Partitioning Behavior of Nb, Ta, and Zr in Fully Lamellar γ/α2 Titanium Aluminides and Its Effect on the Lattice Misfit and Creep Behavior

Abstract

Previous investigations revealed that Nb, Ta, and Zr improve the creep properties of fully lamellar titanium aluminides significantly. The enhanced creep properties can originate from different potential effects of the alloying elements. Herein, the elements’ partitioning between α2-Ti3Al and γ-TiAl and their influence on the lattice parameters of ternary Ti–44Al–5X (X = Nb, Ta, Zr) alloys are investigated in comparison with a binary Ti−44Al alloy by atom probe tomography and high-energy X-ray diffraction. Ta partitions nearly equally between α2 and γ, Nb accumulates slightly in γ, and Zr enriches strongly in γ. As all alloying elements have a larger radius than Ti, a stronger partitioning to γ decreases the lattice misfit more. The synchrotron measurements show a clear influence of the alloying elements on the lattice parameters and resulting lattice misfits in γ- (Formula presented.) and γ- (Formula presented.) direction as well as the c/a ratio. In accordance with theoretical calculations based on the partitioning behavior, the lattice misfit decreases from Ti–44Al–5Ta over Ti–44Al–5Nb to Ti–44Al–5Zr. The c/a ratio decreases from Ti–44Al–5Nb over Ti–44Al–5Ta to Ti–44Al–5Zr. A correlation between the measured lattice misfits of the ternary Ti−44Al−5X with their primary creep strains is found, as the primary creep strain decreases with decreasing lattice misfit.