Phase separation and up-hill diffusion in the ordered α2 compound of a γ- Ti-Al-Nb alloy

Abstract

γ-TiAl alloys are intermetallic materials that are designed for applications in air craft engines. Oftentimes alloying elements are added to improve the corrosion resistance and mechanical properties. Here we investigate microstructural changes that take place when Nb is added to a binary γ-TiAl alloy. Previous studies of the alloy Ti-42Al-8.5Nb have shown that upon Nb addition the orthorhombic O-phase forms out of the parent α2 phase within the alloy’s microstructure. During annealing Nb segregates to the newly formed O-phase while Al and Ti partition to the remaining α2-like matrix phase. The changes these newly formed O-phase domains undergo with time at elevated temperature are not known. Here we use Transmission Electron Microscopy techniques to monitor the distribution of Nb and the size of O-phase domains during long-term annealing between 1 week and 4 weeks at 550°C. The results show that the size of O-phase domains shrinks with annealing time. At the same time the Nb concentration remains largely constant or possibly shows a small decrease.