To investigate the refining phenomena of TiN inclusion and macro-/microstructure through heterogeneous nucleation of δ-Fe at TiN encapsulating oxide during solidification in ultrapure ferritic stainless steel, five samples were prepared, denoted as the initial, low-yttrium-treated, moderate-yttrium-treated, high-yttrium-treated, and magnesium-yttrium-treated samples. Based on the results of TiN inclusions, the minimum value of the average size (2.61 μm) and the maximum value of the number density (346.70 mm−2) were observed after magnesium-yttrium (Mg–Y) treatment. Combined with the mechanism analysis, these results reveal that the TiN can be refined because of the following three reasons: (ⅰ) the pure oxides, possessing low planar and/or linear disregistry with TiN; (ⅱ) the complex oxides, including the phase, which possesses extremely low disregistry with TiN; and/or (ⅲ) the increasing number of the oxides. By comparison, finer macrostructures were yielded with the addition of Y-/Mg–Y-based modifiers because of the decreasing nucleation barrier, and the finest macrostructure with equiaxed grain ratio of 100% and average grain size of 1.099 mm was observed in the Mg–Y-treated sample. Coupled with the results of TiN inclusions, therefore, the refining efficiency of macrostructure increases with the number of TiN. Referring to the results of differential scanning colorimetry, a general rule of solidification for metallic materials has been revealed. In addition, the in-situ nucleation-growth of δ-Fe was observed using high temperature laser confocal microscopy, and the microstructures were refined after Y/Mg–Y treatment. Eventually, the interfacial wetting-lattice mismatch heterogeneous nucleation model is successfully applied to evaluate the refining phenomenon of macro-/microstructures.
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