| Lead-based reactor has become the most promising candidate for Generation IV nuclear system due to its good neutronics,thermal-hydraulics and inherent safety properties and most likely it will be earliest used for commercialization application.The development of core structure materials is one of the key issues for the realization of the lead-based reactor,which will be operated in extremely harsh environments,including high neutron fluxes,high temperature and highly corrosive coolant.9Cr-ODS steels have become one of the most attractive potential structure material,owing to their excellent high temperature mechanical properties and irradiation resistance.The thesis is mainly on the design and study of the Si-containing 9Cr-ODS steel,for the harsh operating condition in the lead-based reactor system.The Si-containing 9Cr-ODS steel with considering of both mechanical properties and corrosion resistance to the liquid Pb-Bi alloy was designed,based on the worldwide researched ODS steels,which can be used for Generation IV nuclear system.And the optimization of Si content has been systematically investigated.The effect of Si content of 9Cr ODS steel on its microstructure,mechanical property and corrosion behavior in static PbBi was also studied.In this study,ODS steel samples with different Si contents(i.e.0,0.3,0.5 and1.Owt%)were fabricated with mechanical alloying and hot isostatic processing(HIP)methods.Firstly,the effect of Si content on the microstructure evolution of 9Cr-ODS steel was carried out.The experimental results showed that the grain structure of 9Cr-ODS steels with different Si contents were typical of a bimodal character in size-level,and the average grain size of 9Cr-ODS steels with different Si contents were 410nm,448nm and 578nm,respectively.HRTEM,TEM-EDX and APT were used to deeply investigate the compositions,grain structures and the formation mechanism of nano-particles with a size range from 5nm to lOnm.The results indicated that the Si element and nano Y2O3 followed the dissolution-reprecipitation mechanism,which meant that Si and Y2O3 both dissolved into the matrix and reprecipitated as a monoclinic cube Y2Si2O7 phase during subsequent HIP.Hence,the average grain size increased with increasing the Si content,and the nanoscale Y2Si2O7 phase formed with the addition of Si.It could be concluded that the lower Si content is,the finer grain sizes of 9Cr-ODS steel are obtained.But the Si content has little effect on the distribution of nano-precipitates.In a word,the Si-containing 9Cr-ODS steels with finer grain sizes and evenly dispersed nano-precipitates were successfully manufactured.Secondly,the effect of Si content on the mechanical property at room temperature and high temperature of 9Cr-ODS steel was investigated.At room temperature,the results showed that the yield strength of Si-containing 9Cr-ODS steels exceeded 1300MPa.The strength mainly derived from the contribution of the fine grain strengthening and second phase strengthening.The 550? high temperature yield strengths of 9Cr-ODS steels with different Si contents(i.e.0,0.3,0.5,and 1.Owt%)were 422.3MPa,795.6MPa,924.3MPa,and 972.8MPa,respectively.Lastly,the effects of Si content on the corrosion behavior of 9Cr-ODS steels in the static lead-bismuth environment with oxygen concentration of 10-6wt%and temperature of 550? were studied.The results showed that the corrosion interfacial structure consisted of three oxide films of Fe3O4/(Fe,Cr)3O4 Cr-rich ribbons of silicon-containing 9Cr-ODS steels.With the Si content increasing from 0 to 0.3%,the thickness of the corrosion layer reduced by over 50%.The addition of Si element can effectively suppress the formation of the internal oxide zone(IOZ)in 9Cr-ODS steels.On the one hand,Si had a strong affinity for oxygen,on the other hand,Si can accelerate the formation of Cr-rich ribbons.The formation mechanism of corrosion interface on the surface of 9Cr-ODS steel was analyzed deeply based on elemental diffusion theory and oxidation corrosion theory. |
PDF Full Text Request