In situ synchrotron radiography of graphite nodule evolution during solidification in ductile cast iron

Ductile cast iron (DCI) is widely used in renewable-energy and transport applicationsbeca use it combines castability, strength, and fatigue resistance. Its mechanical properties are largely governed by the morphology and size distribution of graphite nodules; understanding their evolution during solidification is therefore critical for industrial optimisation. In situ synchrotron radiography was used to capture nodule evolution across remelting cycles. Combined with automated detection–tracking, this approach quantitatively resolves nucleation, flotation, growth, and degeneration. Inclusion dynamics were directly observed, revealing that shrinkage-driven convection strongly governs inclusion motion. The onset of convection coincides with primary nucleation and likely triggers it via local thermal and compositional changes. Graphite flotation occurs continuously throughout solidification: nodules grow rapidly during ascent and slow abruptly upon arrest by the dendritic network. Wide interdendritic channels enable prolonged flotation and lead to the formation of exceptionally large nodules. Primary spheroidal graphite (SG) nucleation shows a rapid initial growth phase followed by a distinct drop in rate. Its final size is strongly influenced by the local dendritic structure, resulting in large variability. In contrast, secondary SG nucleation is diffusion-driven and proceeds at roughly one-sixth the initial growth rate of primary events. This disparity likely reflects stronger undercooling, higher carbon supersaturation, and vigorous convection during early solidification. Across remelting cycles, as magnesium fades, compacted graphite (CG) becomes increasingly prevalent. A morphological transition is observed—from early interconnection-dominant forms to later protrusion-driven growth—accompanied by higher areal growth rates and increased final CG fractions. While the frequency of primary SG graphite nucleation remains largely unchanged, secondary SG nucleation undergoes a marked decline.