Structural anomaly in the high-entropy alloy ZrNbTiTaHf

We present a high-resolution scanning transmission electron microscopy study on the microstructure of a non-equiatomic high-entropy alloy with the composition of Zr12.7 Nb30.8 Ti17.7 Ta30.8 Hf8.0 (at.%). We identify a novel inter-grain phase (IGP) that compositionally and structurally differs from the surrounding body-centred cubic host. In particular, we find that the IGP has a composition of Zr52.8 Nb6.9 Ti4.6 Ta20.6 Hf15.1 (at.%) and that it solidifies in a face-centred cubic crystal lattice structure. The occurrence of the latter is unexpected and remarkable since all possible binary phase diagrams of the involved elements only show body-centred cubic and hexagonal close-packed crystal structures. Therefore, to validate our experimental findings we have conducted parameter-free ab-initio calculations based on density-functional theory and the coherent-potential approximation. The simulations support our experimental findings showing that for the composition of the IGP, the face-centred cubic crystal structure is indeed the most stable one.