Melt superheated treatment can affect the solidification structure, but the direct evidence requires strict experiments. The molecular dynamics simulation method can break the limited experimental conditions, and provide advanced prediction for research. In this study, influences of superheated temperature (Ts) on the nucleation undercooling (ΔT) of metallic melts (Ti, Co, Mg, Al, Ni, Fe, Ag) were studied by the molecular dynamics simulation. The results show that the value of ΔT increases with the rise of Ts until the maximal ΔT approaches. In the curve of ΔT vs. Ts, there is an inflexion region where the nucleus cluster was broken. Above this inflexion region, the number of nucleus clusters decreases with the rise of Ts. Based on the simulated results, a model was proposed for describing the relation of ΔT and Ts, with which the maximal undercooling for metals can be predicted.
The developed model of diffusion-limited and diffusionless solidification of eutectic alloy describes the relation “undercooling (ΔT)–velocity(V)–interlamellar spacing (λ)” for two cases. Namely, if the lamellar velocity V is smaller than the solute diffusion speed in bulk liquid VD, VVD, the solidification is mainly controlled by kinetic and thermal undercoolings. We show an influence of model parameters on the growth kinetics of eutectic solidification. Comparison of the model predictions with experimental data obtained on Fe-B samples processed in melt-glass fluxing is given.
Numerous experimental data on rapid solidification of eutectic systems exhibit the formation of metastable solid phases with the initial (nominal) chemical composition. This fact is explained by suppression of eutectic decomposition due to diffusionless (chemically partitionless) solidification beginning at a high but a finite growth velocity of crystals. A model considering the diffusionless growth is developed in the present work to analyze the atomic diffusion ahead of lamellar eutectic couples growing into supercooled liquid. A general solution of the model is presented from which two regimes are followed. The first presents diffusion-limited regime with the existence of eutectic decomposition if the solid-liquid interface velocity is smaller than characteristic diffusion speed in bulk liquid. The second shows suppression of eutectic decomposition under diffusionless transformation from liquid to one-phase solid if the solid-liquid interface velocity overcomes characteristic diffusion speed in bulk liquid.