Enhanced fracture toughness in Nb<i>x</i>TiZrHf high-entropy alloys by metastability engineering

Compositional tuning of refractory high-entropy alloys (HEAs) is a powerful strategy to modulate their structural stability and mechanical properties. In this study, we investigate the sound velocities, elasticity, and mechanical properties of NbxTiZrHf (x = 0, 0.2, 0.4, 0.6, 0.8, and 1.0) HEAs using ultrasonic interferometry combined with Vickers hardness measurements. Notably, the metastable bcc Nb0.2TiZrHf HEAs exhibits exceptional fracture toughness, reaching up to 12.2 MPa·m1/2, which is 1.7–3.3 times higher than that of other bcc NbxTiZrHf counterparts. The mechanism for the abnormal strengthening in fracture toughness of Nb0.2TiZrHf HEAs is primarily attributed to the stress-induced bcc-to-hcp phase transition, which promotes plasticity/ductility strengthening and crack deflection. These findings provide deep insights into “metastability engineering” for designing refractory HEAs with superior fracture toughness and high strength.