**4. Roles of grain boundaries in oxide scale**

Fe//{110}FeO, <110>Fe//<110>FeO. For a very thin oxide scale, a Fe/FeO orientation relation-

Effects of grain orientation and grain boundary characters on the elevated temperature oxidation behaviour demonstrate the role of grain boundaries in enhancing high-temperature oxidation resistance of various polycrystalline steel alloys. Overall surface energy and tribo-

Grain boundaries can be classified geometrically in terms of the relative misorientation between the neighboured grains. This relative misorientation can be defined by misorientation axis and angle. For instance, 2° ≤ *θ* < 15° misorientations are defined as low-angle grain boundaries (LAGBs), whereas the high-angle grain boundaries (HAGBs) are ≥15°. Certain specific combinations resulting in a coincidence site lattice (CSL), the degree of coincidence is represented by the reciprocal density of common lattice points, denoted as the Σ number. As such, special grain boundaries refer to the low ∑ (∑ ≤ 29) CSL boundaries, even though

In oxidation and corrosion, it is widely believed that HAGBs have undergone hot corrosion and substantial depletion/segregation of alloying elements through the entire cross section. Distribution of grain boundaries in surface layer of oxide scale reveals that the misorientation tends to be large near grain boundaries, particularly at the oxide-substrate interface, where

Low-energy CSL boundaries with higher mobility can enhance the resistance of cracking or oxidation [15, 16]. CSL boundaries with low Σ orientation (Σ ≤ 49) display improved physical and chemical properties relative to general or high CSL boundaries (Σ > 49) [17]. Some studies [18] reported that the resistance to intergranular oxidation of Ni-Fe alloy increased upon increasing the fraction of special boundaries. The extent of oxidation of individual Σ boundaries in Ni-Fe alloys is based on morphological observations. It found that Σ3, Σ11 and Σ19 were

In a oxidised microalloyed low carbon steel [12], a high proportion of low-angle and low-ΣCSL boundaries, magnetite for 60°/<111> (Σ3), and hematite for 57.42°/<1-210> (Σ13b) and

<0001> in the angle range of 27°–63° and <102> in the angle range of 63°–83°. For α-Fe<sup>2</sup>

O3

It is noted that coherent twins have been excluded from this analysis, which results in a significantly lower fraction of Σ3 boundaries. In any case, it becomes clear that these low CSL grain

relatively high densities correspond to 57.42°/<110> (Σ13b) and 84.78°/<010> (Σ19c).

O3

O4

has a profound fraction of Σ7, Σ13b and Σ19c.

for axes near

carries a high

O3, the

84.78°/<0-110> (Σ19c) can be found. Misorientation peaks occur in α-Fe<sup>2</sup>

Furthermore, CSL boundaries distributions in **Figure 4** [17] reveal that Fe<sup>3</sup>

logical behaviour can also be enhanced by grain boundary engineering.

ship was {100}Fe//{100}FeO, <100>Fe//<110>FeO [13].

**3.3. Character distribution of grain boundaries**

there is no physical basis for this assumption [14].

the high fraction of small magnetite grains is accumulated.

more resistant to oxidation than other Σ boundaries [19].

proportion of Σ3, Σ5 and Σ7, whereas α-Fe<sup>2</sup>

*3.3.1. Grain boundaries*

66 Study of Grain Boundary Character

*3.3.2. Special grain boundaries*

The grain boundaries of either wustite, magnetite or hematite play a significant roles in the oxidation of metals and during their processing at high temperature. Three stages can be divided into: (i) diffusion-controlled oxidation of metal alloys; (ii) the plastic deformation mechanism near grain boundaries and resulting fracture of oxide scales; and (iii) tribological properties of oxide scale consisting various different grain boundaries during metal processing. All these above are this section will address.

#### **4.1. Diffusion-controlled oxidation mechanism in grain boundaries**

Grain characters, such as grain shape or grain boundary, highly influence the oxidation kinetics of pure metals and alloys. Grain boundary diffusion is more predominant in iron metal oxidation at high temperature. At low temperatures, the role of grain boundary diffusion as a main factor in comparison with other short circuits remains elusive.

To understand the role of grain boundary in diffusion-controlled oxidation, it is essential to detect which types of grain boundaries are involved. The EBSD/X-ray energy dispersive spectroscopy (EDS) map scanning can analyse the elemental distribution and correlate with the grain boundary character, and hence visualise the type of grain boundaries that are susceptible to hot corrosion or oxidation. **Figure 5** shows the image quality plus grain boundary map of the cross section of the hot corrosion alloy 617 [14] and EDS elemental distribution maps of various alloying elements. Preferential segregation/depletion of alloying elements occurred at grain boundaries: the segregation of Mo, S, Co and Ni at the random HAGBs along with a depletion of Cr after hot corrosion. The presence of S segregations also at intact interfaces and at oxide grain boundaries affects the oxide growth mechanism [20]. ∑3 boundaries show few preferential enrichment/depletion of any alloying element, that is, indicating that these boundaries are resistant to hot corrosion.

**Figure 5.** (a) Image quality plus grain boundary map (colour code: ∑3-red, ∑9-blue, ∑27-green, random HAGBs-black) and EDS elemental map showing distribution of (b) Mo, (c) S, (d) Co, (e) Ni, (f) Cr, (g) Al and (h) O across the cross section of the alloy 617 after hot corrosion testing [14].

