Formation of complex nuclei in graphite nodules of cast iron

The complex nature of nodular graphite nuclei was studied in a partially solidified and quenched spheroidal graphite iron (SGI) that contained small graphite nodules. The reduced graphite to nuclei diameter ratio obtained by the accelerated solidification process increased the probability of nuclei exposure during metallographic cross sectioning. Ten thousand graphite nodules were analyzed to identify nucleation sites using an automated scanning electron microscopy-energy dispersive X-ray (SEM-EDX) system and the resultant ternary plot presents a chemistry of oxides, sulfides and nitrides located inside the graphite nodules. Focused ion beam milling was used to extract thin foil specimens and extensive high-resolution transmission electron microscopy (TEM) analysis of several representative nuclei was conducted. The nucleus of a graphite nodule was composed of a Mg–O core with Mg–Ca–S and Mg–Al–Si–N compounds attached on its sides. The crystallographic orientation relationships between different nucleating compounds and between graphite and nucleating compounds were analyzed using selected area diffraction patterns. For the first time, the direct observation of the nuclei interfaces revealed the presence of nano-structured boundary layers. The formation of a complex nodular graphite nucleus was thermodynamically simulated and the nucleation behavior of the complex compound was linked to the novel experimental results.