Effect Of Zr,Nb And Mo On Microstructure Evolution And Properties Of Fe-Cr-Al Alloys For Nuclear Fuel Cladding

After the Fukushima nuclear accident in 2011,Zr and its alloys are facing severe challenges as nuclear fuel cladding materials,and the development of accident-tolerant fuel cladding materials was put on the agenda.Fe-Cr-Al alloys are considered to be one of the most promising one due to its good irradiation brittleness and swelling resistance,splendid compatibility with coolant,superior thermal stability,satisfactory mechanical properties at high temperature and excellent oxidation resistance.Alloying elements(Zr,Nb,Mo,etc.)are commomly used to improve the strength of Fe-Cr-Al alloys.However,the addition of alloying elements leads to the precipitation of brittle phase under the operating temperature and irradiation conditions of nuclear reactor,seriously reducing the plasticity of the alloys.At the same time,alloying elements influence the high temperature oxidation resistance of Fe-Cr-Al alloys.To solve the problems,three groups of alloys were designed to study the effects of Zr,Nb and Mo contents on the microstructure evolution and properties of Fe-Cr-Al alloys for nuclear fuel cladding material systematically by means of three-dimensional atomic probe(3DAP),transmission electron microscope(TEM),scanning electron microscope(SEM),Raman spectroscopy(RS),X-ray diffraction(XRD)and Thermal-calc software.The effects of Zr,Nb and Mo contents on the type,distribution and volume fraction of precipitations in Fe-Cr-Al alloys after long-term aging at 475?were analyzed.The effects of precipitations on the mechanical properties of Fe-Cr-Al alloys at room temperature were discussed.Meanwhile,the effects of Zr,Nb and Mo contents on the oxidation resistance of the Fe-Cr-Al alloys under the atmosphere of water vapor+argon at 1200?and the room mechanical properties after oxidation were studied.The main conclusions are shown as follows:(1)The volume fraction of the primary Laves phase Fe₂Zr in Fe-13Cr-6Al-2Mo-x Zr(x=0.15,0.25,0.5,1.0)alloys increased with Zr content before aging,leading to the decrease of grain size,increase of yield strength and reduction of ductility with Zr content.When the Zr content reached 1.0 wt%,primary Laves phase mainly precipitated along the grain boundaries.After aging,secondary Laves phase mainly precipitated within in the grains and Mo segregation and?-phase Fe₇Mo₆ occurred on the grain boundaries.The yield strength of the Fe-13Cr-6Al-2Mo-x Zr alloys increased firstly and then kept constant,while the ductility decreased with aging.After the same aging time,the yield strength increased and the ductility decreased with Zr content.The yield strength increase of the alloys after aging was mainly due to the precipitation of secondary Laves phase.When the strength increase caused by precipitations and strength decrease caused by loss of solid solution element reached equilibrium,the yield strength changed little.Oxidation behavior of the Fe-13Cr-6Al-2Mo-x Zr alloys under the atmosphere of water vapor+argon at 1200?showed that the oxidation resistance of the alloys decreased with Zr content.The more Zr content,the more Zr O₂ formation after oxidation.Diffusion of oxygen ions in Zr O₂ was fast,leading to the greater depth of internal oxidation.The HV hardness of the alloys decreased with oxidation and increased with Zr content.(2)The volume fraction of the primary Laves phase Fe₂(Nb,Mo)in Fe-13Cr-6Al-2Mo-x Nb(x=0.15,0.25,0.5,1.0)alloys increased with Nb content before aging,leading to the decrease of grain size,increase of yield strength and reduction of ductility with Nb content.After aging,different kinds of second phases precipitated in Fe-13Cr-6Al-2Mo-x Nb alloys.The precipitation of(Nb,Mo)-Al-rich phase was promoted with Nb content.The dominant precipitation of the?-phase Fe₇(Mo,Nb)₆ transformed to the Laves phase with Nb content.The yield strength of the Fe-13Cr-6Al-2Mo-x Nb alloys increased firstly and then kept constant,while the ductility decreased with aging.The increase of yield strength after aging was mainly due to the precipitation of different second phases.The precipitation and growth of the(Nb,Mo)-Al-rich phase,the Laves phase and the?-phase all led to the decrease of the ductility.The precipitation of the grain boundary?-phase and secondary Laves phase resulted in the transition from ductile to brittle fracture.The oxidation behavior of the Fe-13Cr-6Al-2Mo-x Nb alloys under the atmosphere of water vapor+argon at 1200?showed that when the Nb content was less than 0.5 wt%,it had little effect on the oxidation resistance of the alloy.Nevertheless,the oxidation resistance of the alloys increased with the Nb content when it was more than 0.5 wt%,because large amounts of the Laves phase hindered the diffusion of ions.The HV hardness of the alloys decreased with oxidation and increased with Nb content.(3)The volume fraction of the primary Laves phase Fe₂(Nb,Mo)in Fe-13Cr-6Al-0.5Nb-0.2Y-x Mo(x=2,3,4)alloys increased with Mo content before aging,leading to the decrease of grain size,increase of yield strength and reduction of ductility with Mo content.After aging of the Fe-13Cr-6Al-0.5Nb-0.2Y-x Mo alloys,the(Nb,Mo)-Al-rich phase,the Laves phase and the?-phase mainly precipitated.The precipitation of the?-phase on the grain boundaries and transformation from the Laves phase to the?-phase within the grains were accelerated with Mo content.The?-phase within the grains grew anisotropically.After aging,the increase of yield strength was mainly due to the precipitation of nanoscale(Nb,Mo)-Al-rich phase and the Laves phase.The precipitation and growth of the(Nb,Mo)-Al-rich phase,the Laves phase and the?-phase all led to the decrease of the ductility.The precipitation of the grain boundary?-phase resulted in the transition from ductile to brittle fracture.Oxidation behavior of the Fe-13Cr-6Al-0.5Nb-0.2Y-x Mo alloys under the atmosphere of water vapor+argon at 1200?showed that the oxidation resistance of the alloy decreased with Mo content.The higher the Mo content,the easier the Mo oxidized to Mo O₃.Mo O₃ was very volatile,leading to the formation of micropores in the Al₂O₃-scale,benefiting the diffusion of O to the matrix.The HV hardness of the alloys decreased with oxidation and increased with Mo content.According to study of Zr,Nb and Mo contents on the microstructure evolution and properties of Fe-Cr-Al alloys for fuel cladding material,the comprehensive mechanical properties after long-term aging at 475?and oxidation resistance property at 1200?under the atmosphere of water vapor+argon of the Fe-13Cr-6Al-0.5Nb-0.2Y-2Mo alloy were the best,which was optimized as nuclear fuel cladding material for the further study.