On the growth of graphite lamellae in a high Si GG20 cast iron

In this paper, observations are made on the faceted crystallographic lateral growth structure of graphite flakes in a high Si GG20 cast iron. A 3.82 wt.% Si, 3.25% wt.%C slightly hypereutectic cast iron with large type C graphite flakes embedded in a ferrite + spheroidized pearlite matrix failed catastrophically. Fracture is propagated by debonding the graphite flakes from the metallic matrix, exposing the graphite flakes’ lateral surfaces on the fracture surface. Flake morphology and substructure were observed using scanning electron microscopy (SEM). Very thin and flexible triangular layers, nucleating on a “hexagonal rosette” center point, suggest a growth mechanism involving the incorporation of new carbon add-atoms. Epitaxial precipitation of secondary graphite during solid-state transformation shows a preferred growth habit in the a-direction, producing 2-D sheets of graphene to which carbon atoms can easily attach. Furthermore, the spiral growth of individual sheets, contributing to the thickening of the flake in the c-direction, could be inferred. The results are discussed, considering that graphite crystalline defects may play a decisive role in the spiral growth mechanism and the thickening of the graphite flake during solid-state secondary graphite precipitation.