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

This paper reports an experimental and theoretical study of composition and temperature dependence of α 2 phase decomposition in lamellar γ-Ti−(43∼47)Al−(4∼10)Nb alloys. The α 2 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 α 2 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 α 2 to ω 0 phase transformation takes place in the alloys (the Nb content ≥ 7 at.%) at 800° C. The blocky ω 0 phase is enriched in Nb and the Nb partitioning coefficient between ω 0 and α 2 is about 1.3, indicating that the ω 0 phase transformation is also related to Nb diffusion. The pseudo-binary phase diagram calculated by first-principles correctly predicts the α 2 to O phase transformation at temperature below 750° C and α 2 to ω 0 phase transformation at temperature above 750° C in the alloys. Since the α 2 phase is unstable thermodynamically at intermediate temperature, such kinds of α 2 to O and ω 0 phase transformations are considered necessarily for design of high Nb-containing γ-TiAl alloys. [Display omitted] [ABSTRACT FROM AUTHOR]