Intermetallic alloys based on orthorhombic titanium aluminide

Orthorhombic titanium aluminides represent the youngest class of alloys eme rging out of the group of titanium aluminides. These new materials are base d on the ordered orthorhombic phase Ti2AlNb, which was discovered for the f irst 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 investigat ed 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 titani um-base materials in temperature regimes, where heavy nickel-base superallo ys are the only alternative today. The present understanding of microstruct ure-property relationships for orthorhombic titanium aluminides reveals an attractive combination of low and high temperature loading capabilities. Th ese involve high room-temperature ductility and good formability, high spec ific elevated temperature tensile and fatigue strength, reasonable room-tem perature fracture toughness and crack growth behavior, good creep, oxidatio n, and ignition resistance combined with a low thermal expansion coefficien t. This article reviews the aspects of composition-microstructure-property relationships in comparison to near-alpha titanium, TiAl, and nickel-base a lloys. Special emphasis is also placed on the environmental degradation of the mechanical properties.