Influence of Nb and Mo on microstructure formation of rapidly solidified ternary Ti-Al-(Nb, Mo) alloys

Titanium aluminides containing Nb and Mo are promising structural intermetallics for an increased performance and weight reduction of high temperature components. A liquid droplet rapid solidification technique was applied on ternary Ti-(45-48)Al-(3-7)Nb and Ti-(45-47)Al-(1-2)Mo alloys to study the influence of the ternary additions on microstructure formation. The cooling rate was controlled by droplet size and kept constant at 4.5 × 103 K s−1. The results show the strong dependency of the microstructure formation on alloy composition and the solidification behavior. The β-solidifying Al-lean Ti-(45-46)Al-(3-7)Nb and Ti-(45-46)Al-(1-2)Mo alloys tend to form coarse grained α/α2 microstructures, whereas the Al-rich alloys show a fine dendritic structure consisting mainly of α2− and γ-phase. In the Mo-containing alloys also a significant amount of β/β0-phase was observed in the alloys. Complementary thermodynamic calculations based on the CALPHAD approach show good agreement of the experimental results with predicted solidification and phase transformation paths taking into account non-equilibrium conditions. With the ability to predict and experimentally observe non-equilibrium microstructure formation, alloys with tailored properties for processes involving rapid solidification and cooling can be identified and selected for future processing using beam-based additive manufacturing.