Phase stability and interdiffusivity of the L1 0-based ordered phases in Al-rich portion of the Ti–Al binary system

Abstract

The stability region and interdiffusivity of the L1 0-based ordered phases, namely, γ (TiAl:L1 0), γ 2 (D0′ 23) and TiAl 2(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 γ 2 can be described as a higher-ordered structure of the L1 0, and that the critical boundary of the γ/γ 2 continuous ordering transition is located around 64.5 at.%Al which is almost independent of temperature. The interdiffusion coefficients of the γ, TiAl 2 and γ 2 phases were experimentally determined by the Bolzmann-Matano equation. The interdiffusivity in the γ and γ 2 phases slightly increases with Al concentration and the interdiffusion coefficients are given by D ˜ γ = 1.59 × 10 − 3 exp ( − 304 kJ mol − 1 / R T ) m 2 s − 1 for Ti - 56 at % Al , D ˜ γ = 3.82 × 10 − 4 exp ( − 281 kJ mol − 1 / R T ) m 2 s − 1 for Ti - 60 at % Al , D ˜ γ = 6.03 × 10 − 5 exp ( − 264 kJ mol − 1 / R T ) m 2 s − 1 for Ti - 64 at % Al and D ˜ γ 2 = 2.88 × 10 − 4 exp ( − 268 kJ mol − 1 / R T ) m 2 s − 1 for Ti - 70 at % Al .