Processing and Characterization of TiAl-based Alloys: Towards an Industrial Scale
M. Thomas, M.-P. Bacos
This paper highlights Onera efforts focused on the design of new TiAl-based alloys, the development of a commercially-viable route for the manufacture of aero engine components and the optimization of mechanical properties. The alloy G4, with a duplex microstructure, has been developed with an excellent balance of properties for gas turbine applications up to 800°C. Additionally, a series of TiAl-3(Fe,Zr,Mo) alloys have been designed for applications with good ductility requirements. Since these alloys were developed for the casting route, results show that the minimization of subsequent heat treatments is required for property scatter reduction. Alternatively, the Powder Metallurgy (PM) route was explored with the aim of establishing well defined microstructure-property relationships. Moreover, microstructural changes and related phase transformations were fully clarified in the now well known 47-2-2 alloy. This thorough understanding then enabled us to optimize the microstructure of the 47-2-2 alloy via process parameters, in order to meet industrial requirements in terms of tensile, creep, and fatigue properties. Furthermore, in the last three years, alternative PM processing routes, by means of Spark Plasma Sintering and Direct Metal Deposition processes, were found to provide enhanced tensile properties. Finally, structural factors such as the surface-related embrittlement and the formation of oxide scales that occurs on the degradation of mechanical properties, and in turn on the temperature limitations of the TiAl-based alloys, were identified. Different approaches aimed at improving the oxidation/corrosion resistance are then described to achieve a better environmental durability for TiAl-based alloys.