| Nuclear waste treatment has been the key problem influencing the sustainable development of nuclear power.Accelerator driven subcritical system(ADS),proposed for the transmutation of long-lived radioactive nuclear wastes,has been widely concerned around the world.Reduced activation ferritic/martensitic(RAFM)steels are considered as the promising candidate spallation target structural materials in ADS,due to their high resistance to irradiation swelling,superior thermo-physical properties and mechanical properties.However,the hash service condition in the spallation target puts forward higher requirements on the mechanical properties and lead-bismuth eutectic(LBE)compatibility of structural materials.In this dissertation,9Cr2WVTa RAFM steel is processed by cold-swaging and post-annealing treatment to obtain an ultrafine-grained structure with fine dispersed carbides,and to further improve its relevant properties.The mechanical properties,high-temperature oxidation resistance and LBE compatibility of the ultrafine-grained 9Cr2WVTa steel have been investigated by means of scanning electron microscopy(SEM),transmission electron microscopy(TEM),electron backscatter diffraction(EBSD).electron probe microanalysis(EPMA)and other methods.The results will provide a theoretical foundation for the further properties improvement of RAFM steel.In addition,a novel Fe2Zr phase-strengthened ferritic alloy is designed and fabricated to explore its potential as the structural material which could be used at higher temperature,and its room and elevated temperature mechanical properties and fracture mechanisms are studied.The main research works and conclusions are as follows:(1)The microstructure evolution of 9Cr2WVTa steel during cold-swaging and post-annealing process is investigated to obtain an ultrafine-grained structure with fine dispersed carbides,and its effect on the mechanical properties has been studied.Both grain and carbide sizes in the normal as-tempered sample are effectively refined after cold-swaging process with the cumulative strain of 2.81.After post-annealing at 700 ℃ for 30 min,a microstructure composed of ultrafine grains with the average size of～350 nm and fine dispersed carbides with the average size of～50 nm is obtained,along with a significant reduction in the dislocation density.Grain boundary strengthening and precipitation strengthening of carbides enhance the strength of post-annealed sample,and the presence of fine dispersed carbides not only improves the strain hardening ability,but also retards the initiation and propagation of the cracks,leading to a better ductility.In comparison with the as-tempered sample,the post-annealed sample shows more excellent room and elevated temperature strength and ductility,and better creep-rupture properties at 550 ℃.(2)The evolution of microstructure and mechanical properties in the as-tempered and post-annealed samples during aging at 550 ℃ is investigated.The results reveal that,the low deformation stored energy and the pinning effect of fine dispersed precipitations in the post-annealed sample ensure the stability of grain size during the aging process.The precipitation and growth of M23C6 carbides and Laves phases in the as-tempered and post-annealed samples are enhanced during the aging process.More high-angle grain boundaries in the post-annealed sample not only provide more nucleation sites for the precipitation of Laves phases,but also enhance the fast grain boundary diffusion of W,leading to the higher number density and area fraction of Laves phases in the post-annealed sample,in comparison with the as-tempered sample.The excellent microstructure stability of two samples during aging at 550 ℃ ensures the stability of their mechanical properties.(3)The effects of grain size and Mn content on the oxidation resistance of 9Cr2WVTa steel in air at 650 ℃ are studied.It is shown that grain refinement accelerates the fast outward diffusion of Mn,and promotes the formation of Mn1.5Cr1.5O4 and Mn2O3 oxides,which improves the compactness of oxide scale and leads to a better high-temperature oxidation resistance.Meanwhile,due to quick outward diffusion rate of Mn and high thermodynamic stability of Mn-oxides,as the Mn content in the as-tempered sample increases to 0.93 wt.%,large numbers of Mn-oxides are formed in the oxide scale,which would improve the compactness of oxide scale and lower oxygen partial pressure.As a result,a compact oxide scale composed of Mn1.5Cr1.5O4 and Cr1.3Fe0.7O3 is formed,resulting in the improvement of high-temperature oxidation resistance.(4)The effects of grain size and Mn content on the LBE compatibility of 9Cr2WVTa steel in oxygen-saturated LBE alloy at 550 ℃ are investigated.At the initial stage of corrosion,grain refinement promotes the enrichment of Mn and Cr in the oxide scale,and forms an initial compact oxide scale,which lowers the oxidation rate.However,less intergranular M23C6 carbides in the post-annealed sample lead to the less formation of intergranular Cr2O3 oxides,which has a limited retard effect on the inward diffusion of O;meanwhile,grain refinement increases the number of fast inward diffusion paths for O.Hence,as the corrosion time increases,the internal oxidation in the post-annealed sample is enhanced,and a thicker oxide scale is formed after long-term corrosion.Increasing the Mn content in the as-tempered sample promotes the enrichment of Mn on the surface of alloy,and improves the compactness of oxide scale,leading to a low oxidation rate at the initial stage of corrosion.However,with the increase of corrosion time,the permeation of Pb iinto the oxide scale and the high solubility of Mn in the Pb would lower the compactness of oxide scale,which may be the main reason that 9Cr2WVTa steel with higher Mn content shows a little thicker oxide scale after long-term corrosion at 550 ℃.Besides,in comparison with the as-tempered sample,a thicker protective FeAl layer would be formed on the cold-swaged sample after aluminization treatment,which separates the alloy matrix and LBE,and remarkably improves the LBE compatibility of ultrafine-grained 9Cr2WVTa steel.(5)Fe-Cr-W-Zr ferritic alloys with dispersed Fe2Zr phases are designed and fabricated,and their room and elevated temperature mechanical properties and fracture mechanisms are studied.When tensile at room temperature,increasing the number of Fe2Zr phases remarkably improves the strength of alloy,but the remarkable plastic strain incompatibility between the a-Fe matrix and Fe2Zr phases leads to the fracture of Fe2Zr phases,which decreases the ductility of alloy.When tensile at 700 ℃,the strength of alloy decreases due to thermally activated process.Meanwhile,the sufficient plastic deformation of a-Fe matrix and following Fe2Zr/matrix interface decohesion reduce the stress concentration at the Fe2Zr/matrix interface,which decreases the possibility of fracture in Fe2Zr phases;besides,ductile a-Fe matrix suppresses the fast propagation of cracks,finally resulting in a significantly enhanced high-temperature ductility.In comparison with 9Cr2WVTa steel,Fe-9Cr-2W-1OZr ferritic alloy shows better high-temperature strength-ductility synergy and creep-rupture property,which means it has the potential to apply as the high-temperature structural material. |
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