Effects of electron beam manufacturing induced defects on fracture in Inconel 718

Abstract

The effects of electron beam manufactured (EBM) process-induced defects on local microstructural failure initiation and propagation in IN 718 have been investigated. Predictions for transgranular fracture, based on local cleavage plane stresses, and for intergranular fracture, based on dislocation-grain boundary (GB) interactions and evolving dislocation pileups, were combined with a crystalline dislocation-density plasticity approach to understand the influence of AM process-induced defects, such as porosity, NbC precipitates, and regions of dry powder. High local stresses along the peripheries of pores caused crack nucleation, and mismatches in deformation behavior between NbC precipitates and the surrounding matrix led to local stress gradients that induced crack nucleation and decohesion at precipitate/matrix interfaces. Regions of unmelted powder had significant stress accumulations that initiated failure at low nominal strains. Failure due to high localized stresses near regions of unmelted powder was dominant over precipitate/matrix decohesion and crack nucleation near pore peripheries. Based on the predictions, the mechanical behavior of AM alloys is governed by local dislocation-density evolution near process-induced defects, which preferentially nucleate material failure. Furthermore, interactions between these different defect types can significantly accelerate failure initiation and propagation.