Effect of Ce on solute redistribution in liquid ahead of solid–liquid interface during solidification of Fe–4 wt.%Si alloy

Abstract

The high efficiency of Ce addition in grain refinement of δ-ferrite in a cast Fe–4 wt.%Si alloy was verified. In order to further understand the solute effect of Ce on the grain refinement of δ-ferrite, the conventional directional solidification technique, which enabled to freeze the solid–liquid interface to room temperature, was used to investigate the interfacial morphology and solute redistribution in the liquid at the front of the interface, together with thermodynamic calculation of the equilibrium partition coefficients of Ce and Si in Fe–4 wt.%Si–Ce system using the Equilib module and the FsStel database in FactSage software system. Metallographic examination using a laser scanning confocal microscope showed a transition of the solid–liquid interface from planar to cellular in the Fe–4 wt.%Si alloy after adding 0.0260 wt.% Ce during the directional solidification experiment. Further, electron probe microanalysis revealed an enhanced segregation of Si solute in the liquid at the front of the solid–liquid interface due to the Ce addition. This solute segregation is considered as the cause of planar to cellular interface transition, which resulted from the creation of constitutional supercooling zone. Thermodynamic calculation indicated that Ce also segregated at the solid–liquid interface and the Ce addition had negligible effect on the equilibrium partition coefficient of Si. It is reasonable to consider that the contribution of Ce to the grain refinement of δ-ferrite in the cast Fe–4 wt.%Si alloy as a solute was marginal.