The Thermal Stability Of Refractory-metals Containing γ-TiAl Based Alloys Subjected To A Long-term Exposure 700℃ In Air

The thermal stability of a near lamellar (NL) alloy Ti-44Al-4Nb-4Zr-0.2Si-1B, subjected to exposure at 700℃in air for up to 10000 hours, has been studied in this thesis. The microstructural changes and phase transformation were investigated using scanning electron microscopy and transmission electron microscopy. The corresponding changes in room temperature and high temperature tensile properties and room temperature S-N fatigue limit were assessed along with exposure time. In addition, the creep resistance of NL alloy Ti-44Al-4Nb-4Zr-0.2Si-1B was studied at 700℃in air for up to 1000 hours, and compared to fully lamellar (FL) alloys Ti-44Al-4Nb-4Hf-0.2Si-1B and Ti-44Al-8Nb-1B.In contrast to the widespread decomposition ofα2 lamellae observed in Ti-44Al-8Nb-1B, the decomposition ofα2 lamellae through phase transformationα2â†'γand the formation of B2(ω) through phase transformationα2+γâ†'B2(ω) were found to be quite limited inside theα2+γcolonies during the same exposure. This tends to suggest that the Nb-Zr containing alloy is thermodynamically more stable than Nb-containing alloy.The main change during the long-term exposure was observed to occur in B2(co) grains which retained considerably at lamellar colony boundaries. B2(ω) grains consisted of many micron-sized B82-ωcells, and along the cell walls congregated D88-(o domains/particles as a network. It was observed that theωdomains/particles with highly ordered D88 structure in cell-wall regions grew significantly during the long-term exposure. Such D88-ωparticles were also found to form widely inside the cells. Moreover, micron-sized equiaxed y phase was also precipitated from B2(ω) grains. All these changes are suggested to be responsible for the considerable reduction of tensile ductility observed in the exposed alloy. However, no decrease in 0.1 pct proof stress, tensile fracture stress and high cycle fatigue limit was caused by the long-term exposure. The 0.1 pct proof stress was still maintained at a level of >600MPa, while fatigue limit was increased noticeably after 10000-h exposure. It was found that fully lamellar alloy Ti-44Al-8Nb-1B showed a better creep resistance than fully lamellar alloy Ti-44Al-4Nb-4Hf-0.2Si-1B and near lamellar alloy Ti-44Al-4Nb-4Zr-0.2Si-1B. This maybe attributed to the different microstructure of the alloys.