Refining Mechanism for Tin and Solidification Structure Through a Nucleating Chain in Magnesium-Yttrium-Treated Ultrapure Ferritic Stainless Steel

Abstract

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 and the maximum value of the number density 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 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, the metal with higher latent heat of fusion/crystallization, smaller critical undercooling degree, and narrower solidification interval usually has finer grains. 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.

Keywords: magnesium-yttrium treatment, TiN inclusion, disregistry, macro-/microstructure, heterogeneous nucleation