Atomic-Scale Investigation on the Formation of MgAl₂O₄–MnS Inclusions in 304 Stainless Steel Based on Multi-scale Characterization and First-Principle Calculation

Inclusions in steel have an important effect on steel properties. Al₂O₃ is a common inclusion in steel, resulting in stress concentration in steel products. Mg treatments can transform Al₂O₃ into small and dispersed MgAl₂O₄ inclusions, which can serve as the nucleation center for MnS to form MgAl₂O₄–MnS composite inclusions. In this study, based on multi-scale characterization of the chemical composition, phase, and structure of the interface of the MgAl₂O₄–MnS composite inclusions, the mechanism of the formation of MgAl₂O₄–MnS composite inclusions was revealed by first-principles calculations. Firstly, the chemical composition, morphology, and crystal structures of the composite inclusions were determined by scanning electron microscope and energy dispersive spectrometer (SEM-EDS) and X-ray diffraction (XRD). Secondly, the MgAl₂O₄–MnS inclusions were micro-sectioned by SEM and focus ions beam (FIB) to explore their interface. The high-resolution transmission electron microscopy (HTEM) analysis showed that the orientation relationship between MgAl₂O₄ and MnS was MgAl₂O₄ (111)//MnS (200). Finally, the electronic structures and adsorption energy of the atoms of Mn and S on the low exponential structures ((100), (110), and (111)) of MgAl₂O₄ with different initial positions were investigated by first principles. It is determined that the most stable adsorption structure can be obtained following the path of adsorbing Mn first and then S on the (111) surface of MgAl₂O₄. The characterization from multi-scale revealed the formation mechanism of MgAl₂O₄–MnS composite inclusion. The theoretical calculation and the experimental results are in good agreement.