Study On Low-dose Neutron Irradiation Hardening And Formation Mechanisms For Large Defect Clusters In Zr

Irradiation damage in materials induced by energetic particles would affect seriously service life of materials in reactors,which is an important topic attracting much attention in nuclear materials study.Zirconium(Zr)alloys has a very low thermal neutron capture cross-section,and its alloys are widely used as fuel cladding tube and other in-core components in commercial water-cooled reactors due to their good mechanical properties,aqueous corrosion and irradiation resistances.Irradiation effects of zirconium alloys are very complex because of crystallographic anisotropy,texture,affection from alloy elements,impurities,heat treatment,irradaiton condition and so on.Although irradiation damage of zirconium and its alloys have been intensely studied,there are still unsolved problems on the mechanisms of hardening,growth,creep,multiscale evolution of microstructure and properties induced by radiation.In the present thesis,we mainly investigate the strong hardening of Zr-4 in the initial stage of neutron irradiation and the formation mechanism of large clusters in hcp-Zr resulting in its' hardening.Here,'large clusters' refers to point-defect clusters whose sizes are big enough to be observed in the experiments,and they have a major effect on the irradiation hardening.It is commonly assumed that the large clusters are mainly formed during thermal stage in which the point defects and small clusters produced by the collision cascades in the athermal stage(in the sense that equilibrium thermally activated processes are not significant,i.e.it would also happen if the ambient temperature of the sample would be OK)can migrate and combine with each other,to grow and transform into observable complex defects in long-time scale thermal activated processes.Besides this traditional description,we find two other mechanisms for the formation of large clusters in the athermal stage,which could be used to explain the observed dislocation loops in very low-dose irradiation experiments.Cascade overlap and comparison of irradiation damage induced by neutron with that by ion are also studied preliminarily.The main research and innovations of this dissertation are as followings:(1)Low-dose neutron irradiation experiments are carried out on zirconium alloy Zr-4 in the China Mianyang Research Reactor(CMRR)at about 40? with neutron fluences ranging from 3.0×10-9(?0.02dpa)to 8.0×1020n · cm-2(E>0.1MeV).The irradiation hardening and change in microstructure are characterized following irradiation using tensile testing,SEM and TEM examinations.It is observed that the yield stress and the ultimate tensile strength of Zr-4 strongly increase under neutron irradiation,accompanied by the decrease in the total elongation.The mechanical properties change rapidly at first and then vary slowly to saturation values.In previous researches,the irradiation hardening of Zr-4 is usually attributed to the pinning of dislocations by small point defect clusters and hindering dislocation motion by large clusters dispersed in the Zr matrix through short range interaction,here we find numerous dislocation loops in the irradiated Zr alloys,supporting this mechnism.(2)1-80keV collision cascades in hcp Zr are simulated by classical Molecular Dynamics(MD)method.We observe three types of cascades:Unfragmented(in which the cascade stays as a compact part during the whole process),Connected(in which the cascade keeps compact at the peak of damage but fragments into separated parts during the cool stage)and Unconnected(in which the cascade already fragments at the peak moment),the latter two belonging to subcascade formation.We find that subcascade already forms in 5keV cascades,and the probability of subcascade formation increases along with the energy of the primary knock-on atom(PKA),already reaching to 100%at 80keV.But it is maybe improper to replace a high-energy cascade by serveral low-energy cascades in hcp-Zr.The simulations also reveal that high-energy cascade can significantly promote the formation of defect clusters,and 10%of 80keV cascades at 500K directly create experimental-scale(around 3nm)vacancy clusters.Our results of large clusters is closely related to the very low-dose(about 0.0025dpa)irradiation experimental findings.(3)5-20keV collision cascades close to an 1/3[1120]edge dislocation(ED)in hep Zr are studied by MD.We find that when the cascade zone(or a part of the cascade zone,when subcascades are formed)is heavily asymmetrical around the ED,the pre-existing ED is able to significantly promote the nucleation of the vacancy clusters,and a single 20keV PKA can even directly create an experimental-scale vacancy loop(?3nm)with Burgers vector 1/2[1100].With an additional heat annealing,this vcancy loop can be transformed into a 1/3[1120]dislocation loop,in good agreement with experimental observations.It is the first time that the whole damage process in hcp Zr from a PKA-initiated collision cascade to the formation of an experimental-scale 1/3[1120]vacancy loop is continuously observed by simulations.Our results reveal that intrinsic dislocations in materials can enhance large clusters formation in one single collision cascade nearby,and offer another probable mechanism for the formation of high-density dislocation loops in the low-dose irradiation experiments of hcp Zr.(4)5keV and 10keV cascades overlap during athermal stage at three different temperatures(30K,300K and 600K)in hep Zr are simulated by MD.We find that the total number of point defects mainly depends on the irradiation dose(dpa),unrelated to the PKA energy.During continuous irradiation process without long-time thermal migration of small defects,the vacancy(Vac)clusters are mainly formed from the collapse of displacement cascades.Besides the mechanism analogous to the Vac clusters,the self-interstitial atom(SIA)clusters can also be formed outside the cascade zone by short distance migration and aggregation.This difference in the mechanisms of cluster formation leads to quite different trends of the fraction of Vac and SIA clusters along with the irradiation dose.We also find that in the process of cascades overlap,new cascade can promote the splitting of one cluster or the combination of several clusters,and experimental-scale clusters can be created.We also observe small c-type vacancy loops in the basal plane of hcp Zr,and calculate the formation energy of a-type and c-type clusters under different strain fields by MD.The results offer a possible mechanism for the nucleation of c-type vacancy loops:tensile strain along the<a>direction and compression strain along the<c>direction caused by irradiation growth of hcp Zr can result in a formation energy of c-type loops smaller than that of a-type loops.(5)Because of the simple structure and properties of 6H-SiC compared with that of hcp Zr,equivalent mechanism in total crystal disorder values of SiC irradiated by low-dose neutron and energetic ion are preliminarily studied with Raman spectroscopy,in order to accumulate experience for the afterward comparison of irradiation damage in hcp Zr induced by neutrons and energetic ions.We find that the total disorder of 6H-SiC following neutron irradiations approximately agrees with that caused by energetic ions at the same dpa in the low-dose irradiation stage at near room temperature,because the low-dose irradiation mainly produce dispersed simple Frenkel pairs and small clusters,and these defects are difficult to interaction with each other to produce extended defects.Our results can provide references to the research on the mechanisms of irradiation hardening and the influence of dislocation loops acting as obstacles for dislocation glide in zirconium,which is significant for the development of the study on irradiation effects on materials and of new zirconium alloys.In future,we will continuously study comprehensively the higher-dose irradiation damage induced by neutrons and investigate the evolution of defects in the long-time thermal stage based on the achievements in this dissertation.