Atomic-Scale Investigation of Graphite Nodule Morphology Effects on the Mechanical Behavior of Ductile Iron

The mechanical performance of ductile iron is strongly influenced by the morphology and spatial distribution of graphite nodules. In this study, the tensile behavior of ductile iron was investigated through combined experiments and molecular dynamics (MD) simulations. The experimental results show that specimens with a more uniform graphite distribution exhibit higher strength and ductility than clustered counterparts. To reveal the underlying mechanisms, four ductile iron supercell models with different graphite sizes and spatial distributions were constructed. The simulations show that uniformly distributed graphite promotes a more homogeneous stress field, whereas graphite clustering induces localized stress concentration and accelerates interfacial damage. Fracture occurs through severe strain localization in the ferrite matrix, followed by progressive graphite–matrix debonding and crack propagation along weakened matrix ligaments. Further analysis indicates that the mechanical response is governed by two coupled mechanisms: graphite refinement enhances dislocation activity and strain hardening, while graphite clustering induces higher stress triaxiality at the early stage, leading to a transient strengthening effect but also accelerated damage accumulation. In contrast, refined and uniform graphite distributions reduce stress triaxiality, suppress damage evolution, and enable more sustained plastic deformation, resulting in improved overall performance. These results provide an atomistic understanding of how graphite morphology governs load transfer, plastic deformation, and fracture in ductile iron.