**3.2. Microtexture evolution of oxide scale**

Crystallographic orientation refers to how the atomic planes in a volume of crystal or grain are positioned relative to a fixed reference [9]. These grains present the occurrence of certain

**Figure 2.** EBSD phase maps for wustite, hematite, magnetite and ferrite of the microalloyed steel hot rolled at 860°C with the thickness reductions and then cooling rates of (a) 10%, 10°C/s, and (b) 13%, 23°C/s [7], (c) SEM morphologies of top oxide scale formed over pure iron and (d) the magnification of one of valley surface [8].

orientations caused by heat treatments from melting and subsequently thermomechanical processing. This tendency is known as preferred orientation or texture. *Microtexture* is the conjoining of microstructure and texture [9], referring to the orientation statistics of a population of individual grains and their spatial location, that is, the orientation topography. To avoid ambiguity, a texture that reflects an average value obtained from many different grains is often called macrotexture [9]. Microtexture can generally be representing by pole figure, inverse pole figure or orientation distribution functions (ODF). Pole figure and inverse pole figure can be defined by the projection of orientation space through the crystal to specimen coordinate system or vice versa. ODF in the form of sections through the orientation space express the probability density function of orientations. This is a quantitative evaluation of the microtextures made by means of spherical harmonics method.

Analysis of the microtexture in the oxide phases and their orientation relationship is now being studied. A strong {001} texture may be found in wustite whatever the steel substrate [10], though this fibre texture also evolves in magnetite under low-temperature oxidation [6]. **Figure 3** shows texture development of magnetite and hematite in deformed oxide layers and their intensity distributions along associated fibres or texture components. Magnetite has a cubic structure, and its ODF sections are depicted using the *φ*<sup>2</sup> = 0° and 45° (**Figure 3b**) in terms of the Bunge system. In contrast, for hexagonal hematite, the ODF sections with *φ<sup>2</sup>* = 0° and 30° (**Figure 3d**) are used [9]. *θ* fibre develops in magnetite superimpose on *φ<sup>2</sup>* = 0° section at *Φ* = 0° with the rotations of <100>//ND [6, 11]. **Figure 3a** shows the intensity distribution of the *θ* fibre in magnetite on the samples subjected to various deformation conditions. <10-10> fibre component in α-Fe<sup>2</sup> O3 (**Figure 3c**) lies on *φ<sup>2</sup>* = 30° section and corresponds to orientations along *Φ* = 90°.

**Figure 3.** Development of texture intensity *f*(*g*) along the (a) *θ* fibre of magnetite, (c) <1010> fibre of hematite, of the samples with different thickness reductions (TRs) and cooling rates (CRs), and orientation distribution function (ODF) sections for (b) magnetite and (d) hematite at a TR of 28% and a CR of 28°C/s [12].

orientations caused by heat treatments from melting and subsequently thermomechanical processing. This tendency is known as preferred orientation or texture. *Microtexture* is the conjoining of microstructure and texture [9], referring to the orientation statistics of a population of individual grains and their spatial location, that is, the orientation topography. To avoid ambiguity, a texture that reflects an average value obtained from many different grains is often called macrotexture [9]. Microtexture can generally be representing by pole figure, inverse pole figure or orientation distribution functions (ODF). Pole figure and inverse pole figure can be defined by the projection of orientation space through the crystal to specimen coordinate system or vice versa. ODF in the form of sections through the orientation space express the probability density function of orientations. This is a quantitative evaluation of

**Figure 2.** EBSD phase maps for wustite, hematite, magnetite and ferrite of the microalloyed steel hot rolled at 860°C with the thickness reductions and then cooling rates of (a) 10%, 10°C/s, and (b) 13%, 23°C/s [7], (c) SEM morphologies of top

the microtextures made by means of spherical harmonics method.

oxide scale formed over pure iron and (d) the magnification of one of valley surface [8].

64 Study of Grain Boundary Character

Study on orientation relationship of oxide phase is still less explored thus far. A cube-cube orientation relationship between wustite and magnetite may prevail in undeformed oxide scale possibly due to the defective structure of the wustite. The orientation of the magnetite and substrate was reported {110}Fe//{100}Fe<sup>3</sup> O4 , <110>Fe//<100>Fe<sup>3</sup> O4 in the case of transforming by continuous cooling from 400°C. By contrast, the Fe/FeO orientation relationship was {100}

Fe//{110}FeO, <110>Fe//<110>FeO. For a very thin oxide scale, a Fe/FeO orientation relationship was {100}Fe//{100}FeO, <100>Fe//<110>FeO [13].
