Intermetallic alloys based on orthorhombic titanium aluminide

Orthorhombic titanium aluminides represent the youngest class of alloys emerging out of the group of titanium aluminides. These new materials are based on the ordered orthorhombic phase Ti2AlNb, which was discovered for the first time in the late 1980s as a constituent in a Ti3Al-base alloy. In the 1990s primarily simple ternary Ti-Al-Nb orthorhombic alloys were investigated in countries such as the US, UK, India, France, Japan, and Germany. The drive was mainly provided by jet engine manufacturers and related research labs looking for a damage-tolerant, high-temperature, light-weight material. This follows the aim of further extending the use of lower density titanium-base materials in temperature regimes, where heavy nickel-base superalloys are the only alternative today. The present understanding of microstructure-property relationships for orthorhombic titanium aluminides reveals an attractive combination of low and high temperature loading capabilities. These involve high room-temperature ductility and good formability, high specific elevated temperature tensile and fatigue strength, reasonable room-temperature fracture toughness and crack growth behavior, good creep, oxidation, and ignition resistance combined with a low thermal expansion coefficient. This article reviews the aspects of composition-microstructure-property relationships in comparison to near-alpha titanium, TiAl, and nickel-base alloys. Special emphasis is also placed on the environmental degradation of the mechanical properties.