Microstructure-governed oxidation behavior of Zr-2.5Nb alloy at 500–700 °C: A comparative study on laser powder bed fusion and conventional extrusion

The oxidation behavior of Zr-2.5Nb alloy is significantly influenced by its microstructure. However, the specific role of grain boundaries in the oxidation process, particularly in non-equilibrium microstructures produced by additive manufacturing, remains poorly understood. To address this issue, this study investigated Zr-2.5Nb alloys with differences in grain boundary density prepared by Laser Powder Bed Fusion (LPBF) and conventional extrusion processes. Combined with isothermal oxidation experiments at 500-700°C and first-principles density functional theory (DFT) calculations, the effects of grain boundaries on oxidation kinetics, oxide layer microstructure evolution, and failure mechanisms were systematically explored. The results show that the high-density grain boundaries (GBs) introduced by LPBF significantly accelerate the oxidation process. The LPBF-built alloy exhibits higher mass gains than the extruded alloy at all temperatures. Its oxide layer is thicker but has a higher crack density. DFT calculations reveal the atomic-scale mechanism of grain boundary-promoted oxidation. Grain boundary regions provide more low-energy adsorption sites. Enhanced hybridization between O-p and Zr-d orbitals at GBs directly leads to preferential oxygen adsorption and rapid diffusion. This study establishes a quantitative relationship between grain boundary density and oxidation behavior. It clarifies the GBs -dominated oxidation mechanism and provides a theoretical basis for microstructure design of oxidation-resistant zirconium alloys and service reliability assessment of additively manufactured implants.