CREEP OF ALPHA-2+BETA TITANIUM ALUMINIDE ALLOYS

The interest in titanium aluminide alloys includes elevated temperature applications, for which creep resistance is a primary property. Tests have been made between 650 and 870-degrees-C on a variety of microstructures of Ti-24Al-11Nb and Ti-25Al-10Nb-3Mo-1V (at%) alloys. It has been found that microstructure plays an important role in creep of these materials, so that thermal and mechanical history is important. Stress exponents for power-law creep, and apparent creep activation energies, have been determined for these alloys. As is usually found in structural alloys microstructural characteristics which increase ductility and toughness at low temperature tend to accelerate creep considerably, particularly the presence of beta-phase, and most notably when arranged as locally-continuous beta-films between plates of the alpha-2 phase. Solution treatment in the beta-phase provided optimum creep resistance, but cooling rate effects were different in the two alloys considered. Comparison to near-alpha titanium alloys developed for creep resistance, shows that the aluminide alloys have better performance, especially above 700-degrees-C.