Design of Refractory High Entropy Alloys TiZrHfNbX (X=Cr/Ta) system, using thermodynamic calculations

Refractory high entropy alloys (RHEAs) are materials with novel concept that do not use a specific element as a base material and mixture of multiple elements in near equal amounts. This concept is expected to be adopted for use in aerospace heat-resistant materials that can be used at higher temperatures than currently available. We focused on the TiZrHfNbCr system as a new high-temperature structural material. We carried out thermodynamic calculation using the CALPHAD method and proposed compositions of TiZrHf(NbCr)2 and TiZrHf(NbCr)7 in addition to the equimolar composition (TiZrHfNbCr). These alloys are fabricated by arc melting, and microstructures in TiZrHf(NbCr)7 alloys, predicted by CALPHAD were observed in SEM images. Although some agglomeration of Nb is recognized by observation, microstructures of TiZrHf(NbCr)2 were also good agreement with the prediction by CALPHAD. From the results of TiZrHfNbCr, TiZrHf(NbCr)2 and TiZrHf(NbCr)7, complex oxides were formed and small weight gains were observed compared to other alloys (e.g., ZrTi and NbTa). The effect of formation of complex oxide to the oxidation behavior of these alloys will be discussed in the presentation.