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 Nb 30 Mo 30 Co 20 Hf 20 , Nb 30 Mo 30 Co 20 Zr 20 , and Nb 30 Mo 30 Co 20 Ti 20 (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 (Nb 30 Mo 30 Co 20 Hf 20 and Nb 30 Mo 30 Co 20 Zr 20 ) or fcc (Nb 30 Mo 30 Co 20 Ti 20 ) phases. When tested for uniaxial compression, Nb 30 Mo 30 Co 20 Ti 20 alloy showed a higher yield strength in the temperature range of 22–1000°C than Nb 30 Mo 30 Co 20 Hf 20 and Nb 30 Mo 30 Co 20 Zr 20 alloys. Nb 30 Mo 30 Co 20 Zr 20 alloy did not fail at temperatures of 22–800°C to a given 50% strain, while Nb 30 Mo 30 Co 20 Ti 20 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.