New ternary equi-atomic refractory medium-entropy alloys with tensile ductility: Hafnium versus titanium into NbTa-based solution

We demonstrate that based on NbTa as the base solution, ternary equiatomic HfNbTa and TiNbTa single-phase body-centered-cubic (BCC) solid solutions are achieved at arc-melted alloy ingots. These add interesting new members into the group of ternary refractory medium-entropy alloys (RMEAs), previously available only in NbTiZr. Moreover, HfNbTa and TiNbTa are the first two ternary RMEAs that exhibit tensile ductility, with HfNbTa having a combination of yield strength and tensile strain on par with its high-entropy counterpart, the Senkov alloy TiZrHfNbTa. We also show that alloying Hf into the base NbTa solution manifests a much higher strengthening potency compared with Ti. This difference can be rationalized using empirical parameters and models accounting for the size and modulus misfits among constituent elements.