Influence of Nb on Ti diffusion in γ-TiAl intermetallics studied by mechanical spectroscopy

The development of intermetallic titanium aluminides has been driven by the aeronautic and aerospace industries because of the excellent mechanical properties and low density of γ-TiAl based alloys. Up to now, several generations of γ-TiAl based alloys were developed with increasing complexity of the alloy systems. Nb is one of the most important alloying elements in γ-TiAl alloys and although it is considered as a slow diffuser, its influence has not been fully quantified yet. In this work we demonstrate, through mechanical spectroscopy measurements conducted on several γ-TiAl based alloys with different Nb content, that Nb impedes the diffusion of Ti atoms in the α ₂-Ti ₃Al phase. Internal friction measurements show a relaxation peak P(α ₂), which is associated with short distance diffusion of Ti atoms in the α ₂ phase, involving stress-induced rotation of dipoles Al-V _Ti-Al, whose activation energy is dependent on the Nb content. The increase of the activation energy is quantified as ΔE _a(Ti)= 0.037 eV × at% Nb, being attributed to the next-neighbor interaction of Nb atoms with the local configuration of Ti-V _Ti. This mechanism also produces a further broadening of the relaxation peak, which is attributed to the near-next-neighbor interactions for high Nb contents. Finally, an atomic model for the mechanism responsible for this relaxation is proposed allowing to explain the observed experimental behavior.