Role of oxygen in phase stability and mechanical behavior of the bcc HfNbTaTiZr high-entropy alloy during aging
Yujun Zhao, Maik Rajkowski, Yilun Gong, Stephan Laube, Loïc Perrière, Alexander Kauffmann, Jean‐Philippe Couzinié, Guillaume Laplanche, Tong Li
Abstract
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.