Influence of minimum temperature on the thermomechanical fatigue of a directionally-solidified Ni-base superalloy

It is well understood that thermomechanical fatigue (TMF) lives are significantly influenced by the maximum temperature of the cycle since increasing temperature accelerates both creep and the coupled fatigue-oxidation effects, usually exponentially with increasing temperature. Hence, most TMF experiments focus on the impact of the maximum temperature of the cycle along with the phasing of the temperature and strain. Very little focus has been placed on the role of the minimum temperature of the TMF cycle. Usually the minimum temperature is chosen for experimental expediency and is not based on minimum temperature experienced in actual components. For example, in a gas turbine, the minimum temperature for an extended shutdown is near room temperature. This paper shows that out-of-phase TMF with lower minimum temperature while maintaining the same mechanical strain results in lower life. Possible explanations for the reduction in life include the increase in inelastic strain range due to the increase in elastic modulus at lower temperatures and microstructural changes that occur at elevated temperature, reducing the lower temperature yield strength. Both experiments and simulations using crystal viscoplasticity modeling show that the increase in elastic modulus with decreasing temperature leads to greater inelastic strain range and a commensurate reduction in fatigue life. This effect is just as important to consider as the influence of microstructure changes occurring at the elevated temperatures of the cycle.