Competition for Nucleation and Grain Initiation during Solidification

Without the addition of any grain refiner, the inclusion particles in a melt will induce heterogeneous nucleation and grain initiation during the solidification of metallic materials. However, with grain refiner addition, the exogenous particles (from the grain refiner) and the native inclusions (e.g., oxide particles) will co-exist in the melt, and there will be competition for nucleation and grain initiation among different types of solid particles. In this paper, we analyze such competition in Al and Mg alloys using a numerical solidification model that we have developed previously. The numerical calculations show that the competition for nucleation is strongly dependent on nucleation undercooling of the different types of particles, while the competition for grain initiation is closely related to the sizes of solid particles. Based on the numerical results, the general rules of competition for nucleation and grain initiation have been developed: nucleation starts with particles of minimum nucleation undercooling, followed by particles with progressively larger nucleation undercooling; and grain initiation starts with solid particles of the largest size, followed by solid particles with progressively smaller sizes.