Inhomogeneous Strains and Dislocation Evolution in Surface-Treated Ni-Base Superalloys

Surface treatment processes such as shot-peening create near-surface residual stresses in the component, which retard the initiation and growth of fatigue cracks. These processes, however, also induce considerable amount of strain hardening and alter the sub-surface dislocation structure of the material, which can impact its mechanical and electrochemical properties. Although these effects are critical for part performance, they have not so far been studied for surface-treated Ni-base superalloys, which are widely used for load-bearing applications. To address this, we have developed an X-ray diffraction (XRD) framework based on Convolutional Multiple Whole Profile (CMWP) fitting approach that can be used to quantify dislocation densities in shot-peened Inconel 718 components. The method exploits physics-based mechanisms for size and strain broadening to model these effects and uses dislocation contrast factors to account for deformation-induced crystallographic anisotropy in the diffraction profile. Our results suggest that shot-peening leads to one order of magnitude increase in the dislocation density of Inconel 718 alloys, which is supported by direct evidence from transmission electron micrographs. The approach developed in this work can be extended to other types of Ni-base superalloys, enabling a simple non-destructive method to calculate dislocation densities in this commercially important class of materials.