Preliminary study of the effect of Sn addition on microstructure and creep resistance of a β-solidifying TiAl alloy

In the present work, two β-solidifying γ-TiAl based intermetallic alloys doped with Nb, Zr, Hf (designated TNZ γ-TiAl alloy) and additionally Sn (designated TNZ-Sn) were investigated. Sn is known to be widely used as an alloying element in titanium alloys but almost has not been used for γ-TiAl alloys. In contrast to Nb, Zr and Hf atoms occupying only or preferentially the Ti sublattice, Sn atoms occupy the Al sublattice because Al and Sn belong to the same group of post-transition metals. Two microstructural conditions per alloy, duplex and fully or near lamellar, were obtained in the alloys by upset forging and heat treatments. Microstructure examination by SEM and XRD analysis revealed two basic phases in both alloys, γ-TiAl and α₂-Ti₃Al. The feature of the Sn-containing alloy was the presence of a small amount (0.5–1.5 vol%) of the phase enriched with Sn, Zr and Hf and located along grain/colony boundaries. After upset forging and heat treatment this phase had the average chemical composition 60(Ti + Nb + Zr + Hf)-40(Al + Sn) (at.%) and was identified as the α₂ phase enriched with Sn, Zr and Hf and depleted with Al. The creep tests were performed for the produced microstructural conditions of the alloys. It was revealed that doping with Sn led to a strong increase in the creep resistance at 800 and 850 °C. The obtained effect was attributed to solid solution hardening due to substitution of Al by Sn and probably to a more ionic interatomic bonding taking into consideration an appreciably higher electronegativity of tin as compared to that of other alloying elements.