Microstructures and mechanical properties of Ta–Nb–Zr–Ti–Al refractory high entropy alloys with varying Ta/Ti ratios

The refractory high entropy alloys (RHEAs) containing disordered body-centered cubic (BCC) and ordered BCC (B2) structures often exhibit high strength but low ductility at room temperature, even in the compressive tests. In this study, (Ta_25-xNb₂₅Zr₂₅Ti_25+x)₉₅Al₅ (x = 0, 5, 10) RHEAs are fabricated to investigate the compositional dependence of the microstructures and mechanical properties. All the three alloys exhibit a single BCC structure at the as-cast and solution-treated states, while the basket weave-like microstructures consisting of cuboidal and strip-like phases are formed after aging at 600 °C. The microstructure of the aged alloys is sensitive to the compositions: only disordered BCC1 + BCC2 dual phases are observed in the (Ta₂₅Nb₂₅Zr₂₅Ti₂₅)₉₅Al₅ alloy; additional B2 phases are formed by replacing 5 at% Ta with Ti; a complex microstructure containing BCC1 + BCC2 + B2 + Omega phases are identified in the (Ta₁₅Nb₂₅Zr₂₅Ti₃₅)₉₅Al₅ alloy. Although replacing Ta with Ti results in a reduction of compressive yield strength from 1762 to 1243 MPa, the fracture strain is greatly enhanced from 7.3 to 44.3%, indicating that the strength-ductility balance can be tuned to a large extent in this RHEA system.