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 degrees 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 omega(o) precipitations was observed in the beta(o) region. The un-transformed beta(o) phase, which remained in the omega(o) interval, was examined using high resolution transmission electron microscope (HRTEM). It is shown that the growth of omega(o) precipitations and the morphology of residual beta(o) phases are related to the creep temperature and stress. Certain beta(o) stabilizer elements, such as Nb, segregate into alpha(2) plates and cause omega(o) precipitation during dissolution of alpha(2) plates at high temperature. The interfacial structure between the omega(o) and alpha(2) phases was studied in detail using HRTEM. Meanwhile, texture evolution and dynamic recrystallization of the gamma phase were studied using electron backscatter diffraction (EBSD). Observations on the deformation structure also demonstrate that omega(o) precipitation acts as an obstacle inhibiting crack propagation and growth of the gamma phase. The corresponding formation mechanism and possible effects on creep are discussed.