On the microstructures and cracking modes of Cr-coated Zr-4 alloys oxidized and vacuum-annealed at 1000 °C

Abstract

High-temperature oxidation and elemental diffusion significantly affect the microstructure and mechanical properties of Cr-coated zircaloys under accident conditions, leading to cracking modes different from those of samples with fresh coatings. To clarify the intrinsic mechanisms, in situ three-point bending tests were performed and finite element models were built for the Cr-coated Zr-4 alloys oxidized and vacuum-annealed at 1000 °C. In the case of vacuum-annealed Cr coatings, although numerous microcracks were initiated in the brittle ZrCr 2 diffusion layer under external loading, they could hardly propagate through the Cr coating. Recrystallization and grain growth under the annealing conditions led to significant improvement in the ductility of the Cr coating and its deformation compatibility with the substrate, resulting in the best crack resistance of the coating. In the case of oxidized Cr coatings, oxygen penetrated the Cr coating on the substrate, causing a β-Zr to α-Zr(O) phase transformation beneath the interface at the high annealing temperature. Under external loading, driven by large stresses, the brittle layers of Cr 2 O 3 , ZrCr 2 , and α-Zr(O) were susceptible to microcracking, and some cracks penetrated the Cr coating. However, the crack resistance of the oxidized Cr coating was better than that of the as-deposited coating. • Microstructure evolution and fracture mechanisms of Cr coating were demonstrated. • Different cracking modes of the vacuum-annealed and oxidized Cr coating were observed. • Recrystallization greatly improved the ductility and crack resistance of Cr coating. • Oxidation, Zr-Cr interdiffusion, and phase transformation of Zr promoted crack initiation.