Growth of spheroidal graphite on nitride nuclei: disregistry and crystallinity during early growth

The graphite phase resulting from the stable solidification of iron-carbon-silicon alloys can have a variety of morphologies, from lamellar to spheroidal, including undesired degenerated morphologies. Spheroidal graphite is preferred when high strength and ductility are required. Understanding the nucleation and growth mechanism of graphite will improve control of its morphology during industrial processing. The crystalline structure of spheroidal graphite in iron-carbon-silicon alloys exhibits a multi-regions substructure. Literature opinions on the degree of crystallinity of the core of the spheroid are divergent. Unconvincing explanations are offered. The ability of nitrides to serve as graphite nuclei is questioned because of the high linear disregistry. 

To bring some clarity into the early crystallization of graphite in iron melts, interrupted solidification samples were examined through scanning electron microscopy. Disregistry calculations and thermodynamic analysis demonstrated that certain nitrides are favorable for graphite nucleation. Some unique pictures showing graphite platelets growing out of the nucleus were obtained from deep-etched samples. They bring evidence of the disorientation of graphite platelets growing around the graphite nuclei for the particular case of Mg-Al-Si nitrides. It is suggested that the low crystallinity observed in the core of some graphite spheroids is the result of this random orientation of the graphite platelets growing on nucleating inclusions during early solidification and of subsequent curved-circumferential growth..