Why is Graphite Spherical in Ductile Iron? A Study of Elemental Distributions at Interfaces in Ductile Iron Using Atom P
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Copyright Ó 2020 American Foundry Society https://doi.org/10.1007/s40962-020-00443-0
Abstract 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.
Introduction
documented that an elevated magnesium/cerium/lanthanum content in the cast iron leads to FG–CG–SG transition. A high concentration of Mg (0.040–0.045%) promotes spheroidal graphite in cast iron alloys, and an intermediate level (0.015–0.035%) of Mg will promote formation of compacted shaped graphite, while flake graphite formation is only favorable at a low level of Mg concentration. In addition, during production of ductile iron castings magnesium fade or elevated levels of sulfur associated with the lack of melt quality control will promote the formation of undesired compacted graphite or flake graphite.1 It is challenging to investigate the role of nodulizing and denodulizing elements in graphite morphology transition since detection of trace amounts of elements is difficult. Sulfur is a surface-active element, and it may be adsorbed on the graphite and affect the graphite growth. It has been shown in the literature that a single layer or two layers of sulfur atoms were detected on
Production of ductile iron (DI) with consistent properties is generally accomplished by the addition of nodulizing elements in the iron alloys. Nodulizers such as magnesium, cerium and rare earth elements promote the formation of spheroidal graphite (SG) in iron. Magnesium is the most commonly used nodulizing element in the production of DI. Production of compacted graphite iron (CGI) uses a combination of nodulizing elements and denodulizing elements. Denodulizing elements which promote formation of flake graphite (FG) include sulfur, titanium and oxygen. It is well
This paper is an invited submission to IJMC selected from presentations at the 6th Keith Millis on Ductile Iron held October 23–26, 2018, at the Sonesta Resort, Hilton Head Island,
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