Eliminating intergranular oxidation and microstructural instability in chemically complex intermetallic alloys featuring nano-disorder interfaces

Severe intergranular oxidation and microstructural instability remain major challenges limiting the extensive applications of structural alloys at elevated temperatures. In this study, we propose an innovative strategy by developing a chemically complex intermetallic alloy (CCIMA) based on the L1₂-type Co-Ni-Al-Ti-Nb-Ta-B system. This alloy design incorporates a thermally stable, Co-rich disordered interface nanolayer (DINL) with a face-centered-cubic (FCC) structure, which effectively mitigates these critical issues. The newly developed CCIMA demonstrates exceptional microstructural stability, maintaining its ordered L1₂ matrix and DINLs after the long-term exposure for 336 h at 1000 °C. Grain size remains stable at ∼ 30 μm due to the DINL-induced reduction in the grain-growth driving force. Nanoscale on-axis Transmission Kikuchi Diffraction (TKD) and transmission electron microscopy (TEM) analyses reveal a four-layer oxide-scale comprising NiCo₂O₄, CoAl₂O₄, a mixed layer of (TiNbO₄+Al₂O₃+AlTaO₄), and an inner Al₂O₃ layer. The compact and nanocrystalline morphology of these oxides confers superior oxidation resistance. Notably, intergranular oxidation and the formation of a degradation layer at the alloy/oxide interface occur only within the initial 2 min of oxidation, after which the material exhibits a unique self-healing effect. Supported by density functional theory (DFT) calculations, the underlying atomic mechanism governing this self-healing behavior was unveiled. The present work would provide new insights into the alloy-design strategies for the development of next-generation high-temperature materials with superior structural and oxidation resistance.