Effect of Co and W on the phase transformations and mechanical high-temperature properties of ß-solidifying ¿-TiAl alloys

Titanium aluminide-based alloys (TiAl) are intermetallic alloys with a density of around 4 g/cm³ that are renowned for their exceptional high-temperature properties. The development of the TNM alloy, a \gamma-TiAl alloy that has been expanded to include Nb, Mo and B, enables its use as a high-temperature material with continuous operation at temperatures up to 750°C. This is due to the stabilisation of the ductile \beta-phase, which enables processing via conventional manufacturing methods such as forging. This makes TNM relevant in the aerospace and energy sectors. The range of continuous service temperatures can be further expanded by chemical refinement. However, heat treatments to establish the required microstructures must also be adapted to the altered chemistry and shifted phase fields. This thesis therefore investigates the influence of adding 1 at.% Co (TNM1Co) and 1 at.% W (TNM1W) on phase transformations, microstructural development, and associated thermo-mechanical properties, compared with the reference material TNM. The powder metallurgical states after hot isostatic pressing, along with the heat-treated and water-quenched states, were examined using scanning electron microscopy (SEM) to determine the regions of the individual phase fields within the microstructure. Altered phase transition temperatures due to the different chemical compositions of the TNM variants were determined by differential scanning calorimetry in the laboratory. Correlatively, in situ high-energy synchrotron X-ray diffraction (HEXRD) experiments were carried out. Phase transformations and phase fractions during heating were derived by studying changes in crystal structure and Rietveld refinement, respectively. A temperature range with minimal \beta-phase content was determined for the TNM1W variant, enabling conventional production and development of a microstructure suitable for specific applications. In TNM1Co, the existence of ~3 wt.% \tau-phase was determined for the first time in TiAl alloys with a low Co content. Previously, the \tau phase had only been detected in TiAl alloys at higher Co contents (>1.5 at.%). In situ HEXRD demonstrated the thermal stability of the \tau phase up to 1095°C. The morphology and chemistry of the \tau precipitates were determined using SEM and energy dispersive X-ray spectroscopy, respectively. The size of the precipitates was found to be below 1 µm. The mechanical properties of the \gamma -TiAl alloy variants were determined through tensile tests at room and high temperatures, as well as short-term creep tests. In addition to varying the alloys, the influence of different microstructures was investigated. At room temperature, samples with a lower \beta_o content exhibited higher elongation at break, regardless of their chemical composition. TNM1Co proved to be ductile in high-temperature tensile tests starting at 750 °C. This is contrary to the TNM reference, which is accompanied by a significant decrease in strength to 390 MPa. Furthermore, short-term creep tests were conducted at a test stress of 200 MPa. Here, TNM1W proved to be significantly more creep-resistant than TNM1Co. For TNM1W, the time required for 1% strain to occur was 79.5 hours at 750°C and 8.1 hours at 800°C. The TNM1Co variant exhibited the same strain after only 41.8 hours at 750°C and 2.6 hours. These differences correlate with the adverse effect of increased \beta_o fractions, which allow for increased diffusion due to the B2 crystal structure. Alloying with W in TNM1W slows down diffusion-controlled mechanisms and reduces the negative effect of the increased \beta_o content to some extent. However, as Co forms the \tau phase in TNM1Co, it is bound and cannot act as a diffusion inhibitor. This results in reduced creep resistance compared to the TNM reference.