Thermal stability of a newly developed Mn containing β-solidifying γ-TiAl intermetallic compound at 750 °C

A newly low-cost beta-solidifying gamma-TiAl alloy Ti-44Al-3Mn-0.4Mo-0.4W-0.1B-0.1C (in at. %, named as TMMW) was invented. The rolled bars with a diameter of 12 mm were successfully fabricated by conventional hot rolling process, directly carrying out from the ingot without the pre-forging or hot-packing. The heat-treated TMMW rolled bar, which had a fine-grained nearly lamellar (NL) microstructure, was then air-exposed at 750 degrees C for up to 3000 h. The changes of the microstructure during this exposure were characterized using electron probe X-ray micro-analyzer (EPMA), transmission electron microscope (TEM), electron backscattered diffraction (EBSD) and high-energy X-ray diffraction (HEXRD). The tensile properties of the samples before and after thermal exposure were also tested. The fine-grained NL microstructure is found to be thermodynamically stable during the exposure. Meanwhile, the combined addition of Mo and W shows a positive effect on the stabilization of beta(o) phase at colony boundaries, with only slight precipitation of the nanometer beta(o) phases with no other brittle phases, such as Laves, in the metastable alpha 2 lamellae. It is found that the thickness of the alpha(2) lamellae decreases after 500 h exposure, while remains basically unchanged from 500 h to 3000 h, which is contributed by the effective pinning effect of the precipitated beta o phase in alpha(2) lamellae. As a result, the exposure-induced embrittlement under room temperature and elevated temperature does not take place. Interestingly, the ductility tested at 800 degrees C improves significantly after exposure. As for the tensile strength, it is only reduced by less than 10 % after 500 h exposure, while it remains essentially unchanged when increasing the exposure time from 500 h to 3000 h. Finally, the mechanisms of the precipitation of beta(o) phases in the alpha(2) lamellae and the tensile properties evolution during the exposure were carefully discussed.