Microstructure and creep behaviors of a high Nb-TiAl intermetallic compound based alloy

By means of heat treatment, creep properties measurement and microstructure observation, an investigation has been made into the influence of heat treatment on microstructure and creep properties of the high Nb-TiAl alloy. Results show that the microstructure of as-cast high Nb-TiAl alloy consists of lamellar γ/α2 phases with various orientations. The irregular serrated boundaries with single γ phase are located in between the lamellar γ⧸α2 phases with various orientations. After solution and aging treatment, the microstructure of alloy consists of uniform and regular lamellar γ⧸α2 phases, and the regular boundaries are located in between the lamellar γ⧸α2 phases. Under the applied stress of 200MPa at 800°C, the creep lifetime of the as-cast high Nb-TiAl alloy is measured to be 147h, the one of the alloy after heat treatment is measured to be 297h. In the ranges of the applied temperatures and stresses, the creep activation energy of alloy after heart treatment is measured to be Q=432kJ/mol. The deformation mechanism of alloy during creep is dislocations slipping in the lamellar γ⧸α2 phases, the creep dislocations may be decomposed to form the configuration of the partials plus stacking faults. The deformation of as-cast alloy during creep occurs mainly in the irregular serrated boundary regions with single γ phase. For the heat treated alloy, the primary/secondary slipping systems of dislocations are alternately activated during creep, which results in the bigger plastic deformation of alloy to contort the lamellar α2/γ phases. In the latter stage of creep, the cracks are firstly initiated along the boundaries parallel to the lamellar γ⧸α2 phases, and propagated along the boundaries vertical or at about 45° angles relative to the stress axis up to the occurrence of creep fracture.