A Study of the Structure and Mechanical Properties of Nb-Mo-Co-X (X = Hf, Zr, Ti) Refractory High-Entropy Alloys

Refractory high-entropy alloys (HEAs) are a new class of metallic materials based on group 4–6 elements of the periodic table with possible additions of Al, Si, Re, C, or B. Some single-phase refractory HEAs can maintain high strength up to 1600°C, while multiphase compositions have more attractive specific properties at temperatures up to 1200°C. Here we examine the structure and mechanical properties of refractory HEAs Nb₃₀Mo₃₀Co₂₀Hf₂₀, Nb₃₀Mo₃₀Co₂₀Zr₂₀, and Nb₃₀Mo₃₀Co₂₀Ti₂₀ (at %). The alloys consisted of an intermetallic B2 matrix and particles of a disordered bcc phase, as well as a minor volume fraction of additional bcc (Nb₃₀Mo₃₀Co₂₀Hf₂₀ and Nb₃₀Mo₃₀Co₂₀Zr₂₀) or fcc (Nb₃₀Mo₃₀Co₂₀Ti₂₀) phases. When tested for uniaxial compression, Nb₃₀Mo₃₀Co₂₀Ti₂₀ alloy showed a higher yield strength in the temperature range of 22–1000°C than Nb₃₀Mo₃₀Co₂₀Hf₂₀ and Nb₃₀Mo₃₀Co₂₀Zr₂₀ alloys. Nb₃₀Mo₃₀Co₂₀Zr₂₀ alloy did not fail at temperatures of 22–800°C to a given 50% strain, while Nb₃₀Mo₃₀Co₂₀Ti₂₀ alloy turned out to be brittle. All alloys demonstrated high strain hardening in the temperature range of 22–800°C, and they can compete in terms of specific strength with commercial nickel and cobalt superalloys.