The Transformation Mechanism of Beta Phase to Omega-related Phases in Nb-rich Gamma-TiAl Alloys Studied by in-situ High-energy X-ray Diffraction

In recent years intermetallic gamma-TiAl based alloys with additional amounts of the ternary bcc beta-Ti (Al, Nb) phase attracted increasing attention due to their improved workability at elevated temperatures. Depending on alloy composition and heat treatment the ductile high-temperature beta phase can transform to several ordered phases at lower temperatures. However, currently available phase diagrams of these multiphase alloys are quite uncertain and the precipitation kinetics of some metastable phases is far from understood. In the present study various transformation pathways of the third phase were observed in-situ by means of high-energy x-ray diffraction using synchrotron radiation. A Ti-45Al-10Nb (at. %) specimen was subjected to a temperature ramp of repeated heating cycles (700 degrees C-1100 degrees C) with subsequent quenching at different rates. Depending on the quenching rate reversible transformations of the B2-ordered beta(o) phase to different orrelated phases were observed. At low quenching rates the hexagonal B8(2)-ordered omega(o), phase was formed while at high quenching rates the metastable intermediate trigonal omega " phase could be preserved. The results indicate that the complete transformation from beta(o) to hexagonal B8(2)-ordered omega(o) consists of two steps which are both diffusion controlled but proceed with different velocities.