ABSTRACT
Research on intermetallic alloys and composites has been primarily driven by the demand of the aerospace industry for advanced materials with high specific strength and stiffness, as well as the ability to retain their strength and resist environmental degradation at elevated temperatures. Ordered intermetallics based on silicides and aluminides have exhibited significant promise in the form of attractive properties for high-temperature structural applications, including excellent oxidation and corrosion resistance, relatively light material weight, and superior strength at elevated temperatures.1 The relevant properties of selected silicides and aluminides are presented in Table 5.1.2-10 If Young’s modulus and densities are considered, it is obvious that the lightweight aluminides exhibit higher specific stiffness than the Ni-based superalloys. The results of studies on the evaluation of the physical and mechanical properties of silicideand aluminide-based intermetallic alloys and composites have motivated researchers to attempt further development for several potential applications. The primary motivation for such efforts is the distinct superiority of the properties of selected intermetallic materials compared with those of other conventional alloys. The potential applications include hot-end aeroengine and automotive components, tools and dies, corrosion-resistant piping materials, claddings and coatings for chemical industries, heattreatment fixtures, magnetic and electronic devices, and hydrogen-storage materials.11 Stoloff has discussed the potential applications of various types of intermetallics, which include primarily aluminides and silicides. An account of potential or ongoing structural applications of MoSi2-based composites may be found in the reviews by Petrovic12,13 and Yao et al.14 Furthermore, important design issues, strategies for processing and development, and the advantages and limitations of candidate intermetallic materials including γ-Ti-aluminides, as well as Mo-and Nb-silicides,
have been discussed in a review by Lipsitt et al.15 The basic objectives of research on intermetallics for structural applications at elevated temperatures include the development of materials with the following properties: (i) high melting points, (ii) density preferably less than those of currently used alloys, (iii) brittle-to-ductile transition temperature (BDTT) as low as possible, (iv) room-temperature fracture toughness of ≥15.0 MPa m−2, and (v) impressive oxidation resistance at possible temperatures of exposure. It may be noted that poor fracture toughness adversely affects the resistance to fatigue damage, which in turn is one of the key handicaps in the dynamic load-bearing applications of silicides.
