Effect of Ordered B2 Phase on Deformation Behavior of Al-Nb-Ta-Ti-Zr High Entropy Superalloys

Refractory high entropy alloys composed of group 4-6 transition metal elements are expected to be a potential candidate for high temperature structural materials. However, there are some phase transformations such as phase separation of bcc phase into two bcc phases and formation of hcp phase, which is not suitable for high temperature applications. In contrast, Al_0.25NbTaTiZr high entropy alloys have the (bcc+B2) two-phase structure stable at high temperatures, similar to Ni-based superalloys. Al-doping results in the formation of the B2-(Ti,Zr)₂AlNb phase, while the bcc phase is mainly composed of Nb and Ta. The two-phase structure is formed by the spinodal decomposition and is effective in improving high temperature strength. It is also noted that dislocations after deformation at 1073 K are distributed in both the bcc and B2 phases. This suggests that not only the bcc phase but the B2 phase deforms plastically. In Al_0.25NbTaTiZr alloys, the misfit strain between the bcc and B2 phases is +2.7% and the volume fraction of the B2 phase is nearly 50%. The volume fraction of the B2 phase can be systematically changed from 28% to 74% by changing the chemical composition of the alloys. The yield stress at 873 K increases almost linearly with increasing the volume fraction of the B2 phase. This means that the B2 phase is effective in increasing high temperature strength in Al-doped refractory high entropy alloys. It is also noted that refinement of the two-phase structure leads to an increase in high temperature strength.