A Lattice-misfit-dependent Micromechanical Approach in Ni-based Single Crystal Superalloys

Abstract

In Ni-based single crystal superalloys, the macroscopic properties are intertwined with their complex bi-phased γ (NiAl)/γ’ (Ni3Al) microstructure. This study aims to better understand the coupled effect of lattice misfit and microstructural state on the macro-scale performance by utilizing a faithful representation of the microstructure, via a phase-field model, as a starting point for a micromechanical model embedded in a finite-element crystal plasticity framework. We outlined a micromechanical model with a lattice-misfit-dependent isotropic hardening that takes a unique description depending on the phase: an Eshelby tensor (sphere or penny) for the precipitates (γ’ phase) and a continuous medium for the matrix (γ phase). Finite-element simulations of strain-controlled monotonic tensile and alternate cyclic tests were performed on a set of realistic 3D phase-field microstructures (cuboidal states and directionally coarsened rafted states) defined by their natural (undeformed) or constrained (deformed) lattice misfits and the width of the γ matrix channels. As the magnitude of the natural lattice misfit gets larger for the cuboidal microstructural states, the tensile test simulations predict smaller macroscopic yield stress and strain hardening as well as smaller stress triaxiality at the microstructure level. The model also predicts that having a more pronounced rafted microstructure yields a smaller stress triaxiality at the microscale. The cyclic simulations showed that the averaged stress triaxiality at the microscale during five simulated cycles is the smallest for a natural lattice misfit of -0.3% and a 60% completed rafting process. These insights allow to better understand the experimental studies that investigated the effect of microstructural states on the mechanical response during monotonic and cyclic loading.