Role of oxygen in phase stability and mechanical behavior of the bcc HfNbTaTiZr high-entropy alloy during aging

The present work aims to explore how oxygen impacts the phase stability and mechanical behavior of the initially single-phase, body-centered cubic (bcc) HfNbTaTiZr high-entropy alloy. For this purpose, transmission electron microscopy and atom probe tomography were employed to investigate the structural and compositional evolution in two alloys: HfNbTaTiZr and HfNbTaTiZr-3O (3 at.% oxygen) during aging at 500°C up to 1000 h under an Ar atmosphere. Tensile tests and micro-mechanical tests were performed to study the mechanical properties. In the early stages of decomposition of the bcc parent phase in HfNbTaTiZr, Zr-Hf-rich channel-like body-centered tetragonal (bct) features with a thickness of ∼2.7 nm form along <001> bcc directions, likely driven by lattice relaxations of the bcc solid solution. Meanwhile, a Zr-Hf-rich hexagonal close-packed (hcp) phase of ∼3.6 nm in size forms at the nodes of the bct channels, near which a ∼11.1 nm Ti-rich ω phase is present. As aging proceeds, the ω phase dissolves and the bct phase structurally transforms into a distorted hexagonal phase. Similar phases and microstructural features were also observed in HfNbTaTiZr-3O with finer bct channels of ∼2.1 nm in width, where the bct-to-hcp transformation is hindered due to the stabilized bct channels by oxygen. After longer aging heat treatments, the ω phase persists accompanied by oxygen partitioning from the hcp phase. The microstructures comprised of nanometer-sized bct channels, ω, and hcp phases increase the strength of grain interiors, which can be used to improve the mechanical properties of HfNbTaTiZr in future research.