In spite of many years of research, the physical phenomena leading to the evolution of compacted graphite (CG) during solidification is still not fully understood. In particular, it is unknown how highly branched CG aggregates form and evolve in the semi-solid, and how local microstructural variations at micrometer length scale affect this growth process. We present here the first time-resolved synchrotron tomography combined with a bespoke high-temperature environmental cell that allows direct observation of the evolution of CG and relates this dynamic process to the local surrounding microstructures in a cast iron sample during repeated melting and solidification. Distinct processes are identified for the formation of CG involving the nucleation, growth, development of branches and interconnection of graphite particles, ultimately evolving into highly branched graphite aggregates with large sizes and low sphericities. CG is found to nucleate with a spheroidal or a plate-like shape, developing branches induced by high carbon concentration, e.g. thin melt channels. Additionally, CG grows much faster than spheroidal graphite during subsequent cooling in solid state. The direct visualization of the dynamic solidification process provides unprecedented new insights into formation mechanisms of CG and correlating factors such as local microstructural variations, and guides the development of CG iron solidification models.

尽管进行了多年的研究，但在凝固过程中导致致密石墨 (CG) 演变的物理现象仍未完全了解。特别是，高度支化的 CG 聚集体如何在半固体中形成和演化，以及微米长度尺度的局部微观结构变化如何影响这种生长过程，尚不清楚。我们在这里展示了第一个时间分辨同步加速器断层扫描与定制的高温环境电池相结合，可以直接观察 CG 的演变，并将这种动态过程与铸铁样品在重复熔化和凝固过程中的局部周围微观结构联系起来。确定了 CG 形成的不同过程，包括石墨颗粒的成核、生长、分支的发展和互连，最终演变成具有大尺寸和低球形度的高度支化的石墨聚集体。发现 CG 以球状或板状形状成核，形成由高碳浓度引起的分支，例如薄熔体通道。此外，在随后的固态冷却过程中，CG 的生长速度比球状石墨快得多。动态凝固过程的直接可视化为CG的形成机制和局部微观结构变化等相关因素提供了前所未有的新见解，并指导了CG铁凝固模型的发展。在随后的固态冷却过程中，CG 的生长速度比球状石墨快得多。动态凝固过程的直接可视化为CG的形成机制和局部微观结构变化等相关因素提供了前所未有的新见解，并指导了CG铁凝固模型的发展。在随后的固态冷却过程中，CG 的生长速度比球状石墨快得多。动态凝固过程的直接可视化为CG的形成机制和局部微观结构变化等相关因素提供了前所未有的新见解，并指导了CG铁凝固模型的发展。