Structure and Properties of Titanium–Aluminum Alloys Doped with Niobium and Tantalum

Abstract

Ternary as-cast alloys Ті_55-хNb_хAl₄₅, Ті_53-хNb_хAl₄₇, Ті_55-хТа_хAl₄₅, and Ti_53-хТа_хAl₄₇, where х = 0, 4, 8, 12, and quaternary alloys Ti₄₅Nb_8-xТа_хAl₄₇ and Ті₄₇Nb_8-xТа_хAl₄₅, where х = = 2, 4, 6, melted in a laboratory arc furnace from pure components (~99.9 wt.%) are studied by X-ray diffraction (XRD), scanning and transmission electron microscopy (SEM and TEM), differential thermal analysis (DTA), and compression testing. The alloys are found to consist mainly of superfine lamellar structure of decomposition β → α→ α + γ → α₂ + γ; they all contain tiny islands of the γ-phase and also β-phase at the Nb or Ta content of 12 at.%. The CALPHAD approach is used to obtain a thermodynamic description of the Al-Nb-Ta-Ti system, which adequately reproduces the experimental data. The compression tests performed at ambient temperature show that the ternary alloys with 45 at.% Al are characterized by higher strength compared to the alloys with 47 at.% Al, but the former are inferior to the latter in plasticity. For the Ti₄₆Nb₈Al₄₆ and Ti₄₆Та₈Al₄₆ alloys, high strength in the temperature range 20-800 °C and a drop in plasticity to 2-4% at 200-600 °C are revealed. Among the quaternary alloys examined, the best combination of strength and plasticity (σ₀₂ = 1043 МПа, σ_p = 1612 МПа, ε_пл = 12,4%) is found for the Ті₄₇Nb₄Та₄Al₄₅ alloy.