Globular Suboxide particle in steel

The image illustrates the concentric growth of oxidation products in a steel. The surrounding matrix is composed of titanium alloyed steel, which is an alloy of iron (Fe) with small quantities of titanium (Ti) (~ 0.1%) and other elements such as aluminium (Al), manganese (Mn) in smaller quantities. When a multi-element alloyed steel is exposed to oxygen at high temperatures, oxygen will diffuse into it (high temperature favors faster diffusion). When oxygen enters this alloy, it will begin to recombine with the alloy metals present, following a sequential order based on the stability of the respective alloy metal oxides. Upon reaching the saturation limit, the alloyed metals and diffused oxygen will lead to the growth of the respective metal oxide. As more oxygen becomes available, more metals will reach their oxide saturation limit, and new oxides will form. In this instance, the oxidation sequence begins with alumina Al2O3 (darkest, core) (A) where the inward diffusion of oxygen reacts with the small amount of Al present in the steel. It is followed by a spinel (Hercynite [FeAl2O4] – ulvospinel [Fe2TiO4] mix, mid grey) (S), indicating an increase in oxygen levels that led to the reaction with some of the alloyed Ti. Eventually, the oxygen levels rose sufficiently to initiate quantitative oxidation of the Fe matrix, forming ilmenite (FeTiO3) (brightest grey) (I). Such complex sequential oxide growths are frequently encountered in the oxidation of steels and are known as 'Globular Suoxides' because they are often rounded like in this case, growing outwards from an original nucleation spot. Their detailed structure contains a record of the gradually changing oxygen concentration of the steel around them and therefore is much investigates by metallurgists dealing with steel oxidation.