Aerospace Alloys

In this chapter the main structural aspects and working principles of the gas turbine aero-engines are recalled, with the main aim to highlight the most interesting properties of the materials used for the main components of these engines. Over the years, the new requirements for better performing gas turbine aero-engines have been sustained by the availability of novel materials. The last section is specifically dedicated to the latest developments in aerospace gas turbine engines, in order to highlight the future trends and guidelines on which materials research efforts are concentrated.

This chapter deals with the metallic materials used for structural aircraft components. The main features of fixed-wing aircrafts will be recalled, with a specific focus on the properties and relevant applications of the so-called light alloys. Aluminum and magnesium alloys will be considered, as concerns the main aspects of extraction metallurgy, material processing, and tempers. The applications in aircraft structures will be discussed, also to highlight the development in composition and processing routes that these two classes of alloys have undergone to meet the requirements of the new aircraft structures. In this regard, an important role has been played by laminate composites, using aluminum alloys as metallic component. For these materials too, the main processing steps will be presented, to demonstrate their influence on the mechanical properties of the final products. A separate chapter (Chap. 4) is dedicated to titanium alloys. This choice relies on the fact that, although part of the light alloys group, titanium alloys are used not only for structural parts but also for gas turbine aero-engines.

Titanium and titanium alloys are fundamental constituents of several parts of aircrafts, owing to their unique combination of properties: high specific strength, low coefficient of thermal expansion, moderate density, long fatigue life, creep strength, fracture toughness, and excellent corrosion resistance induced by the spontaneous formation of a TiO₂ surface passivating layer. An indirect proof of the great interest for titanium alloys as fundamental aerospace materials can be inferred from their wide range of applications, from structural components to engine parts. This interest is bound to continue in the future, sustained by the ongoing research focused on the development of new alloys, like Ti-aluminides, exhibiting improved properties, compliant with the design requirements emerging even from novel priorities, like fuel saving and reduction in air pollution. This chapter is entirely dedicated to titanium and its alloys, with particular focus on metallurgical issues and production processes. Furthermore, like for the other light alloys seen in Chap. 3, the main applications in the aerospace field are presented.

The three main groups of superalloys, cobalt-, iron-, and nickel-based, are presented, with reference to their compositional and processing aspects. The main selection criteria for designing the complex alloy compositions are illustrated. Superalloys, since the early stages of their development, have gained an ever-increasing role in the fabrication of components of gas turbine aero-engines, owing to the excellent combination of structural properties and corrosion resistance, both retained up to relatively high temperatures. These aspects have been enhanced further by processing routes, like directional solidification and single-crystal investment casting. Through the elimination of grain boundaries perpendicular to the main stress axis, a significant reduction of the diffusive creep rate has been attained. Diffusive creep is one of the deformation mechanisms in the Ashby maps, discussed in this chapter, together with another physical metallurgy issue: the superalloy strengthening by the precipitation of ordered γ′ phase. The microstructure of the superalloys and, thereby, their mechanical properties are usually refined by thermal treatments, whose main working principles are presented. Eventually the main applications of this fundamental class of alloys are introduced.

The main coating systems used in aerospace gas turbine engines are presented. Coatings are fundamental to protect the surface of the structural components from several degradation factors, like oxidation, corrosion, wear, and erosion. The complexity of the environments and servicing conditions of gas turbine engines requires properties and materials performances that can be attained only through the deposition on structural components and hot parts of composite coating systems. Several of them involve metallic layers, although it is just thanks to a suitable combination of diverse materials, featuring altogether the full range of needed properties, that an effective part protection can be assured. Temperature, as usual, is paramount, since it determines the kinetics of all relevant transformations. Therefore, thermal barrier ceramic coatings are included in this chapter, since they are fundamental for lowering the temperature in the underlying metallic component. Still, the underlayer of a thermal barrier coating, the so-called bond coat, is metallic, and it is paramount as far as an adequate resistance to high-temperature corrosion phenomena is concerned. The main deposition techniques, whose selection is crucial for obtaining a good quality control and durability of the coating systems, are also introduced.

When dealing with metallic alloys, a chapter on high-temperature corrosion is mandatory. Oxidation and hot corrosion are the main phenomenological aspects interesting for gas turbine engine alloys. The reactions of the base metals and coatings with the outer atmosphere, the exhaust gas flow from the combustion chamber, the salt deposit, both liquid and solid, resulting from the condensation of corrosive products, induce important changes in the crystallo-chemical features of the alloy outer layers. The obvious consequences of these changes on the structural and aerodynamic properties of the component render the effects of high-temperature corrosion as important life-limiting factors, and this justifies the relevant research interests. Although high-temperature corrosion is the main topic of the chapter, the general principles of aqueous corrosion are also recalled. Aluminum, magnesium, and other alloys used in aircraft structures may suffer from corrosive phenomena due to the exposure to standard service environmental conditions, not involving high temperatures necessarily. Therefore, general principles of electrochemical corrosion will be recalled, explained, and supported by relevant case histories. Some of the principles exposed in the first section will be useful anyway for the treatment of specific issues of high-temperature corrosion, considered in the second section.

Four groups of materials, which are being considered in view of their potential applications in the aerospace field, are presented. Refractory metal, oxide dispersion strengthened and intermetallic alloys are good candidates for replacing nickel-based superalloys, mainly to increase the gas turbine operating temperatures. The main critical issues for each class of materials and relevant remediation strategies are discussed. Shape memory alloys are the fourth class of materials considered herewith: they are fundamental for the development of structures, exploiting the functional properties of these alloys, and that would otherwise require complex design and more numerous components. This design simplification has several interesting aspects, including increased reliability and weight reduction, as several satellite and spacecraft applications of shape memory alloys have proved already in a number of practical solutions for strategic devices in aerospace applications.