Effects of microstructure variability on intrinsic fatigue resistance of nickel-base superalloys - a computational micromechanics approach

Understanding the effects of microstructure variability on fatigue resistance is a key to selection and design of materials for fatigue applications. The traditional empirical approach rooted in experiments is being increasingly combined with systematic computational modeling. This work is concerned with demonstrating the feasibility of linking effects of microstructure variability on cyclic plasticity at the scale of intrinsic microstructure of a single crystal nickel-base superalloy. The precipitate and the matrix phases of the alloy are modeled explicitly using a physically based crystal viscoplasticity constitutive framework with appropriate scale and spacing effects to reflect dislocation- precipitate interactions. The model is implemented as a user material subroutine within a finite element code. Various realizations of different microstructures are generated using a constrained Poisson point process. Statistical volume elements (SVEs) with random-periodic boundary conditions are simulated under fully reversed cyclic loading at 650 ◦C. Primary cooling γ precipitate size and volume fraction are considered in terms of their effects on the macroscopic stress-strain response and on distributed cyclic microplasticity within the SVE. To compare various microstructures in terms of probability of fatigue crack formation, an appropriate nonlocal measure of cyclic plastic shear strain range is proposed based on percolation of cyclic microplasticity at the scale of the SVE.