Kinetic conditions of graphite morphology transformation in cast iron alloys

The metal melt meeting the thermodynamic conditions of nucleation only provides the possibility for the metal melt to solidify and nucleate, and the metal solidification can proceed smoothly only when it meets the dynamic conditions of nucleation.

During solidification of metal melt, the primary condition for nucleation is that the system must be in a supercooled state to provide a driving force for phase transformation; Secondly, it is necessary to overcome the kinetic energy barrier through fluctuation to form stable nuclei. In general, supercooled metal melts form stable crystalline particles in some small regions through fluctuation. Because the new phase is associated with the interface, the interfacial free energy, which is a thermodynamic barrier, is the main resistance in the nucleation process. According to the characteristics of the interface, there are two different ways of nucleation in liquid metal solidification, namely homogeneous nucleation and heterogeneous nucleation.

During the production of cast iron alloy, the nucleating agent mainly composed of C and Si elements shall be added to the metal melt. Within a certain time after the dissolution of the nucleating agent and its dissolution, a region with high content of C and Si elements shall be formed around the nucleating agent particles and at the position before the dissolution, which greatly improves the carbon equivalent of the region and forces the carbon supersaturated precipitation. There are many non-metallic inclusion particles in the metal melt, which play a role of heterogeneous core under certain conditions, and can be used as an effective heterogeneous nucleation substrate to induce heterogeneous nucleation in the metal melt. The model of heterogeneous nucleation of metals is shown in the figure. Assuming that the crystal nucleus forms a spherical crown on the interface, when it reaches equilibrium, the following relationship is satisfied: