Microstructural Characteristics and Creep Behavior of 45XD TiAl Alloys

A near lamellar microstructure and two fine-grained fully lamellar (FGFL) microstructures in Ti-45Al-2Nb-2Mn+0.8 vol%TiB2 alloy were prepared by selected heat treatments, and the fully lamellar microstructures were aged for stabilizing the lamellar plates. Microstructural examination and tensile creep tests at 760°C showed that the near lamellar microstructure possessed inferior creep resistance due to its coarse lamellar spacing and its larger amount of equiaxed γ grains at colony boundaries. The fine lamellar spacing as well as the fine lamellar colony size gave a major contribution to make the minimum creep rates smaller in the fully lamellar TiAl alloys. Since aging treatments stabilized the lamellar microstructures and delayed the degradation process during creep deformation, the aged samples exhibited lower minimum creep rate and longer creep life than the corresponding samples without the aging treatment. These results suggest that a fine as well as stabilized fully lamellar structure is a critical factor to improve the creep resistance of TiAl alloys in terms of short and long-term creep.