Hierarchical microstructure in high Nb-containing TiAl alloy

Highlights: • The orientation relationship between η precipitate and γ matrix is tetragonal-tetragonal. • The habit plane of plate-like η precipitate is along {022)_γ crystal plane. • The η precipitate is a hierarchical microstructure comprised of alternating arrangement of γ and η plates. • The interface structure between γ and η phase can be described using the classical 'terrace-disconnection' theory. Plate-like η-Nb(Ti,Al)₃ precipitates with hierarchical microstructure comprised of alternating arrangement of γ-(Ti,Nb)Al and η plates in the Ti-55Al-22Nb alloy have been investigated using transmission electron microscopy. In addition, the orientation relationship (OR) between η precipitates and γ matrix is determined to be tetragonal-tetragonal. The habit plane of η precipitates is along {022) crystal planes of γ matrix. And the interface structure between them can be described using the classical 'terrace-disconnection' theory. Furthermore, the phenomenon of phase separation responsible for the formation of this hierarchical microstructure can be well explained by the chemical free energy, that is, Al-depletion and Ti-enrichment in η precipitate drive the splitting of η precipitate. To the authors' knowledge, this is the first work which demonstrates such a hierarchical microstructure in η precipitate in high Nb-containing TiAl alloy.