Effect of ZrO₂ on the Heterogeneous Nucleation of γ-Fe Containing 0.75 mass%C

Abstract

The disregistry between β-ZrO₂ and γ-Fe is small. In this study, the effect of β-ZrO₂ on the nucleation of γ-Fe was investigated by two experiments. First, 20 kg ingots containing 0.75 mass%C with Zr addition or with Al addition were cast and solidification structure was observed. High fraction of equiaxed grain structure was formed in the ingots with Zr addition, but not formed with Al addition. Next, undercooling of 500 g molten steel during cooling was measured. In this experiment a ZrO₂ tablet or an Al₂O₃ tablet was set at the bottom of the crucible. Undercooling with a ZrO₂ tablet was smaller than that with an Al₂O₃ tablet. These two results indicate that β-ZrO₂ acts an effective heterogeneous nucleation site for γ-Fe. Interfacial energy differences between liquid Fe and solid γ-Fe both in cases of facing ZrO₂ and Al₂O₃, which are key elements for heterogeneous nucleation capability, were compared and it was found that the difference in the former case is larger than that in the latter case and that chemical contribution is larger in comparison to structural contribution, i.e., lattice misfit energy, in the former case and vice versa in the latter case.