Insights into multi-effects of single element Mo in Ti-rich Ti₄₀Nb₃₀V_25−xZr₅Mo_x refractory complex concentrated alloys: Strength-ductility synergy and high-temperature strengthening

Refractory high-entropy alloys (RHEAs) or refractory complex concentrated alloys (RCCAs) represent a promising class of materials due to their high strength and unique heat-resistance properties. However, RHEAs or RCCAs often face the challenge that the key factors, including light-weight, ductile, and heat-resistant, are mutually exclusive in a single alloy. The present study aims to achieve an excellent combination of lightweight, strength, and ductility by controlling the Mo element. Ti₄₀Nb₃₀V_25−xZr₅Mo_x (x = 0, 3, and 5, and referred to as Mo0, Mo3, and Mo5) with low densities around 6.2–6.4 g cm⁻³ were designed. After the addition of Mo, the strength and ductility of Mo0 were simultaneously enhanced, where the optimized Mo5 alloy possessed a substantial strain-hardening rate of approximately 2 gigapascals and a final fracture elongation exceeding 25 % at room temperature. Moreover, the tensile strength of Mo5 can still exceed 500 MPa at 1073 K, showcasing the potential for broad-temperature-range applications. According to the experimental analyses and DFT calculations, multi-effects of Mo in the alloy system were revealed: (ⅰ) supplying sufficient solid solution strengthening by introducing large shear modulus mismatch and atomic strain field; (ⅱ) enhancing strain-hardening capabilities by promoting dislocation substructures and facilitating cross-slip mechanism; (ⅲ) enhancing high-temperature strengths by reinforcing atomic interactions and increasing covalent bonding composition. These results fully unleash the potential of the cocktail effect in HEAs rather than relying on overly complex material processing methods, offering new insights into developing novel high-performance single-phase RHEAs or RCCAs.