**5. Methodological potentials**

magnetite/hematite scale [17], the oxide scale is easy to crack in presence of Σ13b and Σ19c in

grain boundaries can provide new insight into means of suppressing propagation of cracks when it is undesirable and into means of producing specific trapped nanoparticles when it

One thing we should consider is to distinguish grain boundary strengthening to steel substrate or to formed oxide scale itself. Extensive studies have been focused on the role of grain boundaries played in the steel substrate. For instance, grain boundary strengthening and precipitation hardening are considered to provide the most to the high-strength properties of the mechanically alloyed oxide dispersion strengthened (ODS) ferritic alloys, containing nanosized (<3.5 nm) oxide dispersions. A higher density of these oxide particles with larger sizes than the ones in the matrix was found at the grain boundaries in ODS Fe–12Cr–5Al alloys

The grain refinement can be used to explain this strengthening process. Whatever grain boundaries works, the essential mechanism should be similar. However, the difference of a protective oxide scale containing reactive element may not simply be due to a site blocking effect in the grain boundary [30]. The contributions of dopant ions to the multiple electrical and ionic processes would provide valuable guidance to elucidate the deformation mechanism of oxide scale thermally formed on steel alloys. That is reason to note the difference between them as the presence of oxide particles at the grain boundaries and the temperature

**Figure 6.** (a) High-angle annular dark field (HAADF)—scanning transmission electron microscopy (STEM) images

) and (b) diffraction patterns after being mechanically alloyed and extruded at

Many different surface properties of metals and alloys will influence tribological performance. These surface properties include surface energy, crystallographic orientation, grain boundaries, texturing of surface and crystal structure. Grain boundaries in the oxide layers can alter the underlying failure mechanisms of formed oxide scale, which affects their tribological performance during metal processing. There are various strained conditions along grain boundaries because many dislocations present to help accommodate the misfit or mismatch

with Σ3. Thus, it is possible that during this time, tailoring specific

α-Fe<sup>2</sup> O3

is desired.

compared to Fe<sup>3</sup>

70 Study of Grain Boundary Character

(Y2O3 + ZrO2) (**Figure 6** [29]).

at which they were formed.

of Fe–12Cr–5Al alloys (Y<sup>2</sup>

950°C [29].

O3 + ZrO<sup>2</sup>

**4.3. Effect of grain boundaries on tribological performance**

O4

Before starting, we need to consider some sample preparations to detect grain boundaries. Two directions are generally used to observe the oxide scale formed on the metal surface. One is the cross-sectional or thickness direction of the oxidised sample, parallel to the direction of oxide growth. Another is the top surface of oxidised sample suitable for the relatively thin oxide layers or the initial oxidation conditions. In order to visualise the grain boundary, the testing sample can be polished using mechanical and chemical-mechanical polishing methods. Particularly, it is necessary to select different etchants for different compositions in the oxide scale or metal substrate. Sometimes, this classical polishing/etching method cannot observe the grain morphologies both the oxide scale and the substrate concurrently. In electron backscattered diffraction (EBSD) technique and transmission electron microscope (TEM), the grain characters can be detected clearly without etching the sample. EBSD can use normal ion milling to prepare the sample. This is can make easier than TEM because TEM need to reduce the thickness of the sample using focus ion milling beam (FIB) or other advanced approaches. In a word, EBSD or TEM is generally used to observe the sample in cross-sectional direction, whereas scanning electron microscopy (SEM) can be used for top surface morphologies of oxidised sample. *In situ* electron microscopy cases are all the same but more challenging for the oxidation investigation.

Experimentally resolving and characterising grain boundary structure often requires a host of techniques: X-ray diffraction (XRD), scanning electron microscopy (SEM), electron backscattered diffraction (EBSD), transmission electron microscopy (TEM), X-ray energy dispersive spectroscopy (EDS) and electron energy loss spectroscopy (EELS). XRD or neutron diffraction normally deals with a texture that reflects an average value obtained from many different grains, that is, macrotexture. This chapter will address some techniques to obtain microtexture involving some individual grains.
