Effects of Nb-doping on the mechanical properties and high-temperature steam oxidation of annealing FeCrAl fuel cladding alloys

FeCrAl alloys have been developed as one of the promising candidates of accident tolerant fuel (ATF) cladding materials for light water reactor (LWR) to replace zirconium-based alloys. In this work, different contents of Nb (0.5, 1.0, and 1.5 wt% Nb) are doped into the Fe-13.5Cr-4.5Al-2Mo (in wt.%) alloy to develop three Nb-doping FeCrAl alloys and the Nb-free alloy is fabricated for comparison. Effects of Nb-doping content on the mechanical properties (at room temperature and 800 degrees C) and high-temperature (1200 degrees C) steam oxidation (HTSO) were investigated. Although annealing temperature increased, the grain size decreased with the increase of Nb content. The volume fraction of Fe2Nb-type Laves precipitation increased with the increment of Nb content. The strength of FeCrAl alloys at room temperature and 800 degrees C were both enhanced by Nb-doping. At room-temperature, Nb-doping improved the mechanical properties through grain matrix strengthening, precipitation strengthening, grain refining strengthening, and dislocation strengthening, according to the theoretical calculation analysis. On the other hand, the precipitation of the Fe2Nb phase and the solution of Nb collectively improved the yield strength of the FeCrAl alloy at 800 degrees C. The weight gain rate and the spallation of Al2O3 scales in high-temperature (1200 degrees C) steam oxidation both increased with the increment of Nb content. Discontinuous Nb oxides at the interface of Al2O3 scales and matrix formed in the Nb-doping FeCrAl alloys, which may be caused by the limited Nb content and/or Nb oxides (Nb2O5) reacted with Al2O3. Besides, Nb oxides also enhanced the stress of Al2O3 scales, resulting in the promotion of cracks and the peeling of Al2O3 scales. Last but not least, 0.5 wt% Nb content was advising for FeCrAl alloys by considering the balance of mechanical properties and HTSO.