Characterization of stress-rupture behavior of nuclear-grade C26M2 FeCrAl alloy for accident-tolerant fuel cladding via burst testing

Abstract

FeCrAl alloys demonstrate excellent high-temperature oxidation resistance compared to zirconium alloys used in LWRs. FeCrAl C26M2 has been down-selected as one of the candidate ATF cladding materials for Gen II LWRs. This study investigates the stress-rupture behavior of the nuclear-grade FeCrAl C26M2 tubing in the temperature range of 753-923 K. Rupture time and uniform strain at rupture were determined as a function of the temperature and applied stress using burst tests. The rupture data was found to obey the Larson-Miller Parameter and the Monkman-Grant relationship. Furthermore, the stress exponent (n) and the activation energy (QC) at lower stresses were determined as 4.4 ± 0.3 and 289 ± 25 kJ/mol, respectively, confirming power-law behavior. Above the normalized stress (σ/E) of 2 × 10−3, the power-law creep transited into the power-law breakdown regime. The steady-state deformation microstructure was examined using transmission electron microscopy (TEM) to detect the rate-controlling creep mechanism(s). Observation of sub-grains with dislocations at the boundaries in the power-law regime along with a stress exponent of 4.4 suggests dislocation climb as the rate-controlling mechanism. Two different fracture modes were observed: the tubing failed either by direct open-up or small crack and pinhole formation. The outcome of this study would provide essential data for the development of informative mesoscale deformation models paving way for discovery of new creep resistant alloys.