Comparison on microstructure and mechanical properties of refractory high entropy alloys of the Hf-Mo-Nb-Ta-Ti-Zr system

Refractory high entropy alloys (RHEAs) consisting of high melting point elements, such as Hf, Mo, Nb, Ta, Ti, Mo, and Zr, have shown promising mechanical properties and phase stability at elevated temperatures and, thus, received increasing attention over the last two decades. In the present study, employing experimental and computational methods, the microstructures and mechanical properties of seven different RHEAs, namely, Hf_16.6Nb_16.6Ta_16.6Ti₅₀ (HEA1), HfNbTaTiZr (HEA2), Hf₂₇Nb₁₂Ta₁₀Ti₂₃Zr₂₈ (HEA3), Hf₃₀Nb₁₄Ta₁₀Ti₂₈Zr₁₈ (HEA4), Hf₁₂Nb₁₆Ta₃₅Ti₂₉Zr₈ (HEA5), HfMoTaTiZr (HEA6), and MoNbTaTiZr (HEA7) were compared. The nonequilibrium solidification curves calculated using CALPHAD demonstrated that Ta, Nb, and Mo tend to solidify first in the dendrite arms, while the liquid phase becomes enriched with Ti and Zr as solidification progresses. However, depending on the Ta content, Hf is proclaimed to solidify in dendrite arms or interdendritic regions, also supported by thorough experimental characterization. Furthermore, the addition of Mo was demonstrated to increase the hardness and strength of the alloys at the expense of ductility. Finally, HEA1, HEA3, HEA4, and HEA5 demonstrate excellent strength-ductility synergy at room and cryogenic temperatures (−80 °C), expanding their service temperature range, promoting their utility in a variety of industrial applications.