Characterization of Microstructure of Crept Nb and Ta-Rich gamma-TiAl Alloys by Automated Crystal Orientation Mapping and Electron Back Scatter Diffraction

Understanding of the creep behavior Nb and Ta-rich gamma-TiAl alloys plays a crucial role towards realization of their potential applications. The present article reports the evolution of microstructural features in the crept gamma-TiAl-based Ti-5Al-8Nb-2Cr-0.2B and Ti-45Al-8Ta-0.2C-0.2B-0.2C alloys. Structural characterizations have been carried out using automated crystal orientation mapping (ACOM) along with precession electron diffraction (PED) in a transmission electron microscope, in conjunction with electron back-scattered diffraction (EBSD) in a scanning electron microscope (SEM) and transmission electron microscopy (TEM). Creep behavior of the fourth generation gamma-TiAl-based alloys has been comparatively investigated under constant load tensile creep tests performed in the temperature range from 800-850?degrees C and applied stresses range of 125-200 MPa. It has been demonstrated that the ACOM with PED technique has accurate and reliable diffraction pattern recognition and higher spatial resolution, and supplements effectively the conventional EBSD technique for characterization of complex microstructural features evolved during creep of multiphase (gamma + alpha(2) + beta)-based TiAl alloys. The results show that the Nb and Ta additions have distinctly different effects on the microstructural instability and phase transformation during the creep deformation. The formation of the Ta-rich intermetallic phase (Ti4Al3Ta, the so-called tau phase) has been detected preferentially along the colony and the gamma-alpha(2) interphase boundaries in the Ta-rich alloy, whilst its isomorph, Ti4Al3Nb intermetallic, has hardly been detected in the Nb-rich alloy. Implications of tau-phase formation and related microstructural instabilities have been discussed with respect to the creep behavior of the two alloys.