**2.1. Iron oxides**

in composition with disrupted atomic bonding. Grain boundary refers to the interface zone between grains of the same phase, while interfaces are boundaries between dissimilar phases. It represents the narrow zone where atomic bonding is disrupted by misalignment of the crystalline grains. This disrupted bonding at the grain boundary is about 5–10 atoms across. Grain boundaries have nanoscale spatial dimensions, which can generate substantial resistance to ionic transport due to dopant (or impurity) segregation. This diffusion provides active paths for atomic motion, particularly at high temperature, during the diffusion-controlled oxidation or corrosion. Thus, composite properties are sensitive to the interface structure and chemistry

This crystallographic structure of a metal alloy is one of the important parameters in determining the oxidation or corrosion behaviour. The characters of grain boundaries in oxide layers formed on substrates influence adhesion and friction behaviour, surface fracture and wear during high temperature steel processing. However, the effect of grain characters on the oxidation behaviour is not fully understood yet. There are still many challenges, one of which is how to engineer grain boundaries to optimise the oxidation resistance of these materials. For this reason, detailed understanding of the processing-structure-property relationships that focus on grain boundaries and interfaces is critical to advanced manufacturing of metals. Furthermore, it is necessary to modify the grain boundary characteristics of this alloy which

In this chapter, an attempt has been made to explore the role and behaviour of grain boundaries in the oxide scale formed on the steel surface during metal processing. In doing so, two things we need to consider for such high-temperature plastic deformation are diffusion mechanism at grain boundaries and resulting boundary migration in the growth of grains.

This section is devoted to the fundamental issues of oxidation mechanism that should be defined and summarised before specific problems are confronted. During metal processing at elevated temperatures, oxidation occurs inevitably on the surface of products. In the case of the pure iron, the oxide scale formed on is a complex mixture of three iron oxide

because iron has divalent and trivalent ions (Fe2+ and Fe3+). The complete oxidation of iron can be divided into three main steps, where iron oxidises to the lowest valence ion Fe2+ and forms the first sub-layer of wustite (Fe1-xO) next to the metal. Then, some of Fe2+ ions oxidise further to Fe3+ and contain both valence iron ions as the intermediate sub-layer of magne-

only consists of the highest valence iron ion Fe3+. This is the case above 570°C (the eutectoid point of the Fe-O system) in the diffusion-controlled growth of multilayered scales on pure iron. Below 570°C, the wustite phase is unstable, and the oxidation of iron directly results in magnetite. In steel, various Fe-C alloys, their oxidation at high temperature can be more complex than pure iron, in particular the segregation of different element at grain

). Under conditions of sufficient oxygen, the outer sub-layer of hematite (Fe<sup>2</sup>

) and wustite (Fe1-xO, *x* = 0.84–0.95) [1]. This is

O3 )

O4

to large potential variations.

60 Study of Grain Boundary Character

affect its oxidation resistance.

**2. High-temperature oxidation**

O3

), magnetite (Fe<sup>3</sup>

phases: hematite (Fe<sup>2</sup>

O4

tite (Fe<sup>3</sup>

boundaries.

In view of the iron cations can exist in the two valence states, iron oxides can have different crystal structures with different Fe/O ratios. These phases include wustite (Fe1-xO), magnetite (Fe<sup>3</sup> O4 ) and hematite (α-Fe<sup>2</sup> O3 ) [2].

Wustite has a defective halite structure, with anion sites occupied by O2− and most cation sites occupied by divalent Fe2+ ions. Cubic close-packed (CCP) array of O2− stacked along the [111] direction. Most of the iron is octahedral with a small proportion of Fe3+ on the vacant tetrahedral sites (**Figure 1a**). A cation-deficient phase written as Fe1−*<sup>x</sup>* O (with 1− *x* ranging from 0.83 to 0.95) exists at 0.1 MPa pressure and temperatures higher than 570°C. Wustite is a p-type (p = positive carrier) semi-conducting oxide with a high concentration of lattice defects. These high cation vacancies result in a high mobility of cations and electron via metal vacancies and electron holes.

Magnetite (Fe<sup>3</sup> O4 ) has an inverse spinel structure containing both divalent and trivalent iron ions. The distribution of its cation is written as (Fe3+) [Fe3+Fe2+] O<sup>4</sup> , where the parentheses denote the tetrahedral sites and the square brackets denote the octahedral sites. In this case, the ferric ion Fe3+ relinquishes half of the octahedral sites to the ferrous species Fe2+, that is, with 8Fe3+ ions located in tetrahedral sites plus (8Fe3+ and 8Fe2+) ions distributed into octahedral sites per unit cell. The structure consists of octahedral and mixed tetrahedral/octahedral layers stacked along the [111] direction. **Figure 1b** shows the sequence of Fe- and O-layers and the section of this structure with three octahedral and two tetrahedral. Magnetite with an excess of oxygen also exists, but this excess is much smaller than that with wustite, and the corresponding concentration of defects is also less.

The crystal system of hematite (α-Fe<sup>2</sup> O3 ) is a rhombohedral structure (**Figure 1c**) with a low concentration of structural defects. Hexagonal close-packed (HCP) arrays of oxygen ions are stacked at the [001] direction. The O-O distances along the shared face of an octahedron are shorter (0.2669 nm) than the distance along the unshared edge (0.3035 nm), and hence, the octahedron is distorted trigonally. The shared Fe-O<sup>3</sup> -Fe triplet structure influences the

**Figure 1.** Crystal structure of (a) wustite (FeO), (b) magnetite (Fe<sup>3</sup> O4 ) and (c) hematite (α-Fe<sup>2</sup> O3 ).

magnetic properties of the oxide because hematite is an n-type (n = negative carrier) semiconducting oxide in which the diffusion of anions is dominant.

In a word, these iron oxides have the different crystal symmetries representing by a different space group: ferrite, *Im-3m*; wustite, *Fm-3m*; magnetite, *Fd-3m*; hematite, *R-3c*, and different lattice parameters: ferrite, wustite and magnetite in cubic symmetry with lattice parameters (*a*) of 0.287, 0.431 and 0.840 nm, respectively, and hematite in a trigonal structure with *a* = 0.504 nm and *b* = 1.377 nm [3].
