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Arc-melted Ti100–xMo2Alx alloys (where x is 44, 46, 48, and 50 at.% Al) that were produced from pure components were examined by X-ray diffraction and scanning electron microscopy with electron microprobe analysis (SEM/EDX). The melting points and solid-phase transformations were studied by differential thermal analysis (DTA) and the mechanical properties by fracture toughness, bending, and compression tests. The CALPHAD approach was used for thermodynamic calculation of phase equilibria in the composition region under study. The cast alloys mainly consist of a lamellar structure formed by lamellas of γ-TiAl and α2-Ti3Al phases with submicron thickness and a cubic Ti55Mo4–6Al39–40 phase of A2 type (β) or B2 type (β0). In the γ-TiAl-based alloys, molybdenum behaves as a low-melting dopant, enriching the grain periphery. The standard mechanical characteristics of the alloys were determined and their structural sensitivity was analyzed. All studied alloys demonstrate excellent high-temperature strength. The yield stress is 400–600 MPa in the temperature range 20–750°C. The strength slightly increases at 300 and 600°C under dynamic strain ageing. The strain hardening parameters were established over a wide range of test temperatures. The temperature dependence of the strain hardening coefficient and strain hardening index was analyzed for alloys in different phase and structural states. In the temperature range from 20 to 600°C, the strain hardening index and coefficient were found to vary slightly. The strain hardening index increased from n = 0.6 to n = 0.95 with aluminum content changing from 44 to 50 at.%, which indicates that the strain hardening mechanism changes with variation in the alloy phase constituents.

Quaternary as-cast Ti 96–x Nb 2 Cr 2 Al x and Ti 93–x Nb 5 Cr 2 Al x alloys, where x = 44, 46, 48, and 50, melted from pure components (~99.9 wt.%) in a laboratory arc furnace, were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM/EDX), and differential thermal analysis (DTA). Bending testing and fracture toughness measurements were performed at room temperature, and compression testing was carried out in a range from room temperature to 750°C. The alloys were found to consist mainly of a superfine lamellar structure formed during decomposition of hightemperature phases. In addition, the alloys with 50 at.% Al contain grains of the γ phase, and all others have precipitates of the cubic β/β 0 phase at boundaries of the high-temperature phases because of double enrichment of the last melt drops with Cr. The tests show that the Ti 96–x Nb 2 Cr 2 Al x alloys with 46 and 48 at.% Al possess the most balanced properties.

Ternary as-cast Ti55−x Nb x Al45, Ti53−x Nb x Al47, Ti55−x Ta x Al45, and Ti53−x Ta x Al47 alloys, where x = 0, 4, 8, and 12, and Ti45Nb8−x Ta x Al47 and Ti47Nb8−x Ta x Al45 quaternary alloys, where x = 2, 4, and 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 a superfine lamellar structure of decomposition β → α → α + γ → α2 + γ and contain tiny islands of the γ phase and also the β phase at 12 at.% Nb or Ta. 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 room temperature show that the ternary alloys with 45 at.% Al are characterized by higher strength but lower plasticity than the alloys with 47 at.% Al. For the Ti46Nb8Al46 and Ti46Ta8Al46 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 MPa, σult = 1612 MPa, epl = 12.4%) is exhibited by the Ti47Nb4Ta4Al45 alloy.