Composition and temperature dependence of α2 phase decomposition in high Nb-containing lamellar γ-TiAl alloys: Experiments and first-principles calculations

Abstract

This paper reports an experimental and theoretical study of composition and temperature dependence of α2 phase decomposition in lamellar γ-Ti−(43∼47)Al−(4∼10)Nb alloys. The α2 phase decomposes to nano-sized orthorhombic (O) phase in the alloys (the Nb content ≥ 5.5 at.%) at temperatures of 550∼750° C. The transformation temperature decreases with increasing the Al content, but increases with increasing the Nb content. The Nb partitioning coefficient between O and α2 typically equals to 2, and decreases with increasing the Al content and temperature, confirming that the O phase transformation is controlled by Nb diffusion. The α2 to ω0 phase transformation takes place in the alloys (the Nb content ≥ 7 at.%) at 800° C. The blocky ω0 phase is enriched in Nb and the Nb partitioning coefficient between ω0 and α2 is about 1.3, indicating that the ω0 phase transformation is also related to Nb diffusion. The pseudo-binary phase diagram calculated by first-principles correctly predicts the α2 to O phase transformation at temperature below 750° C and α2 to ω0 phase transformation at temperature above 750° C in the alloys. Since the α2 phase is unstable thermodynamically at intermediate temperature, such kinds of α2 to O and ω0 phase transformations are considered necessarily for design of high Nb-containing γ-TiAl alloys.