A full-field crystal plasticity model including the effects of precipitates: Application to monotonic, load reversal, and low-cycle fatigue behavior of Inconel 718

Abstract

This work advances a recently developed high-performance, full-field elasto-viscoplastic fast Fourier transform (MPI-ACC-EVPCUFFT) formulation to model large plastic deformation and low-cycle fatigue behavior of Inconel 718 (IN718). Specifically, the recently developed model of Eghtesad et al., 2020 [1] incorporating a strain rate, temperature, and strain-path sensitive hardening law based on the evolution of dislocation density and a slip system-level back-stress law influencing the resolved shear stress is advanced in several aspects. First, strengthening effects due to grain size and shape, solid solution, shearing of small precipitates, and Orowan looping around larger precipitates are incorporated into the initial slip resistance, which evolves with the hardening law including the latent hardening. Second, the resolved shear stress on the slip plane in the direction of slip is altered by accounting for the two orthogonal shear stress components and the three normal stress components, in addition to the slip system-level kinematic effects. The model is used to interpret the complex mechanical behavior and microstructural data for samples of alloy IN718 in additively manufactured (AM) forms before and after hot isostatic pressing (HIP) and in a wrought form. Voxel-based microstructural cells consistent with the characterization data are synthetically constructed to initialize the model setups for predicting simple compression, tension, load reversal, and low cycle fatigue behavior of the alloy. Variation in the microstructural features such as the distribution of grain size and shape, crystallographic texture, content of annealing twins, and precipitates among the samples facilitated reliable calibration and validation of the model parameters. Predicted anisotropy, tension/compression asymmetry, non-linear unloading upon the load reversal, the Bauschinger effect, reverse hardening, texture evolution as well as cyclic hardening/softening along with the mean stress relaxation during low-cycle fatigue are in good agreement with the corresponding experimental data for the alloy. Furthermore, the simulations based on the high-resolution microstructural cells facilitated the discussion of the mechanical fields induced by microstructure, and especially annealing twins.