Formation and toughening of metastable phases in TiZrHfAlNb medium entropy alloys

In this work, the microstructure, phase transformation, and tensile properties were investigated for Ti_55-xZr₂₀Hf₁₅Al₁₀Nb_x (x = 1, 2, 3…, 9, at%) medium entropy alloys (MEAs). It has been found that the phase composition of the as-cast MEAs changes from hexagonal α′ to orthorhombic α″, then to metastable β, finally to stable β phase at x = 2, 4 and 9, respectively. The phase transformation of Nb4–Nb8 MEAs evolved in the sequence of β + α″→ω + α″ + β→α + β′→β during heating process. The phase constitution and microstructure of these MEAs strongly influence their tensile properties. The formation of metastable α″ and β phase in certain compositional and temperature range results in “double yielding” phenomenon and remarkable work-hardening effect. Extraordinary static toughness represented by the product of strength and elongation has achieved for MEA with 8% Nb. The deformation mechanisms for these MEAs changes with the phase composition. For the alloy with most α″ phase, the deformation is dominated by de-twinning of martensite variants; for the alloys with most β phase, stress induced martensitic transformation from metastable β to α″ phase and the refinement of the newborn martensite by the dislocation have been verified. These integrated effects are considered to endow the MEAs excellent deformation ductility.