Development of TiAl alloys on the demand of additive manufacturing and high-temperature application

In the last decades, the successful alloy development in the field of titanium aluminides (TiAl) has led to the replacement of heavy Ni-base alloys in aerospace and automotive propulsion systems. Thereby, the used TiAl alloys, based on the intermetallic γ-TiAl phase, reach maximum service temperatures of about 750 °C and are manufactured via casting and/or forging. To increase the field of application for γ-TiAl based alloys, this thesis aims for higher application temperatures as well as new processing routes, i.e. predominantly powder metallurgical approaches like additive manufacturing. However, the additive manufacturing via electron beam melting (EBM) of established alloys presents process-related challenges. Therefore, the thesis deals with the process-microstructure-property relationship of EBM-manufactured TiAl and uses this knowledge to design new process-adapted TiAl alloys of the 4th generation. This requires a detailed microstructural characterization of the samples built with different process parameters with advanced methods as transmission electron microscopy, high-energy X-ray diffraction studies, energy dispersive X-ray spectroscopy, and mechanical testing up to 850 °C. The results show that the new alloys were chemically as well as microstructurally isotropic after manufacturing EBM samples with optimized parameters and a designed subsequent heat treatment. In parallel, the alloy powders were investigated after densification via spark plasma sintering (SPS), a manufacturing method that allows the evaluation of new alloys and their microstructures completely on solid-state processes without re-melting and, more specifically, the impact of varying Al contents, as it is pronounced in EBM specimens. Based on ex- and in-situ experiments, the kinetic and stability of nm-sized precipitates could be established for optimal high-temperature properties. Finally, the entire gained knowledge enabled the manufacturing of a resilient prototype of a new γ-TiAl based alloy of the 4th generation for high-temperature aerospace applications.