Thermomechanical fatigue behavior of the directionally-solidified nickel-base superalloy CM247LC

Due to the extreme operating conditions present in the combustion sections of gas turbines, designers have relied heavily on specialized engineering materials. For blades, which must retain substantial strength and resistance to fatigue, creep, and corrosion at high temperatures, directionally-solidified (DS) nickel-base superalloys have been used extensively. Complex thermomechanical loading histories makes life prediction for such components difficult and subjective. Costly product inspection and refurbishment, as well as capital expense required in turbine forced outage situations, are significant drains on the resources of turbine producers. This places a premium on accurate endurance prediction as the foundation of viable long-term service contracts with customers. In working towards that end, this work characterizes the behavior of the blade material CM247LC DS subjected to a variety of in-phase (IP) and out-of phase (OP) loading cycles in the presence of notch stress concentrations. The material response to multiaxial notch effects, highly anisotropic material behavior, time-dependent deformation, and waveform and temperature cycle characteristics is presented. The active damage mechanisms influencing crack initiation are identified through extensive microscopy as a function of these parameters. This study consisted of an experimental phase as well as a numerical modeling phase. The first involved conducting high temperature thermomechanical fatigue (TMF) tests on both smooth and notched round-bar specimens to compile experimental results. Tests were conducted on longitudinal and transverse material grain orientations. Damage is characterized and conclusions drawn in light of fractography and microscopy. The influences of microstructure morphology and environmental effects on crack initiation are discussed. The modeling phase utilized various finite element (FE) simulations. These included an anisotropic-elastic model to capture the purely elastic notch response, and a continuum-based crystal visco-plastic model developed specifically to compute the material response of a DS Ni-base superalloy based on microstructure and orientation dependencies. These FE simulations were performed to predict and validate experimental results, as well as identify the manifestation of damage mechanisms resulting from thermomechanical fatigue. Finally, life predictions using simple and complex analytical modeling methods are discussed for predicting component life at various stages of the design process.M.S.Committee Chair: Dr. Richard W. Neu; Committee Member: Dr. David L. McDowell; Committee Member: Dr. W. Steven Johnso

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

AbstractIt 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

Low cycle fatigue of a directionally solidified nickel-based superalloy: Testing, characterisation and modelling

Low cycle fatigue (LCF) of a low-carbon (LC) directionally-solidified (DS) nickel-base superalloy, CM247 LC DS, was investigated using both experimental and computational methods. Strain-controlled LCF tests were conducted at 850°C, with a loading direction either parallel or perpendicular to the solidification direction. Trapezoidal loading-waveforms with 2 s and 200 s dwell times imposed at the minimum and the maximum strains were adopted for the testing. A constant strain range of 2% was maintained throughout the fully-reversed loading conditions (strain ratio R = −1). The observed fatigue life was shorter when the loading direction was perpendicular to the solidification one, indicating an anisotropic material response. It was found that the stress amplitude remained almost constant until final fracture, suggesting limited cyclic hardening/softening. Also, stress relaxation was clearly observed during the dwell period. Scanning Electron Microscopy fractographic analyses showed evidence of similar failure modes in all the specimens. To understand deformation at grain level, crystal plasticity finite element modelling was carried out based on grain textures measured with EBSD. The model simulated the full history of cyclic stress-strain responses. It was particularly revealed that the misorientations between columnar grains resulted in heterogeneous deformation and localised stress concentrations, which became more severe when the loading direction was normal to a solidification direction, explaining the shorter fatigue life observed