Investigation Of Dislocation Loops In Reduced-activation Martensitic Steels And Austenitic Steels By Hydrogen And Helium Ion Irradiations

Reduced-activation ferritic/martensitic(RAFM)steels are currently the most promising class of candidate alloys for structural materials,not only in early fusion applications but also in accelerator-driven spallation(ADS)devices due to their superior swelling resistance and excellent mechanical properties.However,in fusion power reactors,and more significantly in ADS system,high concentration hydrogen and helium will be generated by transmutation nuclear reactions under high energy neutron irradiations.Both hydrogen and helium are known to enhance the growth and nucleation of voids and dislocation loops and thereby have a strong adverse impact on materials mechanical properties.And dislocation loop is one of important factors causing hardening of materials Therefore,we performed the research to investigate the hydrogen/helium synergistic effect on dislocation loops in RAFM steels.In addition,ion beam irradiation is an effective method to investigate the radiation resistance of materials.Therefore,in this work,we also investigate the irradiation resistances of two type of modified 310S steels by proton irradiations at 563 K.The details of those researches as follow:1.Hydrogen can be induced via various ways into reduced-activation ferritic/martensitic(RAFM)steels when they are used as the structural materials for advanced nuclear systems.However,because of the fast diffusion of hydrogen in metals,the effect of hydrogen on the evolution of irradiation induced defects was almost neglected.The effect of hydrogen on the evolution of dislocation loops was investigated with transmission electron microscope.Specimens of RAFM steels were irradiated with hydrogen ions to 5×10²⁰ H⁺·m^-2 at 523-823 K and to 1×10²⁰ H⁺·m^-2-5×10²⁰ H⁺·m^-2 at723 K.Experimental results reveal that there is an optimum temperature for dislocation loop growth,which is?723 K and is greater than the reported values for neutron irradiations(623K).Surprisingly,the sizes of loops produced by hydrogen ions,which are 93 nm and 286nm respectively for mean and maximum value at the peak dose of 0.16 dpa under 723 K,are much larger than that produced by neutron and heavy ion irradiation at the same damage level and temperature.The results indicate that hydrogen could enhance the growth of loops.Moreover,47.3%1/2 a₀<111>and 52.7%a₀<100>loops were observed at 523 K,but 1/2a₀<111>loops disappeared and only a₀<100>loops existed above 623 K.Compared with neutron and ion irradiations,the presence of hydrogen promoted the formation of a₀<100>loops.2.Single-beam and sequential-beam irradiations were performed to investigate the hydrogen/helium synergistic effect on dislocation loops in reduced-activation ferritic/martensitic(RAFM)steels.The irradiations were carried out with 10 ke V H⁺,18ke V He⁺and 160 ke V Ar⁺,alone and in combination at 723 K.He⁺single-beam irradiation induced much larger dislocation loops than that induced by both H⁺and Ar⁺single-beam irradiation.H⁺post-irradiation after He⁺irradiation further increased the size of dislocation loops,whilst He⁺post-irradiation or Ar⁺post-irradiation following H⁺irradiation only slightly increased the size of dislocation loops.The experiment results indicate that pre-implanted H⁺can drastically inhibit the growth while post-implanted H⁺can significantly enhance the growth of dislocation loops induced by He⁺irradiation.The mechanisms behind the complex synergistic phenomena between hydrogen and helium and the different roles that hydrogen and helium played in the growth of dislocation loops are discussed.3.The bubble-loop complexes induced by helium irradiation and effect of pre-irradiation and post-irradiation of hydrogen in reduced-activation ferritic/martensitic steel are investigated.High concentration hydrogen and helium were transmuted from 14 Me V neutron irradiation and have been one of the most challenging issues for RAFM steels for future fusion reactors.After low dose(0.18 dpa)high concentration(5000 appm)single-ion helium irradiation at 723 K,very large dislocation loops were observed,and the majority of bubbles were inside dislocation loops,forming bubble-loop complexes.These bubble-loop complexes defects were also presented in hydrogen/helium and helium/hydrogen sequential-ion irradiated steels.Pre-irradiated hydrogen ion effectively inhibited the later growth of loops induced by helium post-irradiation,and the higher the ratio of hydrogen to helium fluence,the greater the effect of inhibition.At high fluence of hydrogen pre-irradiation,the structure of bubble-loop complexes disappeared.On the other hand,hydrogen post-irradiation promoted the growth of loops induced by helium pre-irradiation,and the higher the ratio of hydrogen to helium fluence,the greater the effect of promotion.The mechanisms for hydrogen/helium synergistic effects are discussed.4.An effective method to improve the irradiation resistance of austenitic stainless steels is addition of oversized solutes into steels.The irradiation resistances of two type of modified 310S steels,in one of which Zr was added and in another Nb,Ta,and W were added,were investigated by proton irradiations at 563 K.Irradiation induced vacancy-type defects was characterized with positron annihilation spectroscopy(PAS),while dislocation loops and bubbles whose size are greater than 1nm are characterized with transmission electron microscopy(TEM).It is found that the relative S parameter?S/S extracted from PAS is more effective than S parameter in evaluating the quantity of vacancy-type defects.It was revealed from?S/S that more vacancy-type defects produced in(Nb,Ta,W)-added steels than that in Zr-added steels,and this trend became more obvious with the dose increasing.S-W curves reveal that proton irradiation induced two kinds of vacancy-type defects,i.e.vacancy clusters and proton-vacancy clusters.TEM observation shows that the density of small bubbles induced by proton in(Nb,Ta,W)-added steels is much higher than that in Zr-added steels.Both 1/3<111>and 1/2<110>dislocation loops were observed with TEM in all of the specimens.The mean size and number density of dislocation loops in(Nb,Ta,W)-added steels are slightly larger than that in Zr-added steels,and increased with increasing irradiation dose.Both PAS and TEM observations shows that irradiation damage in Zr-added steels is less serious than that(Nb,Ta,W)-added steels,and the possible mechanisms are discussed through the enhancement of point defect recombination by oversized solute atoms.