In-situ TEM Research On The Bebavior Of FeCr Alloys During Ion Irradiation And Annealing

FeCr alloys are primary candidate materials used as fuel claddings in the reactors because of their excellent mechanical properties,good resistance to irradiation damage and corrosion.When FeCr alloys served in nuclear reactors,neutron irradiation and the resulting in helium atoms((n,α)reaction)will produce a large number of irradiation defects such as vacancy clusters,dislocation loops and helium bubbles,causing an obvious degradation of FeCr alloy properties.Therefore,understanding the microstructure evolution and mechanism of FeCr alloys under irradiation is of great significance for their service life assessment and the development of irradiation theory.Therefore,in this thesis,the behaviors and mechanism of insitu ion irradiation and in-situ post-irradiation annealing of new types of Fe9Cr1.5W alloy and Fe13Cr5.6Al alloy used as the candidate of fuel claddings in nuclear reactors were studied using Multiple Ion Beam In-situ TEM Analysis Facility.The evolution characteristics and behaviors of dislocation loops,helium bubbles,vacancy clusters and precipitates induced by irradiation were observed and analyzed in-situ.On the basis of quantifying characteristics of irradiation defects,the relative mechanisms were discussed.The matrix of Fe9Cr1.5W alloy consisted mainly of body-centered cubic lath martensite with the width of 200-300 nm.The martensitic laths became thicker with the increase of Si content.A large number of dislocation lines were distributed in Fe9Cr1.5 W alloy.There were many Cr23C6 carbides distributed at the martensite boundaries and fine TaSi2 and V(C,N)particles distributed in the matrix.Fe13Cr5.6Al alloy was BCC ferrite crystal structure with a grain size of over 1 μm and with many dislocation networks and lines.Meanwhile,many precipitates of mainly AlN,Al-O and ZrC phases with different sizes could be founded in Fe13Cr5.6Al alloy.The evolution of irradiation defects such as helium bubbles,dislocation loops and voids in Fe9Cr1.5W alloy was observed by in-situ TEM under different irradiation conditions(irradiation ions,irradiation temperatures,irradiation dose,etc.).The results showed that the larger the irradiation temperature and dose were,the larger the average size of helium bubbles and dislocation loops would be,but the lower the density.During He+ irradiation at room temperature,the irradiation defects were mainly small-sized black spots.When the irradiation temperature increased,the dislocation loops grew by two mechanisms:absorption point defects and defect clusters or the combination of dislocation loops which led to their density reduction.Under H2+/He+ irradiation at 500℃,it was found that new dislocations could be produced by the Frank-Reed source.Under Fe+irradiation at 450℃,with the increase of irradiation dose,dislocation loops would merge with other loops or pre-existing dislocation lines and gradually evolved into dislocation networks.The proportion of a<100>loops and a/2<111>loops were 77.25%and 22.75%at 1.12 dpa,respectively.Under H2+irradiation at 600℃,the grain boundary obviously affected the distribution characteristics and morphology of dislocation loops.At the irradiation dose lower than 0.035 dpa,there were high density dislocation loops in the region far from the grain boundary.However,when the dose exceeded 0.035 dpa,loop density decreased due to the their aggregation.In He+ irradiation experiments,He atoms introduced by irradiation combined with vacancies produced by lattice damage to form HenVm,clusters.Helium atoms,vacancies and defect clusters were preferentially nucleated and grew at dislocation lines,grain boundaries,and precipitates.As the size and density of helium bubbles increased,the density decreased significantly due to the merging of helium bubbles with each other.As the irradiation dose increased,the distribution of helium bubbles in He+ irradiation experiments at 450℃ changed from the initial single-peak distribution to a bimodal distribution.In the H2+/He+ irradiation experiments,<100>loops and helium bubbles together formed bubble-loop complexes.In the Fe+irradiation experiments at 450℃,the M23C6 carbides in the matrix still maintained an crystal structure although the dose reached 31.14 dpa.The voids preferentially nucleated and grew at grain boundaries and precipitations and had a larger size.Fe9Cr1.5W alloy irradiated by He+ at 500℃ was then annealed at 600℃.The bubbles at the grain boundaries formed slender necks between them,which promoted their aggregation through Migration and Coalescence(MC)mechanism controlled by surface diffusion,forming the super large bubbles with the size over 100 nm.Annealing induced the aggregation of helium bubbles at the dislocations to form bubble lines.Meanwhile,some helium bubbles began to transform from their original spherical shape to preferential faceted shape.To investigate the effects of annealing temperature and time on the behavior of irradiated defects,the gradient annealing experiments were performed on the Fe13Cr5.6Al alloy pre-implantd with a fluence of 4.3×1016 He+/cm2 at room temperature,and it was found that the diameter of helium bubbles increased with the rise of annealing temperature and time under the control of surface diffusion mechanism.When the temperature increased to 1072 K,the Brownian motion of bubbles induced by the temperature effect promoted the coalescence and coarsening of bubbles under the MC mechanism,leading to a decrease in the bubble density.