Thermal Stability And Irradiation Tolerance Of Cu-based Nanostructured Metals

With the development of the nuclear reactors,high-performance alloys with ideal irradiation tolerance,high chemical stability,thermal stability and mechanical properties are under great demand.It has been well investigated that materials with induced sinks(e.g.grain boundaries(GBs)or interfaces and free surfaces),especially those with high sink strength or high density are particularly effective to enhance irradiation tolerance.However,nanocrystalline materials with large volume fraction of GBs always suffer from poor thermal stability because of their increased GB free energy and the associated strong driving force for grain coarsening.It has been reported that in nanocrystalline materials,significant grain growth could be observed at much lower temperature than that of their coarse-grained counterparts.This resulted in severely degrading mechanical properties and irradiation tolerance,making it challengeable for the structural materials as a candidate for nuclear reactors.In this work,nanocrystalline Cu and dilute Cu-W alloys and carbon nanotube(CNT)-Cu/(Cu-W)hybrid with different grain sizes and compositions were prepared by magnetron sputtering technologies.The microstructure evolution during annealing at elevated temperatures and ion irradiation were examined.The intrinsic mechanism of thermal stability,irradiation tolerance and corresponding microstructure evolution were investigated.The main conclusions/results are as follows:The effect of substrate temperature on the microstructure of Cu and dilute Cu-W alloys was studied.Thermal stability of the samples with different W contents was investigated by annealing at 673 K to 873 K for 3 h.Thermal stability could be significantly enhanced with W addition,since the GB migration could be inhibited by the W particles segregation during annealing at elevated temperature.With substrate temperature raised,the grain size of Cu and dilute Cu-W alloys increase,which can be derived from the promotion of adatom mobility and surface diffusion Grain size of the Cu-W alloys is significantly smaller than that of the Cu deposited at the same substrate temperature,which is related to the restriction of the Cu atomic diffusion in Cu-W alloys by the addition of W atoms with low diffusion coefficient.The sink strength of nano-GB is quantitatively investigated through the nano-grained Cu and dilute Cu-W alloys with the average grain size ranging from～20 to～50 nm by～300 keV He ions irradiation at room temperature(RT)and 673 K,respectively.Size,volume ratio and number density of voids are highly temperature and grain size dependent.With the increase of temperature and the decrease of grain size,the stable nano-GBs exhibit a behavior of "ideal" defect sink due to their high-volume ratio and the increased point defect recombination probability,which offers a guidance for designing nano-grained structural materials with optimum anti-irradiation performance for future fusion reactors.Cu nanograins were further deposited on the CNT network by a magnetron sputtering method.To investigate the structural thermal stability,the CNT-Cu hybrids were annealed at 473 K to 1073 K for 3h.After annealing at 473 K,the Cu nanograins obviously suffered from serious grain coarsening and evolved into a sphere-like morphology due to the higher Cu/CNT interfacial energy,indicating the poor structural thermal stability,which is similar to the Ostwald Ripening phenomenon.After irradiation under 300 keV He ions,the size and number density of irradiation-induced voids are much smaller than that of the conventional dense films after irradiation under the same condition.This is related to significantly increased GB volume fraction,specific surface area,and decreased diffusion rate of irradiation-induced defects.The growth in grain size of the CNT-Cu hybrids is much lower than that of the conventional dense films,since the one-dimensional substrate(CNT)could control the GB migration perpendicular to the CNT axial direction.In addition,CNT-Cu/(Cu-W)hybrids with and without coated a carbon layer were further prepared to investigate the effect of W atoms and surface/interface complexions on the structural thermal stability and irradiation tolerance.CNT-(Cu-W)hybrids with different W contents were annealed at 473 K to 873 K for 3 h.With the W content increasing,initial temperature of W atoms precipitated and grain size was reduced,indicating an enhanced structural stability.This is resulted by the decrease of atomic diffusion,the enhancement of solute atom drag on GB and the increase of precipitated phase volume fraction.A carbon layer was coated on the surface of the hybrids by dehydrogenating an organic vapor of paraffin based mineral oil under He-ion irradiation at RT.The carbon coated samples(CCS)with large density of GBs and interfaces presented unprecedentedly enhanced structural stability and irradiation tolerance.In this work,nanocrystalline Cu and dilute Cu-W alloys and CNT-Cu/(Cu-W)hybrid with different grain sizes and compositions were prepared by magnetron sputtering technologies.The mechanism of the effect of W atoms on enhancing thermal stability was explored.The intrinsic mechanism of thermal stability,irradiation tolerance and corresponding microstructure evolution were investigated.The influences of GB complexions on the sink strength were systematically studied,and the influence mechanism of grain size on the irradiation tolerance was revealed,providing both experimental data and theoretical instructions for the research of advanced structural materials,and promoting the development of nuclear power generation industry in china.