From flake to nodular: A new theory of morphological modification in gray cast iron

A modified Bridgman technique was used to grow a nickel-alloyed gray cast iron. During the experiments, morphological modifications (MMs) were induced by the change in chemical composition of the melt in front of the solid/liquid interface. This was achieved by means of an “in-situ” addition of commercial modifiers, maintaining the thermal-gradient-to-growth-rate ratio. Optical and scanning electron microscopy (SEM) and X-ray diffraction were used to characterize the resulting microstructures. From the analysis of the evolution of the microstructures during our unidirectional experiments, correlated with results of other researchers, a new explanation for MMs observed in cast iron is proposed; it covers the whole range of morphologies from flake to nodular graphite. The explanation emphasizes the following points. (1) Graphite always grows like sheet crystals. (2) The MM of flake cast iron starts when the triple-contact line between austenite-graphite-liquid disappears, provoking the separation of austenite from graphite during eutectic growth. (3) Vermicular and nodular morphologies arise from the compacting action of solid austenite and liquid, respectively, exerted upon sheet crystals.