Technology, properties and applications of intermetallic γ-TiAl based alloys

This paper reviews industrial scale thermomechanical and near-net shape processing of γ-TiAl based alloys (γ alloys). The progress achieved in forging of large ingots, rolling of sheets from forged ingots as well as HIPed powders, single and multistep extrusion of ingots and HIPing of prealloyed powders to near-net shape parts is presented. The impact of the different prematerial routes on process economy and the quality of semi-finished products will be discussed. The problems which arise when heat treatments are transferred from small lab furnaces to large industrial furnaces are outlined. The superplastic behaviour of γ-TiAl sheet material is described and results of superplastic forming are presented. Different joining techniques have been screened for γ-TiAl based alloys including laser welding, brazing and solid-state diffusion bonding. Although the oxidation resistance of γ alloys is remarkably higher than that of Ti alloys the need for reliable oxidation resistant coatings for long-term application at temperatures > 700°C is anticipated. Results of cyclic oxidation tests on coated γ-TiAl samples are presented. Finally, some potential application fields for γ-TiAl alloys are outlined.