Influence of Extremum Temperatures on TMF of a Ni-base Superalloy

Significantly reducing the minimum temperature while maintaining maximum temperature of thermomechanical fatigue (TMF) cycles can reduce the life even when mechanical strain ranges are similar. This applies to in-phase (IP) and out-of-phase (OP) TMF cycles. This reduction in life has generally been attributed to a combination of changes in microstructure arising from aging and increases in the cyclic inelastic strain promoted by increases in the elastic modulus as the minimum cycle temperature is reduced. TMF cycles under both IP and OP conditions were conducted with maximum cycle temperatures within the 750-950 degrees C range and with minimum cycle temperatures of either 100 or 500 degrees C. A reduction in minimum temperature was observed to promote a decrease in TMF life by as much as a factor of ten for all TMF experiments. The reduction in TMF life is primarily controlled by increases in the inelastic strain range associated with increases in the elastic modulus that arise when the minimum temperature is reduced.