Exploring the Synergistic Enhancement Mechanism of Crack and Oxidation Resistances of Pre-Annealed Cr Coating on Zr-4 Alloys

Abstract

The present study aims to unveil the mechanism for the synergistic enhancement of crack resistance and oxidation resistance in pre-annealed Cr-coated Zr-4 alloys. In-situ three-point bending tests revealed that the crack initiation strain of Cr coating reached 1.12 % for 650 °C-annealed coating and 5.21 % for 700 °C-annealed coating, which was larger than in as-fabricated (0.47%). EBSD analysis indicated that pre-annealing led to recrystallization and grain growth, which reduced residual lattice strain and dislocation density in Cr grains. The equiaxed grains were favorable to hinder the propagation of interfacial microcracks, which enhanced the crack resistance of Cr coating. Oxidation tests showed that pre-annealing delayed the oxidation process of Cr coating and significantly reduced its weight gain compared to the as-deposited coating. Further characterization indicated that less grain boundaries in pre-annealed samples led to fewer inward diffusion paths of oxygen and ZrO2 precipitation, which prolonged the diffusion distance of oxygen and benefited the oxidation resistance of Cr coating. However, an interlayer of ZrCr2 would be formed if annealed above 700 °C, which might weaken the adhesion of coating and substrate. This study has provided a new strategy for synergistic enhancement of the fracture and oxidation resistance of Cr coating by heat treatment.

Keywords: Accident tolerant fuel, Cr coating, Annealing, Crack resistance, Oxidation resistance