Temperature-Induced Transformation of Oxidation Mechanism in HfZrTiTa_0.5Al_0.5 Refractory High-Entropy Alloy: The Role of Elemental Segregation and Lattice Defects

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_0.5Al_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 HfO₂ 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.