Radiation Damage Studies Of The Novel Cr/W Metallic Multilayer Nanocomposites

Nuclear power is a kind of clean and high efficiency moden green energy,which is vigorously developed in China.In order to ensure the smooth development of the nuclear power technology and safe,efficient operation of the nuclear power systems,it is important to solve the problem of the key materials in nuclear power engineering.The proposition of lifetime extension of nuclear power plant and the rapid development of the advanced nuclear energy system in recent years make the irradiation resistance standards of the nuclear reactor structural materials become higher.The successful deployment of advanced nuclear energy systems depends critically on the development of new materials that possess greater radiation tolerance.As novel nanostructure radiation tolerant materials,the nano-scaled metallic multilayer composites show excellent self-healing performance against irradiation damage and are regarded as one of the potential structural materials of the fuel cladding,core structure and fusion reactor first wall structure materials in the next generation of nuclear reactor systems.Focusing on the investigation of irradiation damage resistance of Cr/W multilayers under extreme radiation environments,this dissertation presents the explanation of design consideration of selecting the Cr/W system to organize multilayer composites firstly,and then discusses the radiation responses of Cr/W multilayers irradiated by various types of ions.The detail studies include four aspects:characterization of the neutron induced damage effect in Cr/W multilayers based on Monte Carlo simulation,preparation and the ions irradiation experiment methods on Cr/W multilayers,surface damage and mechanical properties degradation of Cr/W multilayers irradiated by Xe ions,and hydrogen/helium behavior in Cr/W multilayers.The main results and conclusions were summarized as follows:(1)The present study investigated the neutron irradiation displacement damage rate,spectra of the primary knocked-on atoms(PKAs)produced in the cascade collision,and the H/He ratio in Cr/W metallic multilayer nanocomposites versus neutrons' energy.Results suggest that the displacement damage rate of Cr/W multilayers is in the range of 1 × 10-22-2.3 × 10-21 dpa/(n/cm2)and increases with the increasing of energy of incident neutrons.The transmutation hydrogen and helium accumulation rate is very low when energy of the incident neutron is 1 Me V.With the increasing of neutron's energy,the accumulation rate become higher,but the yield of hydrogen and helium is only 0.022 /n and 0.005 /n when irradiated by 14 Me V neutrons,respectively.(2)Cr/W multilayer films were synthesized by magnetron sputtering at room temperature.Xe20+ ion irradiation experiments were performed on the Cr/W multilayer films to investigate the radiation responses and evolution of surface morphology and mechanical properties in neutron irradiation with the same dpa value.The XRD and TEM results reveal that interfaces act as strong barriers to grain coarsening and the irradiation-induced swelling.After Xe20+ ion irradiation,the decrease of hardness and Young's modulus of the Cr/W multilayer films was also less than the pure W films,due to the strong inhibiting effect of the Cr/W interface against grain coarsening.(3)The formation process of hydrogen bubbles in Cr/W multilayers was investigated in the dissertation.A special multi-energy hydrogen ions irradiation experiment was designed based on the calculation results of SRIM.The effects of annealing and modulation period of Cr/W multilayers on the nucleation and growth of the hydrogen bubbles were discussed by using XRD,TEM and nano-indentation.Results reveal that the hydrogen bubbles can be observed in Cr/W(50 nm)multilayers only after annealing at 600 ? when the hydrogen concentration reaches 3 at.%.The Cr/W(5 nm)multilayers remain unchanged in the same condition.This result indicates that the hydrogen interstitial atoms are trapped by the interface and their migration capacity decrease significantly.This phenomenon may help to the delay of hydrogen bubble formation.(4)The formation process of helium bubbles in Cr/W multilayers was also investigated in the dissertation by multi-energy helium ions irradiation experiment.Results reveal that the Cr/W interfaces not only can trap helium interstitial atoms,but also act as strong barriers to the growth of helium bubbles.The hardness and Young's modulus of the Cr/W multilayer films decrease when the multilayers are irradiated by the hydrogen and helium ions.The higher density interface structure seems helpful in keeping the mechanical properties of films stabe.(5)The efficient trapping capabilities of hydrogen and helium atoms at the Cr/W interface were further studied by neutron reflectometry(NR).Results show that the NR data from the initial Cr/W(50 nm)multilayers can be well fitted by the 6 layered slab model.However,layered slabs with high H/He concentration were detected to be localized at the first four Cr/W interfaces near the surface from cases of H/He ions irradiated Cr/W(50 nm)multilayers.This phenomenon provides clear evidence for the role of Cr/W interfaces as sinks for the H/He atoms.This dissertation presents important theoretical foundation and experimental support for the radiation damage resistance study of the Cr/W multilayer nanocomposites,and brings it closer to the practical application in advanced nuclear energy systems.