Influence of Ta/Zr minor-alloying on the high-temperature microstructural stability of cladding Fe-Cr-Al ferritic stainless steels

The Fe-Cr-Al-based ferritic stainless steels have attracted more attention as accident-tolerant-fuel cladding materials of nuclear reactors due to their prominent comprehensive properties. The present work investigated systematically the influence of Ta and Zr minor-additions on the microstructural stabilities at high temperatures (HTs) of Fe-Cr-Al-Mo-Nb alloys. Alloy compositions were designed in light of a cluster formula approach. Alloy ingots, in turn, were homogenized at 1473 K for 2 h, hot-rolled at 1073 K, aged at 1073 K for 24 h, and then retreated at different temperatures (1273-1473 K) for 1 h. The microstructures at different heat-treatment states were characterized with optical microscopy (OM), scanning electron microanalysis (SEM), electron probe microanalysis (EPMA) and transmission electron microscopy (TEM), respectively. The experimental results indicated that fine second-phase precipitates (primarily Laves phase) are distributed homogeneously in the ferritic matrix of aged alloys. These precipitates would be re-dissolved into the matrix when these aged alloys were retreated at a HT above 1273 K. Most of the Laves precipitates are dissolved into the matrix in the Mo/Nb/Ta-containing alloys after retreatment at 1,473 K for 1 h, while the further addition of minor Zr (about 0.1 wt %) can retard this dissolution effectively, resulting in a desirable microstructure with second-phase precipitates uniformly distributed into the refined matrix grains at HTs. Moreover, it was also found that the Zr addition can contribute to the formation of core-shell particles, in which the inner-core is enriched by Zr to form a cubic Zr2Fe phase and the out-shell exhibits a Laves phase structure segregated by Ta and Nb. (C) 2019 Elsevier B.V. All rights reserved.