High Temperature Corrosion of Intermetallics

Intermetallic compounds can be simply defined as ordered alloy phases formed between two or more metallic elements. These materials have different crystal structures from those of the constituent metallic components and exhibit as long-range ordered superlattices. Their relatively low density, high melting point, high specific strength and due ductility make them the promising high temperature structural materials for aviation and aerospace applications. Among the big family of intermetallics, Fe-Al, Ni-Al and Ti-Al systems are attracting most of the attention. The objective of studies is to develop and utilize these intermetallic compounds as a type of important structural material whose overall properties is between nickel-based superalloys and advanced ceramics. However, a balance cannot always be achieved between mechanical and environmental properties. For example, iron aluminides have excellent resistance against oxidation and hot corrosion, however, their strength is relatively low. The higher specific strength and modulus than conventional Ni-based superalloys make Ti-Al intermetallic compounds of interest for aero-engine components, but the oxidation resistance of Ti-containing intermetallics is much lower than desirable; thus a key factor in increasing the maximum temperature in service is enhancing their oxidation and hot corrosion resistance while maintaining the excellent mechanical properties. This book is then intended to give an overview on the major efforts made over the last 20 years on high temperature oxidation and protection of intermetallic compounds including Fe-Al, Ni-Al and Ti-Al. In particular, the focus will be given to Ti-Al systems. After a general introduction on the structural and mechanical properties, the studies on the oxidation behaviours of these intermetallic compounds will be summarised based on the experimental observation reported in open literature. The emphasis will be put on the effects of alloying element, microstructure and coating/surface treatment. It should also be noted that only high temperature oxidation properties in air or oxygen are addressed; no discussion on hot corrosion, carburization, nitridation and sulfidation.