**6. Conclusions and future directions**

This chapter covers the recent advance associated with grain boundaries in the oxide scale formed on metal alloys during metal processing. A number of benefits include (i) characterisation of grain boundaries ranging from microstructure, preferred orientations and different types of grain boundaries; (ii) the role of grain boundaries in the oxide scale playing in diffusion-controlled oxidation, deformation mechanism and tribological performance; and (iii) introducing the experimental techniques and analytical methodology underpinning this subject.

Some specific results can be concluded ranging from micotexture and grain boundaries characters. The (100) plane of magnetite is much more sensitive to the oxidation. In the coincident site lattice (CSL) boundaries, the ∑3 in magnetite and ∑13b in hematite are dominant in the oxide scale. These findings suggest that low-angle grain boundaries and low-energy CSL boundaries can be used to prevent the initiation and propagation of cracks, further to enhance oxidation resistance of the materials.

Three current challenges dominate in the characterisation, mechanism and techniques for investigation the grain characters in oxide scale during metal processing at high temperature. First, grain characters consisting of grain shape and size, phase grain boundaries within oxide scale and orientation relationship between oxides are also need to be considered. Second, to delve which types of (special) grain boundaries to enhance the oxidation/corrosion resistance and then to tailoring them. Finally, a combination of the various advanced techniques provides the frameworks for future investigation on the oxidation of the other metal alloys even bulk ceramics.
