Constitutive modeling and life prediction in nickel-base superalloys

Microstructural features at different scales affect the constitutive stress-strain response and the fatigue crack initiation life in Ni-base superalloys. While numerous efforts have been made in the past to experimentally characterize the effects of these features on the stress-strain response and/or the crack initiation life, there is a significant variability in the data with sometimes contradictory conclusions, in addition to the substantial costs involved in experimental testing. Computational techniques can be useful tools to better understand these effects since they are relatively inexpensive and are not restricted by the limitations in processing techniques. This research investigated the microstructure-dependence of the stress-strain response and the fatigue crack initiation life of two Ni-base superalloys; DS GTD111, which has been developed by GE Energy for use in gas turbine blades, and IN100, developed by Pratt and Whitney, which is used in turbine discs. The focus was on developing multiscale constitutive models and computational frameworks for life prediction.; Physically-based constitutive models were formulated and implemented as user material subroutines in ABAQUS using the single crystal plasticity framework which can predict the material stress-strain response with the microstructure-dependence embedded into them. The model parameters were calibrated using experimental cyclic stress-strain histories. A computational exercise was then employed to quantify the influence of idealized microstructural variables on fatigue crack initiation life. Understanding was sought on the influence of inclusion shape, size, and spacing and the primary and neighboring grain orientations on the variability in fatigue crack incubation life in DS GTD111. A computational scheme was also formulated to predict the fatigue crack initiation life in IN100 for slip band based crack initiation and a material design framework was developed and used for the preliminary design of fatigue resistant microstructures. Lastly, it is noted that crystal plasticity models are often too computationally intensive if the objective is to model the macroscopic behavior of a textured or randomly oriented 3-D polycrystal in an engineering component. Homogenized constitutive models were formulated and implemented as user material subroutines in ABAQUS, which can capture the macroscale stress-strain response in both DS GTD111 and IN100. Even though the study was conducted on two specific Ni-base superalloys; DS GTD111 and IN100, the objective was to develop generic frameworks which should also be applicable to other alloy systems.