Microstructure and properties of high-Nb containing TiAl-base alloys

This paper summarizes our recent studies of a high-Nb containing TiAl-base alloys in the areas of the phase relationships, microstructure evaluation, microstructure dependence of mechanical properties, and deformation characteristics. It was found that the Nh addition increases both the melting range and eutectoid temperature of the Ti Al alloy system, but decreases the alpha transus and the minimum Al solubility in gamma phase. Consequently, the addition extends the beta-phase field to higher Al concentration, but contracts the a-phase field. A gamma+alpha(2)+B2 ternary phase field was determined to exist between 1100 degrees C and 1250 degrees C. The presence of B2 (omega) phase in Ti-45Al-10Nb was found to be detrimental to their room temperature (RT) ductility and high temperature (HT) strength. Two important deformation features in gamma phase containing high Nb contents were analyzed to be high critical resolved shear stress (CRSS) for ordinary slip (similar to 180MPa at 900 degrees C), and relatively low stacking fault energy (SFE) (45-55 mJ/mm(2)). The high CRSS and the difficulty in dislocation movement due to low SFE are believe to be the major factors responsible for excellent HT strength-retention of Ti-45Al-10Nb alloys.