Microstructure and mechanical properties of Ti₃₅V₃₅Nb₁₀Mo₂₀Al_x refractory high entropy alloys: An experimental and computational study

To develop novel structural materials with high specific strength and excellent plasticity, this study employs the CALPHAD method to design and fabricate Ti₃₅V₃₅Nb₁₀Mo₂₀Al_x (x = 0, 5, 10 and 15) refractory high-entropy alloys (RHEAs) through vacuum arc melting. The effects of Al addition on the microstructure and mechanical properties of these RHEAs were systematically investigated, with a particular focus on the underlying solid solution strengthening mechanisms. The results demonstrate that except for the Ti₃₅V₃₅Nb₁₀Mo₂₀Al₁₅ RHEA, which exhibits a minor precipitation of the B2 phase, all other RHEAs show a single-phase, disordered body-centered-cubic solid solution. Room-temperature compression tests demonstrate that Ti₃₅V₃₅Nb₁₀Mo₂₀Al₅ RHEA exhibits the highest specific yield strength of 166.41 kPa·m³·kg⁻¹ with plastic strain exceeding 50 %, outperforming most reported RHEAs. This superior mechanical performance is primarily attributed to solid solution strengthening arising from elastic misfit and chemical/electronic interactions. Electronic structure analysis reveals that the larger pseudo-gap of Ti₃₅V₃₅Nb₁₀Mo₂₀Al₅ RHEA indicates stronger covalency and the localization of electrons near Al atoms promotes the formation of covalent bonds. Furthermore, the addition of Al effectively reduces the elastic anisotropy of the RHEAs, thereby mitigating crack initiation. This work provides valuable guidance for designing high-performance RHEAs and reveals the electronic-structure-driven strengthening mechanism of Al additions.