Effect of Catalyst on Heterogeneous Nucleation in Fe–Ni–Cr Alloys

Abstract

In order to elucidate the nature of heterogeneous nucleation, a differential scanning calorimetry (DSC) thermal analysis of Fe–Ni–Cr alloys (Ni+Cr content=30 mass% (constant); Ni content: 7.5 to 29.3 mass%) containing TiN and Al₂O₃ was conducted. Then, special attention was paid to the difference in the phase of the primary crystal nucleated by the triggering effect of a catalyst (nucleating agent). The solidification and transformation mode appearing during cooling in these alloys is classified into three cases: F mode, FA mode, and A mode. The change of modes and the critical undercooling (ΔT) depend on the kind of catalyst used as well as on the chemical composition. In addition, in spite of the kind of primary crystal, the value of ΔT is always smaller in the order of TiN<Al₂O₃. These findings are the same as those for Fe–Ni alloy, although Fe–Ni–Cr alloys have the advantage that the increase of Cr content can trigger the crystallization of the primary crystal of the δ phase, which consequently can reduce ΔT and extend the region of low ΔT. The total concentration of catalyst molecules (C₁), as well as the interface free-energy ratio ((γ_CL−γ_SC)/γ_LS), is one of the most significant factors in explaining undercooling for heterogeneous nucleation. (γ_CL−γ_SC)/γ_LS is related to the kind of catalyst used and the chemical composition, and C₁ is related to the crystallized content of catalyst. As a matter of fact, the number of catalysts supplied to the solidification front has a practical effect on the control of the solidification-structure.