Formation of an ordered phase in hcp precipitates during aging of bcc HfNbTaTiZr high-entropy alloy

The role of O in the precipitation behavior at high temperatures of the equiatomic body-centered cubic (bcc) HfNbTaTiZr high-entropy alloy remains unclear and hence is investigated in this work. We employed transmission electron microscopy and atom probe tomography to reveal structural and compositional evolutions of precipitation in HfNbTaTiZr upon isothermal aging at 1200 °C. Due to the presence of O in the atmosphere, a Zr-Hf-rich hexagonal closed-packed (hcp) phase precipitates from the bcc matrix after 200 h. Due to the gradual uptake of O by the alloy from 200 h to 500 h, the Ti and O concentrations of the hcp phase increase. This results in the formation of an ordered phase in the hcp phase with a structure similar to Ti 3 O, whose number density and size slightly increase with time. As the hcp phase is increasingly reinforced by oxygen solutes and oxide precipitates, it becomes harder.