Growth Mechanism of Graphite Spheroids from Nano- to Micro-Scale

Graphite spheroids are one of the most intriguing forms of aggregation of graphite. They can be found as natural graphite in meteorites and volcanic rocks (metamorphic graphite caused by exposure to heat and pressure in the earth's crust), and in man-made materials such as steel and cast iron, but also in pyrolytic graphite, in carbon nanosheets obtained through vapor deposition, and others. While growth of graphite has been studied in depth during the last decades, the understanding of its physics continues to evolve as modern investigating equipment pushes the knowledge horizon. The complexities of the solidification of cast iron and its dependency on undercooling, a function of the cooling rate and of the nucleation potential, are discussed in conjunction with the asymmetric eutectic coupled zone. A sequential chart summarizes the various stages of graphite aggregation, as the graphite begins growing in contact with the nucleus at the beginning of solidification till it reaches its final crystallography at room temperature. It sets the basis for an in-depth analysis of the multi-mechanisms involved in the growth of graphite in cast iron during solidification and the subsequent solid-state transformation, with emphasis on compacted and spheroidal graphite. It discusses the significance of turbostratic graphite that appears mostly, but not only, at the nucleus/graphite interface and in the initial stages of graphitization, as revealed by recent electron microscopy studies.