Mechanical properties of (HfCo)100−x(NbMo)x refractory high-entropy alloys with a dual-phase bcc-B2 structure

The common concept for the development of the high-performance metallic materials is associated with the combination of a soft disordered matrix and hard ordered reinforcements. However, unusual combinations of a hard disordered phase and a soft(er) ordered phase are also possible. In this work, we explored structure and mechanical properties of (HfCo) 100-x (NbMo) x ( x = 0; 10; 25; 60; 75; 100 at.%) refractory high-entropy alloys. The binary HfCo and NbMo alloys had ordered B2 and disordered bcc structures, respectively. The (HfCo) 100-x (NbMo) x (10 ≤ x ≤ 75) alloys had mostly a dual-phase structure of disordered (Nb, Mo)-rich bcc and ordered (Hf, Co)-rich B2 phase. Almost fully a eutectic-like structure was found in the (HfCo) 75 (NbMo) 25 alloy. "Cube-on-cube" orientation relationships between the bcc and B2 phases ((110) bcc ||(110) B2 , [001] bcc ||[001] B2 ) were detected. In addition, a small amount of Nb-rich bcc and/or Hf-rich fcc oxide phases were detected in some alloys. The B2 HfСo alloy was ductile but extremely soft, while, the NbMo alloy was hard and brittle. Simultaneous improvement of strength and ductility in the dual-phase (HfCo) 100-x (NbMo) x (10 ≤ x ≤ 75) was revealed. The (HfCo) 90 (NbMo) 10 alloy with a close to single-phase B2 structure exhibited a yield stress anomaly (YSA) at 500°C. The YSA was associated with a change of dominant slip mode from a <100> to a <110>. Relationships between the chemical composition, microstructure, and the mechanical properties of the (HfCo) 100-x (NbMo) x alloys were discussed. • The (HfCo) 100-x (NbMo) x ( x = 0; 10; 25; 60; 75; 100 at.%) refractory high-entropy alloys were introduced. • The HfCo alloy had a B2 structure, and it was ductile yet soft. • The NbMo alloy possessed a bcc structure and high strength but low ductility. • The (HfCo) 100-x (NbMo) x (10 ≤ x ≤75) alloys with dual-phase bcc-B2 structures showed a better strength-ductility synergy than the base alloys. • The (HfCo) 90 (NbMo) 10 alloy demonstrated a yield strength anomaly, associated with a change of the dominant slip mode from a <100> to a <110>.