Two dominant diffusion can occur at grain boundary or lattices [8]. Surface diffusion happens at lower temperatures compared to grain boundary diffusion, and volume diffusion is active only at very high temperatures. With a small grain size, the higher grain boundary area naturally increases grain boundary diffusion [21]. Grain boundary diffusion is more sensitive to grain size when compared to volume diffusion. In contrast to lattice diffusion, the control of elemental diffusion at the grain boundaries can be effective to have a thin and compact oxide scale on the Fe-Cr alloy surface [22]. This suggests that the grain boundary diffusion is confined at the initial of oxidation, while the oxide layer is relatively thin.

In a stainless steel of cyclic steam oxidation, the previous results [23] indicate that grain boundaries not only promote the chromium outward diffusion, but also provide the fast diffusion paths for the oxygen penetration. The grain boundaries promote the iron outward diffusion, accompanied with the fast growth of interfacial voids between two oxide layers. In a Ni-5Cr alloy, intergranular selective oxidation also accompanied by local chromium depletion and diffusion-induced grain boundary migration. Recently, coupled transmission electron microscope (TEM)/APT surface and grain boundary oxide compositions were identified, and Ni enrichment was observed around the oxides. The data provide novel information on the role of the minor impurities and the formation of early-stage oxides in 304 stainless steel [24]. However, copper diffusion along grain boundaries is not the main mechanism in this case. A high-resolution characterisation of the oxide–metal interface has shown the presence of a Fe-rich oxide, less dense than the original Cr-rich oxide [25]. It is reasonable that the Gibbs free energy reduction with Cu spinel solid solution formation in hematite at high oxygen partial pressure induces the bulk diffusion of Cu through hematite grains to the top surface of external oxide [26].

Various diffusion mechanisms can differ from types of grain boundaries in different oxidised substrates, for example, CSL special grain boundaries in ferritic stainless steel [18], whereas high-angle grain boundaries in Al<sup>2</sup> O3 [27]. Diffusion-controlled oxidation mechanism of the oxides thermally grown on the metal surface is similar to the pure oxides in bulk ceramics, ranging from a point effect mechanism to migration of disconnections, grain boundary ledge defects [27].

#### **4.2. Deformation mechanism near grain boundaries**

Two dominant diffusion can occur at grain boundary or lattices [8]. Surface diffusion happens at lower temperatures compared to grain boundary diffusion, and volume diffusion is active only at very high temperatures. With a small grain size, the higher grain boundary area naturally increases grain boundary diffusion [21]. Grain boundary diffusion is more sensitive to grain size when compared to volume diffusion. In contrast to lattice diffusion, the control of elemental diffusion at the grain boundaries can be effective to have a thin and compact oxide scale on the Fe-Cr alloy surface [22]. This suggests that the grain boundary diffusion is

**Figure 5.** (a) Image quality plus grain boundary map (colour code: ∑3-red, ∑9-blue, ∑27-green, random HAGBs-black) and EDS elemental map showing distribution of (b) Mo, (c) S, (d) Co, (e) Ni, (f) Cr, (g) Al and (h) O across the cross

In a stainless steel of cyclic steam oxidation, the previous results [23] indicate that grain boundaries not only promote the chromium outward diffusion, but also provide the fast diffusion

confined at the initial of oxidation, while the oxide layer is relatively thin.

section of the alloy 617 after hot corrosion testing [14].

68 Study of Grain Boundary Character

This section will discuss the internal stress state after diffusion-controlled oxidation of metal alloys and plastic deformation of oxide scales during metal processing. The occurrence of concomitant grain boundary sliding in the thermally grown oxides may be evidenced leading then to the corresponding microscopic strain. Local strain caused by the oxidation of magnetite to hematite can cause inter crystalline microcracks. These microcrackings can induce plastic deformation under differential contraction and to open diffusion paths inducing grain boundary diffusion.

Cracking propagation can roughly attribute to alloying elements segregation at grain boundaries. To delve, then which types of grain boundaries will occur these elements accumulation, and which types of alloying elements would be detrimental to crack propagation? For example, the Co oxide enriches at the boundaries of high stacking fault (SF)/low SF grains [28] and the Ni/Ti/Al-rich oxides at normal grain boundaries. But the enrichments of these elements have slightly influence on crack initiation and propagation in some Ni-based superalloy.

The mechanical stresses in the oxide scale play a significant role in its integrity. Generally, internal stresses are induced by the growth of oxides, thermal expansion mismatch and applied forces [6], some of which originate from many different causes. The formation and propagation of cracks generally occur along grain boundaries of oxide scale. The stress is the greatest at the tips of small cracks in the material, and consequently, the reaction proceeds at its greatest rate from these tips. To alleviate the propagation of cracks, low-angle and low-ΣCSL boundaries in microstructure can offer obstacles, because they minimise the solute effects and reduce the interaction between the interfaces and glissile dislocation. In the case of magnetite/hematite scale [17], the oxide scale is easy to crack in presence of Σ13b and Σ19c in α-Fe<sup>2</sup> O3 compared to Fe<sup>3</sup> O4 with Σ3. Thus, it is possible that during this time, tailoring specific 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 is desired.

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

**Figure 6.** (a) High-angle annular dark field (HAADF)—scanning transmission electron microscopy (STEM) images of Fe–12Cr–5Al alloys (Y<sup>2</sup> O3 + ZrO<sup>2</sup> ) and (b) diffraction patterns after being mechanically alloyed and extruded at 950°C [29].

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 at which they were formed.
