Abstract
Rare earth (RE) inclusions with high melting points as heterogeneous nucleation in liquid steel have stimulated many recent studies. Evaluating the potency of RE inclusions as heterogeneous nucleation sites of the primary phase is still a challenge. In this work, the edge-to-edge matching (E2EM) model was employed to calculate the atomic matching mismatch and predict the orientation relationship between La2O2S and γ-Fe from a crystallographic point of view. A rough orientation relationship (OR) was predicted with the minimum values of and as follows: ∥ and ∥. The interface energy and bonding characteristics between La2O2S and γ-Fe were calculated on the atomic scale based on a crystallographic study using the first-principles calculation method. The calculations of the interface energy showed that the S-terminated and La(S)-terminated interface structures were more stable. The results of difference charge density, electron localization function (ELF), the Bader charges and the partial density of states (PDOS) study indicated that the La(S)-terminated interface possessed metallic bonds and ionic bonds, and the S-terminated interface exhibited metallic bond and covalent bond characteristics. This work addressed the stability and the characteristics of the La2O2S/γ-Fe interface structure from the standpoint of crystallography and energetics, which provides an effective theoretical support to the study the heterogeneous nucleation mechanism. As a result, La2O2S particles are not an effective heterogeneous nucleation site for the γ-Fe matrix from crystallography and energetics points of view.
Highlights
Previous studies showed that adding the rare earth element La/Ce into steels resulted either in a decrease in the proportion of the columnar grains or the average grain size of the equiaxed grains or both in the case of as-cast austenitic [1,2,3], ferritic [4,5,6] and other engineering steels [7,8]
The most generally accepted effect mechanisms of La/Ce are the heterogeneous nucleation effect of the primary δ-ferrite/γ-austenite on La/Ce-containing inclusions with high melting points, including oxides, oxysulfides and sulfides [1,2,4,5,6,9], or/and the solute effects [2,10] attributed to the solute enrichment or depletion in the liquid phase ahead of the solid-liquid interface
A potency evaluation criterion based on the two-dimensional lattice disregistry model was proposed [11]: when the lattice disregistry value (δ-value) is below 6%, the potency is the most effective, and the grains can be significantly refined; when the δ-value is between 6% and 12%, the potency is moderately effective; when the δ-value is above 12%, the potency is poor, and the grains cannot be refined
Summary
Previous studies showed that adding the rare earth element La/Ce into steels resulted either in a decrease in the proportion of the columnar grains or the average grain size of the equiaxed grains or both in the case of as-cast austenitic [1,2,3], ferritic [4,5,6] and other engineering steels [7,8]. A potency evaluation criterion based on the two-dimensional lattice disregistry model was proposed [11]: when the lattice disregistry value (δ-value) is below 6%, the potency is the most effective, and the grains can be significantly refined; when the δ-value is between 6% and 12%, the potency is moderately effective; when the δ-value is above 12%, the potency is poor, and the grains cannot be refined Based on this model, researchers reported the heterogeneous nucleation potency of La/Ce oxides, oxysulfides and sulfides for the primary δ-ferrite and/or primary γ-austenite during steel solidification, examined using the two-dimensional disregistry model [2,6,12,13]. Yu and co-workers [15] calculated the lattice disregistry between La2O2S and γ-austenite in an Fe-43Ni expansion alloy and reported the lowest δ-value of 5.42% between the (0001) crystal plane of La2O2S and the (100) crystal plane of γ-austenite This implied a high heterogeneous nucleation potency of La2O2S for γ-austenite.
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