Rhombohedral Graphite Phase in Nodules from Ductile Cast Iron

Abstract

A significant amount of rhombohedral graphite has been found by X-ray diffraction in spherical carbon particles (nodules) extracted from ductile cast iron using an original technique that consists in a sequencial acid dissolution of the metallic matrix. This clarifies the underlying mechanism acting to produce the necessary bending of basal planes to form a spherical cap. The nodules have a multilayer structure with [00.2] parallel to the radial direction. Plastic deformation would produce twining in hexagonal graphite with {11.1} planes acting as twin-matrix boundaries. A subsequent dissociation of this boundary creates a region where the stacking sequence corresponds to the rhombohedral phase bounded by two hexagonal graphite ones, tilted by 24 deg to each other. The existence of rhombohedral graphite in ductile cast iron nodules gives further support to the theory according to which the change of morphology of graphite from lamellas to spheroids, passing through worm like intermediate form, is a consequence of an increment of the interfacial energy between graphite planes and the melt. In this theory an essential stage to produce the conformation of crystalline conglomerates having rounded final forms requires mechanical work on thin faceted sheets of graphite giving the opportunity for rombohedral phase formation.