Study on high temperature creep behavior of the accident-resistant cladding Fe–13Cr–4Al-1.85Mo-0.85Nb alloy

Abstract

The Fe–13Cr–4Al-1.85Mo-0.85Nb alloy, as accident-resistant fuel cladding material in light water reactor (LWR), was prepared by a vacuum induction melting, forging and hot rolling process. In this study, the creep behaviors of the alloy at the temperature range of 400–450 °C were studied. The effects of rolling direction (RD), transverse direction (TD), creep temperatures and stresses on the creep properties were systematically investigated. The results show that the steady-state creep rates of the FeCrAl alloy increase with the increase of the loaded stress, which vary in the range of 8.81 × 10 −10 s −1 to 8.40 × 10 −8 s −1 in the RD and 8.30 × 10 −9 s −1 to 5.17 × 10 −7 s −1 in the TD. Based on the analysis of the real stress exponent n equaling to 5.0, the creep processes in the both RD and TD are controlled by dislocation climbing mechanism, as evidenced by the microstructure of dislocations tangled with precipitations after creep test. At 540 MPa and the temperatures of 400–450 °C, the creep activation energies in the RD and TD are 421.06 kJ mol −1 and 424.7 kJ mol −1 respectively. Before creep test, the dispersion of fine Mo-, Nb-, and Si-rich Laves phases is observed. After creep test, ‘spheroidization’ of these secondary Laves phases obviously occurs. EBSD analysis shows that the grains in the TD are smaller and equiaxed, while the grains in the RD are large and deformed. This is one of the reasons on the big differences of the creep rate of the alloy in the TD comparing with the RD. The average geometrically necessary dislocations (GNDs) of the cross-sectional structure of the alloy decrease after creep test in both the RD and TD. Moreover, the creep rate in the RD is about 1/10 of that in the TD, mainly due to the large elongated grains with less grain boundaries and higher GNDs in the RD. The obtained conclusions provide researchers and engineers with useful references for the engineering application of the FeCrAl alloy as accident-resistant fuel cladding material. • Creep process of the FeCrAl alloy is controlled by dislocation climbing mechanism. • Creep properties and reasons on anisotropy of creep rates of the alloy are revealed. • The creep rates of the alloy in the RD is around 1/10 of that in the TD. • GNDs of the cross-sectional structure of the alloy decrease during the creep process. • Creep activation energies in the RD and TD are 421.06 and 424.7 kJ mol −1 respectively.