Intermetallic ¿-TiAl based alloys are an innovative class of materials dedicated to applications in the automotive and aeronautic industry. Especially their low density, high specific yield strength and excellent resistances against creep and oxidation make them a suitable choice for structural high-temperature components in combustion engines. However, in order to conquer new application areas and facilitate cost-effective production, a further improvement of their properties and processability is required. While the incorporation of additional alloying elements is a promising possibility, their respective impacts on the phase transformations and phase equilibria need to be considered. The overall aim of this doctoral thesis is the exploration of phase transformations in several ¿-TiAl based alloying systems. Especially the questions as to how technically important alloying elements affect the phase equilibria in the Ti¿Al system and how the associated phase transformations emerge during powder metallurgical processing are investigated. Answers to these questions are obtained through the synergy of several advanced experimental techniques. In-situ high-energy X-ray diffraction grants time-resolved insights into the present phases, their respective volume fractions and their lattice parameters as a function of temperature and alloy composition, while the phase transition temperatures are complementarily assessed by differential scanning calorimetry. Advanced microstructural characterization, e.g. scanning and transmission electron microscopy as well as electron back-scatter diffraction, allows the correlation of the present microstructural features with the phase transformation pathway. The studied elements Mo, Zr and Si are found to exhibit quite different effects with respect to the phase transformation behaviour of ¿-TiAl based alloys. The ß-stabilizing Mo significantly changes the microstructure evolution during solidification and subsequent heat treatments by evoking phase transformations not present in the binary Ti¿Al system. Both Zr and Si increase the stability region of the ¿ phase at the expense of the ¿ and ¿2 phase and lower the material¿s solidus temperature. However, while Zr constrains the single ¿ phase field region at high temperatures by lowering the ß-transus temperature, the precipitate-forming Si increases all solid-solid phase transition temperatures. With respect to the powder metallurgical processing, the presence of the high-temperature ¿ and ß phase in the powder of a W-containing ¿-TiAl based alloy is evidenced. Sufficient thermal exposure results in an equilibration through ordering of these phases and the formation of ¿ phase. Additionally, in-situ high-energy X-ray diffraction and a newly developed experimental setup are used to study the powder densification by spark plasma sintering with respect to the phase evolution for the first time. In addition to the equilibration of the non-equilibrium phase distribution of the powder, several equilibrium phase transformations determining the material¿s microstructure are observed. The knowledge obtained in this thesis provides both fundamental and processing-related insights into the phase transformations and phase equilibria in ¿-TiAl based alloys. Furthermore, future alloy developments and the accuracy of thermodynamic calculations will benefit from the assessed phase diagrams, transition temperatures and volume fraction evolutions.
|Translated title of the contribution||Untersuchung von Phasenumwandlungen und Phasenstabilitäten in Titanaluminid-basierten Legierungen|
|Publication status||Published - 2022|
Bibliographical noteno embargo
- Titanium aluminides
- Phase Transformations
- Phase diagram