Investigation on the formation mechanism of composite inclusions by multi-scale characterization in high manganese steel with yttrium addition

The rare earth modification of inclusions is an effective method to solve the critical issue of brittle inclusions leading to the failure of high manganese steel railway frog as one of the most essential assemble units of the railway operating system. The high manganese steel with rare earth yttrium was produced using a vacuum induction furnace. The multi-dimensional structural analysis of composite inclusions in the high manganese steel was performed to clarify the composition, crystal structure, and interface structure of the composite inclusions at multi-scales ranging from micrometers and nanometers to atoms and electrons. The chemical composition and morphology of composite inclusions were characterized by scanning electron microscopy with energy dispersive X-ray spectroscopy. After modification with rare earth yttrium, the MnS-Al2O3 composite inclusions in the steel transform into Y2S3-Y2O2S composite inclusions with smaller dimensions and approximately spherical shapes. The interface structure and crystallographic orientation of the Y2S3-Y2O2S composite inclusions were revealed as Y2O2S (100) // Y2S3 (5 0 3) by the focused ion beams scanning electron microscopy and the high-resolution transmission electron microscopy analysis. Moreover, the first-principles calculations were performed to obtain the adsorption energies and electronic structures of Y and S atoms at various positions on different terminations of the three low-index surfaces ((1 0 0), (110), and (111)) of Y2O2S, which reveals the adsorption pathway of Y and S atoms and formation mechanism of enclosed phase Y2S3 on the core phase Y2O2S that the adsorbate Y and S atoms grew parallel to the Y2O2S (1 0 0) surface and formed specific hexagonal ring structures of Y2S3 crystal. •A multi-scale characterization method is built to determine the find structure of the Y2O2S-Y2S3 composite inclusions.•The formation of composite inclusions is studied by first-principles calculation at the atomic level.•The formation path of the Y2S3 is determined according to the adsorption energy of the single Y and S atoms.