Why is Graphite Spherical in Ductile Iron? A Study of Elemental Distributions at Interfaces in Ductile Iron Using Atom Probe Tomography and Transmission Electron Microscopy

Atom probe tomography (APT) with high sensitivity and spatial resolution was applied to study the elemental distributions in spheroidal graphite in ductile iron. A specimen of quenched ductile iron with some graphite nodules in direct contact with liquid phase was used in this study to understand the role of alloying elements in spheroidal graphite formation. Compositional analysis was performed at the graphite/matrix interface and graphite/nuclei interface using APT and EDX (energy-dispersive X-ray). Distributions of various alloying elements at interfaces were obtained. Compositional gradients were observed at both the graphite/liquid interface and the graphite/nuclei interface. The spheroidal graphite nuclei were differentiated as the complex compound made of oxide, nitride and sulfide. The graphite–nuclei interface was shown to be semi-coherent/incoherent using the transmission electron microscopy, with a high density of crystallographic defects and high curvatures in the graphite basal planes growing off the nuclei. Intercalation of large nodulizing elements within and in between the graphite basal planes may account for the spherical morphology of the graphite in ductile irons.