Development Of Irradiation Damage And Irradiation-induced Defects In Multi-component Low Alloy Ferritic Steels

The nuclear reactor pressure vessel is a critical component for the safe operation of nuclear power plants.Nowadays,the A508-Ⅲ steel,with low cost in production and excellent mechanical properties is widely used to manufacture the reactor pressure vessel.During the lifetime of nuclear power plants in operation,the pressure vessel will suffer with neutron irradiation,resulting in the degradation of mechanical properties,such as irradiation hardening,due to the formation of irradiation-induced Mn Ni Si clusters and dislocation loops which hinder the movement of dislocations.Recenlty,some nuclear power plants have approached the initially designed service lifetime of 40 years,and the demands for life extension are increasing day by day.Therefore,it is necessary to understand the evolution of the microstructure and mechanical properties under high doses.In addition,the stable service temperature of reactor pressure vessel is 270 ～ 290 ℃.But based on the type of the nuclear pwer plants and actual working conditions,the maximum temperature can reach about 330 ℃(pressurized water reactor),and the minimum temperature can reach about 180 ℃(boiling water reactor).Currently,most studies are carried out around 290 ℃ and foused on the evolution of irradaiton hardening and irradaiton defects at a low dose level(around 0.1 dpa).They also concentrate on exploring the effect of Mn and Ni on formation of Mn Ni Si clusters.There is relatively less research addressing the effects of high doses and irradiation temperatures on irradiation damages.Moreover,the effect of Si on the irradaiton hardening and irradaiton defects remains to be explored systematically,given that Si is an important element in Mn Ni Si clusters.To avoid complex interactions among multiple elements,simple binary,ternary and quaternary model alloys have been employed to investigate the simple effect of each single element as well as combined effects of two elements.Hence,based on the A508-Ⅲ steel,the Fe-Mn-Ni(-Si)model steels are made for detailed investigation in this research project.Nanoindentation test(NI),backscattered electron diffraction(EBSD),focused ion beam(FIB),transmission electron microscopy(TEM)and atom probe tomography(APT)are employed to study the changes of the microstructures and mechanical properties under irradiation at different temperartures(room temperature,250℃,350℃)and doses(0.1 dpa,0.6 dpa,1.5dpa)in the two steels.The quantitative analysis of the Mn Ni(Si)clusters are carried out to reveal the evolution of the clusters under different irradiation conditions,and to explore the influence of Si on clustering.The dispersed barrier hardening(DBH)model is applied to establish the relationship between the microstructure and mechanical properties,in order to study the contribution of clusters and dislocation loops for irradiation hardening,furtherly to clarify the effect of Si.Additionally,the effects of irradiation temperature,irradiation dose and Si for grain boundary segregations are studied.The main conclutions of this investigstion are as following:(1)The relationships among the number density,volume fraction and equivalent radius of the irradiation-induced Mn Ni(Si)clusters are systematically investigated for the first time under different irradiation doses and temperatures,revaling that the concentrations of clusters in different irrdaition temperature regions are different.At the same time,the coalescence of adjacent clusters could be one reason for the rapid growth of Mn Ni(Si)clusters.(2)The slow evolution of the composition of irradiation-induced Mn Ni(Si)takes precedence over the rapid growth of the cluster.At the same time,the addition of Si prolongs the time required for the adustement of cluster composition,and delays the of rapid growth of Mn Ni Si clusters to 1.5 dpa or later.(3)The addition of Si mainly decreases the irradiation hardening from the high number density of dislocation loops at different irradiation temperatures,due to the reduction in size.At the same time,with the temperatures and doses increase,the increasing high number density of Mn Ni(Si)clusters is the main source for the increment of irradiation hardening.When the number density of clusters is much higher than the number density of dislocation loops,the strengthing meachanism of part solute clusters could be shearing mechanism,resulting in decrease of the serviceability of DBH model.(4)The increase of temperatures and doses could promote the segregation of Mn,Ni and Si elements at the high angle grain boundaries.The increase of irradiation temperature may reduce the segregation of C at the high angle grain boundaries,but further increase in irradiation dose above 0.1 dpa may not lead to reducation of the low segregation level of the C element.At the same time,the Ni and Si atoms may exhibit competition for lattice sites at the high angle grain boundaries.Additionally,the Mn-enriched precipitates with a stable Mn content(equivalent radius of 3 nm)are discovered on the high angle grain boundary in Fe-Mn-Ni sample at 350 ℃ with 0.1 dpa.