Phase stability and compressive properties of low-density (Zr₅₀Ti₃₅Nb₁₅)_100-xAl_x high entropy alloys

In the present study, phase stability and compressive mechanical properties of the (Zr₅₀Ti₃₅Nb₁₅)_100-xAl_x (x = 0 to 30 at.%) high-entropy alloys were investigated. It was found that the crystalline structure of cast alloys changed from the initial disordered body-centered cubic (BCC) structure to an ordered body-centered cubic (B2) structure as the Al concentration was increased. Their microstructure and phase evolution after long-term annealing up to 1000 h at temperature ranging from 900 °C to 1100 °C were analyzed. Annealing has no obvious influence on the matrix phase of Zr₅₀Ti₃₅Nb₁₅, which keeps BCC structure during all the annealing processes. For the alloys (Zr₅₀Ti₃₅Nb₁₅)₈₀Al₂₀ and (Zr₅₀Ti₃₅Nb₁₅)₇₀Al₃₀ with higher Al content, secondary phase (Al₃Zr₅) precipitates after annealing at 900 °C. With the increasing annealing temperature to 1000 °C or 1100 °C，there is a tendency that the amount of the Al₃Zr₅ phase decreases slightly, indicating that Al₃Zr₅ phase is unstable at high temperature. Accompanied by the phase change, there were also corresponding variations in compressive properties. The secondary phase precipitated in the (Zr₅₀Ti₃₅Nb₁₅)₈₀Al₂₀ and (Zr₅₀Ti₃₅Nb₁₅)₇₀Al₃₀ decreases the compressive plasticity, but has little influence on the yield strength. Meanwhile, the compressive properties of as-cast (Zr₅₀Ti₃₅Nb₁₅)₉₀Al₁₀ and (Zr₅₀Ti₃₅Nb₁₅)₈₅Al₁₅ were also tested to investigate the effect of Al content on the yield strength. Our findings provide a new insight in developing low-density high entropy alloys containing Al.