Aluminides are intermetallic compounds. A wide range of potentially useful alloys of these have been investigated with attention now concentrated on gamma (Ti-Al) and orthorhombic alloys. Alpha and super-alpha compositions (TiAl3 based) are no longer of significant interest.
Aluminide base alloys offer superior high temperature performance with low weight and non-burn. Gamma aluminide turbine blades based on Ti47Al2Cr2Ni have been tested and are strong as nickel based alloys up to 760°C (1400°F) and half the weight.
Aluminides have generally proved difficult to process, have limited heat treatability, and generally low ductility at room temperature. Work continues however in an attempt to optimise these characteristics and deliver useful alloys for specific applications.
Alloys based on orthorhombic phase Ti2Nb are candidate materials for use in future aeroengine gas turbines, due to their low density, high strength retention at elevated temperatures and their potential as matrix materials for fibre reinforced matl matrix composites (MMC’s). They have a desnity of 5.5g/cm3 (0.2lb/in3), which makes them lighter than steels or nickel based alloys they would be likely to replace. They do not suffer from the poor ductility and fracture toughness of gamma nased alloys, and possess better burn resistance compared to traditional titanium alloys. The most significant weakness appears to be poor oxidation and corrosion resistance, especially with the presence of some alloying elements. Research continues to fcus on understanding phase equilibria and stability; optimising composition, mechanical properties and processing routes; and enhancing the oxidation and corrosion resistance.