Microstructure and mechanical properties of the new Nb25Sc25Ti25Zr25 eutectic high entropy alloy

The novel quaternary Nb25Sc25Ti25Zr25 (at.%) high entropy alloy with dual-phase structure was developed and cast by arc melting of elemental precursors. The as-cast state comprised of lamellar eutectics with alternating α(Sc, Zr) with HCP lattice plates having thickness of 30-50 nm, length up to 600 nm and inter-lamellar spacing of 5-40 nm, embedded in the β-NbZrTi matrix with BCC structure. While the α(Sc, Zr) plates showed reduced content of Nb, the β-NbZrTi matrix was particularly depleted in Sc. The unique mixture of fine hexagonal α(Sc, Zr) plates within the β-NbTiZr matrix in as-cast state led to relatively high combination of strength and ductility: the alloy with initial hardness of 418 HV reached compressive strength of 1250 MPa, yield strength of 1020 MPa and compressibility of 8.2%. The phase composition exhibited high stability with no significant changes taking place after annealing at 1000 ˚C for 24 h. The high-temperature exposure caused only coarsening of α(Sc, Zr) hexagonal plates, which reached 3 μm in width and 5-10 μm in length. The plastic strain of the annealed alloy with hardness of 202 HV increased to 20% while compression strength decreased to 670 MPa. As opposed to the as-cast state, the annealed alloy exhibited limited strain hardening during room-temperature compression.