Nicolas Vincent Barbi
This thesis is an exploration of deformation and fracture properties of a novel two-phase α2 + γ Ti46Al8Ta (at.%) alloy featuring a "convoluted" microstructure, produced by partners within a European Union project named IMPRESS. Properties of this alloy are compared with those of other two-phase TiAl alloys, to assess the performance of the novel convoluted microstructure in critical, load-bearing high-temperature applications. Uniaxial compression tests featuring repeated relaxation cycles are conducted from room-temperature to 1073K on three different α2 + γ TiAl alloys, namely Ti-46Al with 8Ta, 8Nb or 7Nb (at.%). Of these, the first two are novel "convoluted" cast titanium aluminides developed within the project, while the last is a more conventional alloy displaying a duplex microstructure. The data include measurements of the flow stress and activation volume of these alloys at 293, 673, 873, 923, 1023 and 1073 K. It is found that, despite differences in composition, processing and microstructure, the values and evolution with temperature or with stress of these parameters are relatively similar for the three alloys. This observation, coupled with the agreement of present results with data in the literature for similar alloys, leads to conclude that the strain-rate dependence of plastic flow in alloys of this class is governed by features of the γ-phase that are relatively insensitive to the α2 phase morphology. The convoluted alloys furthermore feature a yield stress anomaly previously documented for this class of intermetallic alloy. Fracture toughness measurements are conducted from RT to 923K on the Ti46Al8Ta (at.%) alloy, and compared with similar data gathered on two alloys featuring a lamellar microstructure, namely the same Ti46Al8Ta alloy heat-treated to feature a coarse lamellar structure, and a boron-refined industrial alloy of the XD class, of composition Ti45Al2Mn2Nb (at.%) + 0.2 (vol. %) TiB2, also heat-treated to display a lamellar structure. The convoluted alloy displays an initiation fracture toughness ranging from 12MPa√m at RT to 20MPa√gm at 923 K. Comparatively, at room temperature it slightly outperforms the fine lamellar XD alloy, while the coarse lamellar alloy of similar composition is significantly tougher. At 923 K, the convoluted alloy outperforms the fine lamellar XD alloy, the fracture toughness of the coarse lamellar is still superior but to a lesser extent. All alloys, including the convoluted alloy, show pronounced R-curve behaviour at both room temperature and 923 K. Examination of the crack path by cross-sectioning of samples having undergone interrupted toughness tests and slightly reloaded in the scanning electron microscope show roughly similar fracture characteristics for all three alloy. These observations, coupled with the fracture toughness data of this work, show that the convoluted alloy is toughened by the same mechanisms, of crack deflection, microcracking, and shear ligament toughening, as two-phase TiAl alloys having the lamellar structure, known to be optimal from the perspective of fracture toughness. The effects of temperature and atmosphere (vacuum, dry air and atmospheric air) are assessed on the Ti46Al8Ta alloy featuring a convoluted microstructure. Fracture toughness data suggest a marginal difference between dry or atmospheric air; however, the difference is within experimental error. During the tests, however, clear evidence was found to document that ambient humidity causes subcritical crack growth, presumably due to hydrogen embrittlement.
EPFL2010
