Effects Of Minor Alloying Elements Addition On High Temperature Microstructural Stability Of Fe-Cr-Al Ferritic Stainless Steels

Zr alloy has been dominated in fuel cladding materials of PWR due to its low thermal neutron absorption cross section,excellent corrosion resistance and structural integrity.However,with the development of reactor technology and the increasing requirement for operational safety,more stringent requirements are put forward for the stability of cladding materials at high temperature.Zirconium alloy claddings work well under the normal operation conditions but typically burst at temperature more than 900^°C,however Fe-Cr-Al ferritic stainless steel has better neutron irradiation resistance and better oxidation resistance,corrosion resistance and mechanical properties at high temperature than Zr alloys,which is expected to become the next generation accident-tolerant fuel（ATF）cladding material.Guided by the structure model of"Cluster plus Glue Atom",this dissertation includes the composition characteristics of Fe-Cr-Al-M alloys and the regularity of adding minor alloying elements M（M=Mo,Nb,Zr,Ta）,and then designs Fe-Cr-Al-M alloys according to the cluster model.The structure characterization and performance test of the designed alloys are carried out systematically.The effect of Fe-Cr-Al-M alloying elements on the stability of microstructures and the evolution of microstructures with temperature were studied,so as to provide a candidate material for new ferritic stainless steel for Accident-resistant fuel cladding materials.The experimental results indicated that fine second-phase precipitates（primarily Laves phase）are distributed homogeneously in the ferritic matrix of aged alloys.The second phase precipitates start to dissolve in to the matrix after retreatment of these alloys at high temperature above 1000^°C.Laves phase precipitates dissolved entirely almost,into the ferrite matrix,in alloys containing minor alloying elements as Mo/Nb/Ta after 1200^°C/1h retreatment,while the alloys further minor alloyed with addition of Zr（about 0.1 wt.%）can help to retard laves phase dissolution very effectively,resulting in a stable microstructure with second-phase precipitates distributed into the refined matrix grains uniformly after high temperature retreatment of these alloys.It was also found that the Zr addition contributed towards the formation of core-shell second phase particles.These kind of precipitates were enriched by Zr at inner-core to form a cubic Zr₂Fe phase and the outer-shell exhibited a Laves phase structure segregated by Ta and Nb.