# Examination of Spheroidal Graphite Growth and Austenite Solidification in Ductile Iron

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## Abstract

Microstructures of a ductile iron alloy at different solidification stages were captured in quenching experiments. Etched microstructures showed that spheroidal graphite particles and austenite dendrites nucleated independently to a significant extent. Growth of the austenite dendrite engulfed the spheroidal graphite particles after first contacting the nodule and then by forming an austenite shell around the spheroidal graphite particle. Statistical analysis of the graphite size distribution was used to determine the nodule diameter when the austenite shell was completed. In addition, multiple graphite nucleation events were discerned from the graphite particle distributions. Majority of graphite growth occurred when the graphite was in contact with the austenite. Circumferential growth of curved graphene layers appeared as faceted growth fronts sweeping around the entire surface of a spheroidal graphite particle which was at the early growth stage. Mismatches between competing graphene growth fronts created gaps, which divided the spheroidal graphite particle into radially oriented conical substructures. Graphene layers continued growing in each conical substructure to further extend the size of the spheroidal graphite particle.

## Notes

### Acknowledgments

The authors would like to express their gratitude to Dr. Simon N. Lekakh for his help on experimental design and technical discussions. The authors wish to gratefully thank Dr. Mingzhi Xu for his help on computational simulations, metal melting, sampler modifications, and thermal analysis. Marc Harris, Daniel Field, Seth Rummel, and Terrell Webb are acknowledged for their help with sampling and specimen preparations. Jessica Terbush and Clarissa Wisner are gratefully acknowledged for their guidance on HRSEM operations.

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