The morphology and microstructural evaluation of Y phases in AISI 321 (a Ti-stabilized stainless steel) were characterized after hot deformation. The electronic structure and phase stability of titanium carbosulfide were further discussed by first-principle calculations. It was found that Y phases, like curved strips or bones in AISI 321 stainless steel, mostly show a clustered distribution and are approximately arranged in parallel. The width of the Y phase is much less than the length, and the composition of the Y phase is close to that of Ti₂SC. Y phases have exceptional thermal stability. The morphology of Y phases changed considerably after forging. During the first calculations, the Ti₂SC with hexagonal structure does not spontaneously change into TiS and TiC; however Ti₄S₂C₂ (Z = 2) can spontaneously change into the two phases. The Ti-S bonds are compressed in Ti₄S₂C₂ cells, which leads to poor structural stability for Ti₄S₂C₂. There is a covalent interaction between C/S and Ti, as well as an exchange of electrons between Ti and S/C atoms. Evidently, the mechanical stability of Ti₄S₂C₂ is weak; however, Ti₂SC shows high stability. Ti₂SC, as a hard brittle phase, does not easily undergo plastic deformation.
Keywords: Ti-containing steels; electronic structure; first principles; phase stability; structural evaluation.