Investigation on the graphite nucleation and growth mechanism of the compacted graphite iron

Abstract

This paper investigates the three-dimensional spatial characteristics of graphite in compacted graphite iron (CGI), at different crystallization stages, by interrupting the solidification process through experimental technique and taking three-dimensional X-ray diffraction (XRD) images through contrast tomography technique. The nucleation, growth mechanism and degeneration of graphite were analyzed by electron probe microanalysis technique (EPMA), scanning electron microscope with energy disperse spectroscopy (SEM-EDS), and transmission electron microscopes (TEM). The relationship between the maximum volume, connectivity and duration of crystallization was established by the analysis of the growth dynamics. The results have established a further relationship of nucleation between compacted graphite and spheroidal graphite. It was confirmed that the nucleation of graphite in CGI has a mechanism identical to that in the spherical graphite iron. The investigation proves that Ce2S3, La2S3, and Ti2S3 have nucleation capacity and that the growth mechanism of compacted graphite involves the evolution from spheroidal to cluster-like and then to interconnected graphite. The investigation also proves that the formation of compacted graphite is based on the graphite growth and graphite degeneration, which finally forms a worm-like morphology. According to the analysis of growth dynamics, the connectivity can be used as a key index for the CGI graphite nodularity and will affect the comprehensive properties of the material such as strength, thermal fatigue, thermal conductivity. The results are expected to provide a better understanding of compacted graphite iron as well as a better enrichment of the nucleation and growth mechanism of CGI.