Experimental kinetic study of interdiffusion behavior and intermetallic compound Zr(Fe,Cr)2 formation at the Cr/Zry4 interface under elevated temperatures

Interdiffusion behavior at the Cr-Zr interface under elevated temperatures in Cr-coated Zr-based accident tolerant fuel cladding is a key phenomenon for understanding its microstructural stability during the reactor's long-term operation. The interdiffusion behavior of the Cr/Zircaloy-4 (Zry4) diffusion couple and microstructural evolution at the interface were studied under vacuum conditions in the temperature range 973 K to 1323 K. Results show that interdiffusion results in the formation of a Zr(Fe,Cr)2 intermetallic compound layer irrespective of the annealing temperature. Segregation of Fe in the Zr(Fe,Cr)2 interlayer is confirmed, which is enhanced by annealing temperature within the α-Zr phase temperature regime and dropped greatly within the β-Zr temperature regime. The kinetic growth model for Zr(Fe,Cr)2 is found to follow the parabolic law in both α-Zr and β-Zr phase regions, and the diffusion activation energy (Q) of Cr in Zr(Fe,Cr)2 is determined as 7.23×104Jmol−1 for α-Zr phase annealing. Extrapolation of the diffusion coefficients measured within the α-Zr phase predicts a much lower value within the β-Zr phase compared to the experimental result, indicating that the α→β(Zr) phase transformation promotes the growth rate of Zr(Fe,Cr)2 interlayer formation, likely due to the enhancement in both diffusion ability and solubility of Cr in β-Zr relative to α-Zr. The insights obtained in this study will help to improve the understanding of the cladding's microstructure stability and its related degradation behaviors during practical application.