The equiatomic body-centered cubic (bcc) TiZrNbHfTa high-entropy alloy (HEA) is one of the promising candidates for high-temperature applications beyond Ni-based superalloys and other conventional refractory alloys. The bcc phase was initially thought to be stable due to the high mixing entropy. However, a hexagonal closed-packed (hcp) phase precipitates from the bcc TiZrNbHfTa upon annealing at ~800 °C [1]. Also, the role of oxygen in the phase stability of the bcc matrix or hcp precipitate remains unclear. In this context, the aim of this study is to investigate the effects of oxygen on the precipitation sequence in TiZrNbHfTa bcc HEA. Atom probe tomography and transmission electron microscopy were employed to study the compositional and structural evolution during annealing at 1200 °C. We found out that the Zr-Hf-rich hcp phase precipitates in the bcc matrix after annealing for 200 h, due to penetration of oxygen through the porous glassy tube in the Ar atmosphere. As annealing proceeds, an oxygen-rich ordered phase with a structure of Ti3O was observed in the hcp precipitates and gradually populates with increasing sizes and volume fractions. This corresponds well with an increased Ti concentration in the hcp phase, indicating a gradual formation of the oxygen-rich ordered phase in the hcp precipitates. Our preliminary results reveal the hcp structural instability induced by oxygen upon annealing TiZrNbHfTa bcc HEA at high temperatures.

Reference:

[1] B. Schuh, et. al, Thermodynamic instability of a nanocrystalline, single-phase TiZrNbHfTa alloy and its impact on the mechanical properties, Acta Materialia 142 (2018) 201-212.