Effect of complex alloying on the phase composition and oxidation resistance of O-Ti2AlNb based intermetallic alloys

The effect of complex alloying on the phase composition and oxidation resistance of intermetallic alloys based on the Ti2AlNb phase has been studied. The following experimental alloys (in at.%) were investigated: Ti-22Al-25Nb-0.15B, Ti-22Al-21Nb-5.5(Zr, Mo, Ta, Sn, Hf)-0.3Si-0.15B, Ti-25Al-21Nb-5.5(Zr, Mo, Ta, Sn, Hf)-0.3Si-0.15B, Ti-23.5Al-23Nb-4(Zr, Mo, Ta, Hf)-0.3Si and Ti-23Al-25.5Nb-2.5(Zr, Mo)-0.3Si. The first alloy was used as a reference alloy. The complex alloys were alloyed with metallic elements having a high melting point and / or a higher atomic radius in order to improve oxidation and creep resistance, and to provide solid solution strengthening in a wide temperature range. The ratio between Al content and β-stabilization (in Mo-equivalent) of the alloying elements varied around the ratio value corresponding to the ternary Ti-22Al-25Nb alloy. As-cast alloys were annealed at 900°C followed by furnace cooling in order to thermodynamically equilibrate the as-cast ingots. The alloys based on Ti-22Al, Ti-23Al and Ti-23.5Al consisted mostly of two basic phases, O(Ti2AlNb) and β / B2, whereas the complex alloy based on Ti-25Al was near completely single-phase and contained only insignificant amount of the β / B2 phase. The volume fraction of the β / B2 phase in the alloys containing 22 – 23.5 Al was in the range of 17.6 – 27.5 %. Microalloying with boron led to formation of borides but did not refine the as-cast microstructure. DSC analysis showed that for all alloys, the equilibrium phase transformation sequence is as follows: L → β / B2 → β / B2 + α2(Ti3Al) → β / B2 + α2(Ti3Al) + O(Ti2AlNb) → O(Ti2AlNb) + β / B2. The oxidation behavior was evaluated for the annealed conditions of the alloys at 700°C / 600 h. The oxidation resistance increased due to complex alloying, with an increase in the aluminum content and with a decrease in the β / B2 phase content. The highest oxidation resistance showed the alloys based on Ti-25 and 23.5Al. In these alloys, the mass gain after oxidation at 700°C / 600 h was around 0.5 mg / cm2. Potential advantages of the complex alloying of O-Ti2AlNb based alloys are discussed.

1. Ministry of Science and Higher Education of the Russian Federation - Контракт ИПСМ РАН №13.2251.21.0285