Phase stability and interdiffusivity of the L10-based ordered phases in Al-rich portion of the Ti-Al binary system

The stability region and interdiffusivity of the L10-based ordered phases, namely, γ (TiAl:L10), γ2 (D0′23) and TiAl2(oC12) at 900-1350 °C in the Al-rich part of the Ti-Al system were investigated by electron probe microanalysis (EPMA) using diffusion couples, differential scanning calorimetry (DSC) and high-temperature X-ray diffraction (XRD). The results of the phase equilibria among these ordered phases are basically in agreement with the recent data reported by Palm et al. It is established that the atomic configuration of γ2 can be described as a higher-ordered structure of the L10, and that the critical boundary of the γ/γ2 continuous ordering transition is located around 64.5 at.%Al which is almost independent of temperature. The interdiffusion coefficients of the γ, TiAl2 and γ2 phases were experimentally determined by the Bolzmann-Matano equation. The interdiffusivity in the γ and γ2 phases slightly increases with Al concentration and the interdiffusion coefficients are given byD˜γ=1.59×10−3exp(−304kJmol−1/RT)m2s−1forTi-56at%Al,D˜γ=3.82×10−4exp(−281kJmol−1/RT)m2s−1forTi-60at%Al,D˜γ=6.03×10−5exp(−264kJmol−1/RT)m2s−1forTi-64at%AlandD˜γ2=2.88×10−4exp(−268kJmol−1/RT)m2s−1forTi-70at%Al.