The transformation mechanism of β phase to ω-related phases in Nb-rich γ-TiAl alloys studied by in situ high-energy X-ray diffraction

摘要

In recent years intermetallic γ-TiAl based alloys with additional amounts of the ternary bcc β-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 β 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 ℃ - 1100 ℃) with subsequent quenching at different rates. Depending on the quenching rate reversible transformations of the B2-ordered β_o phase to different co-related phases were observed. The results indicate that the complete transformation from β_o to hexagonal B8_2-ordered ω_o consists of two steps which are both diffusion controlled but proceed with different velocities.