Temperature‐Induced Transformation of Oxidation Mechanism in HfZrTiTa 0.5 Al 0.5 Refractory High‐Entropy Alloy: The Role of Elemental Segregation and Lattice Defects

ABSTRACT Refractory high‐entropy alloys (RHEAs) face severe oxidation challenges at elevated temperatures, despite their outstanding mechanical properties. Consequently, understanding the oxidation mechanisms of RHEAs is crucial for their high‐temperature applications. In this study, the temperature‐induced transformation of oxidation mechanism in HfZrTiTa 0.5 Al 0.5 RHEA was investigated through detailed microstructural characterisation of the oxidation layers and theoretical calculations. Below 600°C, exceptional oxidation resistance was observed (oxide layer thickness < 5 μm). From 600°C to 800°C, external oxidation governed by the outward diffusion of metal ions predominated, leading to the formation of a dense oxide layer approximately 45 μm in thickness, consisting of nanocrystalline grains and an amorphous phase on the RHEA surface. The temperature‐induced transformation of the oxidation mechanism occurred at 1000°C. Above 1000°C, the internal oxidation mechanism dominated by inward oxygen diffusion took precedence, and the oxide layer thickness increased to approximately 255 μm. Needle‐like structures grew from the pre‐formed external oxide layer into the alloy matrix, giving rise to an internal oxide layer. During the internal oxidation process, needle‐like structures consisting of HfO 2 preferentially grew along the direction perpendicular to the alloy/oxide interface. Severe lattice defects with distorted 9R structures and twins formed at the tips of the needle‐like structures due to the lattice mismatch between the oxides and the matrix. In turn, these severe lattice defects further promoted the segregation of oxygen and Hf. Consequently, a dynamic growth mechanism was established, enabling the continuous extension of needle‐like structures into the interior of the alloy.