Orthorhombic Titanium Aluminides: Phases, Phase Transformations and Microstructure Evolution

The development of light weight high temperature materials with high strength and high creep resistance is strongly driven by turbine engine manufacturers. Titanium aluminides represent an important class of alloys, which provide a unique set of physical and mechanical properties that can lead to substantial payoffs in future aircraft engines. The class of orthorhombic titanium aluminides based on the intermetallic phase Ti AlNb represents the youngest material emerging out of the group of titanium aluminides. The monolithic Ti AlNb base material exhibits an attractive combination of low temperature ductility and high temperature tensile strength and creep resistance. The mechanical property profile and the high chemical compatibility of orthorhombic alloys with SiC fibers in titanium matrix composites make them also very attractive for high temperature use. In this paper the phases, phase transformations, and microstructure evolution by thermomechanical processing of orthorhombic alloys is described based on investigations of the Ti AlNb base alloy Ti–22A1–25Nb (at.%). Thermomechanical processing was successful in producing equiaxed as well as defined coarse and fine grained lamellar microstructures. Depending on heat treatment two and three phase microstructures were produced, i. e. beside the ordered orthorhombic O phase, the ordered β phase (B2) and the α phase (DO ) were determined.