Experimental and First-Principles Study on the Heterogeneous  Nucleation Mechanism of Acicular Ferrite Induced by MgTi2O4 Oxides

Abstract

In this study, the microstructure evolution, inclusion characteristics, and acicular ferrite (AF) heterogeneous nucleation mechanism were investigated in Ti-Mg-treated low-carbon steels through experiments and first-principles calculations. The results showed that the modification path of the oxide followed Mg2SiO4→Ti2O3+Mg2SiO4→MgTi2O4 with increasing Ti content from 0 wt% to 0.01 wt%, and the as-cast microstructure was mainly consisted of AF and polygonal ferrite (PF) in the Ti-Mg-treated steels. The number density of effective inclusions and the AF nucleation probability reached maximum values in the Ti-Mg-treated steels with 0.01 wt% Ti. First-principles calculation showed that the Mg vacancies formed more easily in MgTi2O4, and Mn atoms in γ-Fe could be spontaneously absorbed into these Mg vacancies, resulting in an Mn-depleted zone (MDZ) with a width of approximately 130 nm around the MgTi2O4 oxide. A Baker-Nutting (B-N) orientation relationship was observed between MgTi2O4 and the adjacent AF, with a lattice misfit of 4.3%. For the MgTi2O4(100)/α-Fe(100) interface under the B-N relationship, the TiO-Fe and TiO2-Fe interfaces were more stable than the Mg-Fe and O-Fe interfaces due to their larger adhesion work and lower interfacial energy, with both interfaces featuring metallic bonding. The interfacial energies of the TiO-Fe and TiO2-Fe interfaces were both lower than the interfacial energy of γ-Fe/α-Fe when the chemical potential of Ti reached −0.92 eV and −5.48 eV, respectively. Therefore, the AF laths nucleated on MgTi2O4 due to the combined effect of the enhanced driving force from MDZ formation and the low interfacial energy of the MgTi2O4/α-Fe interface in the Ti-Mg-treated steel with 0.01 wt% Ti.

Keywords: acicular ferrite, MgTi2O4, heterogeneous nucleation, Mn-depleted zone, interfacial energy