Precipitation behavior of ωo phase and texture evolution of a forged Ti-45Al-8.5Nb-(W, B, Y) alloy during creep

Creep rupture tests of the as-forged Ti-45Al-8.5Nb-(W,B,Y) alloy with a duplex microstructure were carried out at 750-850 °C and 130-300 MPa. An investigation was performed to study the effects of temperature and stress on the microstructure and texture evolution, as well as on the creep properties of the forged high Nb-containing TiAl alloy with a duplex microstructure. The results show that the stress exponent n = 2.8-3.3, and the creep activation energy values are in the range of 300-400 kJ/mol. After creep, a regular arrangement of ωo precipitations was observed in the βo region. The un-transformed βo phase, which remained in the ωo interval, was examined using high resolution transmission electron microscope (HRTEM). It is shown that the growth of ωo precipitations and the morphology of residual βo phases are related to the creep temperature and stress. Certain βo stabilizer elements, such as Nb, segregate into α2 plates and cause ωo precipitation during dissolution of α2 plates at high temperature. The interfacial structure between the ωo and α2 phases was studied in detail using HRTEM. Meanwhile, texture evolution and dynamic recrystallization of the γ phase were studied using electron backscatter diffraction (EBSD). Observations on the deformation structure also demonstrate that ωo precipitation acts as an obstacle inhibiting crack propagation and growth of the γ phase. The corresponding formation mechanism and possible effects on creep are discussed.