Empowering oxidation resistance of Cr-coated zirconium alloys via hydrogen-driven interface polymorphic transformation

Cr-coated zirconium alloy is being considered as a promising accident-tolerant nuclear fuel cladding, and empowering its oxidation resistance is critical to ensure superior performance under high-temperature steam conditions during loss of coolant accidents (LOCA). Here, we report a unique C15-C14 polymorphic transformation of interfacial ZrCr₂ Laves intermetallics induced by hydrogen during high temperature oxidation, which enhances the Cr-coating oxidation resistance. Atomic resolution characterization reveals that the C15-C14 transition of ZrCr₂ Laves phase is facilitated by the persistent synchroshear process of the Zr-Cr-Zr triple layers on {0001} lattice plane, with stacking faults and localized C36-type stacking preserved as atomic footprints. The density functional theory calculations demonstrate that the polymorphic transformation is attributed to the hydrogen-induced drop in the C15-C14 transition temperature and significant reduction in stacking fault energy, which collaboratively allow for a stacking fault-mediated phase transition via the synchroshear mechanism. The C14 ZrCr₂ phase with elevated fracture toughness plays a critical role in strengthening the interfacial layer and inhibiting the inward diffusion of oxygen. Our finding provides an optimization strategy for tailoring interface polymorphic transformation and enhancing the high-temperature oxidation resistance performance of Cr-coated zirconium alloys.