| The safety of nuclear power depends on the performance of nuclear materials.This work makes the SA-240 304,SA-738 Gr.B(QT)and SA-738 Gr.B(SR)steels as study objects.Using microscope,transmission electron microscope and positron annihilation spectroscopy researches the microstructure and radiatioin resistance of three kinds of nuclear steels,and then explores annealing evolution behavior of irradiated defects and relevant mechanism.The microstructure of unirradiated SA-240 304 steel is of austenite phase,and the micro-grain size number is 5.Network dislocations and twin bands are intertwined in the matrix and uneven distributed.Large size carbide particles are not observed.The morphology of SA-738 Gr.B(QT)steel and SA-738 Gr.B(SR)steels is a mixture of lath-shaped and equiaxial bainite with a small amount of medium temperature ferrites and the latter steel has longer lath bainite.The precpitates of both steels are large carbides with cubic crystal structure,including C,Mn,Fe and Ni,and small Al-rich phase.After 400keV H+ ion irradiated with different influence,the TEM samples are prepared by FIB and electroplating and then twin-jet electropolishing.The dislocation loops of sample prepared by FIB are more than that prepared by latter.It means additional defects would be introduced by FIB.Some voids with about several nanometers are observed in TEM samples of both steels prepared by electroplating and twin-jet electropolishing.Statistical analysis shows positive correlation between the density of voids and irradiation fluence.The voids density of SA-738 Gr.B(SR)steel is smaller than SA-738 Gr.B(QT)steel,indicating the SA-738Gr.B(SR)steel has higher irradiation resistance ability than SA-738Gr.B(QT)steel.The PAS results show that all the mean life values of both steels are larger than that of annealed iron and the long life values are between 150-210 ps which means irradiation produces small size defects.The T2 of SA-738 Gr.B(SR)steel do not change greatly with dose showing that vacancy clusters do not aggregate or grow.On the one hand,.there are not enough kinetic conditions for migration and growth On the other hand,the internal pressure of vacancies is large,the growth of which needs to overcome the surface energy.The τ2 of SA-738 Gr.B(QT)steel first decrease and then grow slowly.The difference of τ2 values change from the number of original vacancy defects introduced in the production process.The relative intensities I2 of both steels increase with the increase of irradiation fluence as the vacancy nucleation conditions are different The S-W curve of the irradiated steels show that only small defects are produced in steels.Grazing incidence small angle XRD indicates that small amount of austenite phases of SA-240 304 steel change into ferrite phases after Fe+ ion irradiation.Using H+ and Fe+ ion irradiation to accelerate simulated neutron radiation,the annealing evolution behavior of radiation defects are studied and the result indicates that fine and dense helium bubbles are observed in the matrices of cross-sectional samples irradiated by He+.The number density of irradiated defects decreases while the effective diameter increases with the increase of annealing temperature and the helium bubbles tend to polygonal shape.By calculating the apparent activation energy of helium bubbles,we find the mechanism of helium bubble growth is migration-polymerization mechanism in the annealing temperature of 450℃-600℃.No vacancy clusters or dislocation loops are found in SA-240 304 steel irradiated by Fe+ ion.After annealing at 450 ℃，a small number of voids begin to appear,the number density of which is still low.The size ofvoids increases while the number density decreases after annealed at 550 ℃ and 650 ℃. |
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