Microstructure and Creep Behavior of As-cast TiAl-Nb Alloy

The creep behaviors and deformation features of the as-cast TiAl-Nb alloy were investigated at temperature 890 similar to 900 degrees C by creep properties measurement and microstructure observation. Results show that the microstructure of as-cast TiAl-Nb alloy consists of gamma/alpha(2) phases with the lamellar feature, and the boundaries with irregular serrated configuration consist of single gamma phase, and locate between the lamellar gamma/alpha(2) phases with different orientations. The as-cast TiAl-Nb alloy displays a better creep resistance and a longer creep life at high temperature. The deformation mechanism of the alloy during creep is significant amount of dislocations shearing into the lamellar; gamma/alpha(2) phases in the form of dislocation rows. Moreover, a large number of dislocations slipping in the matrix may react to form the dislocation networks, which may promote the climbing of dislocations to retard the stress concentration and to improve the creep resistance of the alloy. Compared to alpha(2)-Ti3Al phase, the gamma-phase possesses a weaker strength; therefore, the crack is easily initiated along the boundaries with single gamma phase which is at about 45 angles relative to the stress axis, and propagate along the boundaries being parallel to the orientation of the lamellar gamma/alpha(2) phases up to the occurrence of creep fracture, which is thought to be the fracture mechanism of the alloy during creep. Thereinto, the tearing edges formed on the surface of the fracture are inclined with the lamellar gamma/alpha(2) phases, which is attributed to the alpha(2)-Ti3Al phase with better strength hindering the crack propagation during creep.