in the area of water cooled gas turbine design have created a need for low cycle fatigue test data for alloy 718 in the temperature range of 204 to 649 °C. To support this need, data were generated in the room temperature to 649 °C range. As noted by previous investigators, there was a crossover in fatigue lives at low strain depending on temperature. At high strain ranges the lowest fatigue life was exhibited at the higher temperatures. However, in the low strain, long life regime this trend reversed with the fatigue life at a given strain range exhibiting a peak at some intermediate temperature. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) studies were conducted on the fatigue specimens to determine the nature of the cyclic deformation process as a function of strain range and temperature, the principal mode of deformation was by mechanical twinning. However, at the two highest temperatures, the primary process for deformation was slip. The principal difference between the strain-life behavior of the specimens cycled at 538 and 649 °C, and those cycled at the three lower temperatures (204, 316, 427 °C) is interpreted in light of this change in deformation process with temperature.