Effect of a catalyst on heterogeneous nucleation in pure and Fe-Ni alloys
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8/28/03
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Effect of a Catalyst on Heterogeneous Nucleation in Pure Fe and Fe-Ni Alloys KEIJI NAKAJIMA, HAJIME HASEGAWA, SAKHOB KHUMKOA, and SHOZO MIZOGUCHI In order to elucidate the nature of the heterogeneous nucleation, a differential scanning calorimetry (DSC) thermal analysis of pure Fe and Fe-Ni alloys (Ni content: 1.0 to 29.3 mass pct) containing TiN, Al2O3, and Ti2O3 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 (T) depend on the kind of catalyst used as well as the chemical composition (Ni content). In addition, in spite of the kind of primary crystal, the value of T is always small in the order of TiN, Al2O3, and Ti2O3. As a matter of fact, only TiN has a practical effect as a catalyst on the triggered nucleation of the primary crystal of the d phase. None of them has a practical effect on the nucleation of the primary crystal of the g phase.
I. INTRODUCTION
VARIOUS phenomena of solidification-structure formation caused by the heterogeneous nucleation have been encountered in material production processes and have been studied extensively. Studies on the heterogeneous nucleation have been performed from the viewpoint of controlling these phenomena. However, the heterogeneous nucleation mechanism is not yet understood sufficiently, owing to the experimental difficulty and the complexity of the phenomena. In particular, most of the previous studies have been focused on other metals and their alloys rather than on iron and its alloys. Although there are a very few reports on iron,[1–4] some differences still exist regarding the critical undercooling among researchers. This difference in data mainly comes from the difference in the experimental methods used. Bramfitt[1] first investigated systematically the effect of a catalyst (nucleating agent) on the heterogeneous nucleation of an iron melt. He inoculated ultrafine particles of carbides and nitrides (325 mesh), instead of the crystallized substrate, into the melt. The temperature was measured by a Pt-PtRh thermocouple sheathed with an alumina tube, which was immersed in the center of a 100 g melt in the alumina crucible. He then concluded that the lattice disregistry between the catalyst and the nucleated primary crystal of the d phase was the key factor. He has proposed the simple experimental expression of undercooling for carbide and nitride catalysts. Using a similar method, Ohashi et al.[2] investigated the heterogeneous nucleation by oxides crystallized in the 300 g iron melt. They found that Bramfitt’s expression was applicable to oxide catalysts too. In order to elucidate the nature of heterogeneous nucleation, an experimental approach different from others was KEIJI NAKAJIMA, Associate Professor, HAJIME HASEGAWA
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