| During the service of the reactor,tungsten(W)and iron(Fe)based key materials will be irradiated by high-energy particles,leading to the presence of displacement defects such as vacancies(Vs)and self-interstitial atoms(SIAs).These defects further develop into large-size clusters(SIAns/Vns),resulting in the degradation and even failure of the macroscopic properties of materials.Some nanostructured materials are found to exhibit improved radiation resistance.This arises from the high-density grain boundaries(GBs)in such materials,which serve as sinks for defects.In order to prepare more excellent radiation-resistant materials,it is crucial to understand the defect-GB interaction mechanism.However,GB character,radiation conditions and the inherent multiscale characteristics of defects development make the microstructure evolution of radiation damage elusive.On this basis,in this work,taking W and α-Fe as the modeling systems,the accumulation and evolution of radiation defects in nanocrystalline/polycrystalline(NC/PC)systems were explored by using multiscale simulation techniques including molecular mechanics methods,group intelligence algorithms,rate theory(RT),kinetic Monte Carlo(KMC)and stochastic cluster dynamics(SCD).The full text is mainly divided into four parts.1.The development of simulation methods(1)Based on the stochastic simulation algorithm,the SCD simulation program has been developed,which can simulate the microstructure evolution process of displacement damage in metal systems at different doses.(2)By combining group intelligence algorithms and machine learning(ML)methods,a support vector machine(SVM)model based on differential evolution(DE)and an extreme learning machine(ELM)model based on particle swarm optimization(PSO)have been developed.(3)A DE-based framework for searching the ground-state configurations of defect clusters has been developed.2.The dependence of radiation damage accumulation in grain interior on material characteristics and radiation conditionsBy combining molecular statics(MS)calculations and steady-state RT,the defect accumulation in W/Fe systems with different grain sizes under different radiation conditions has been investigated.Under the condition of accumulative radiation,due to the biased absorption of the GB for SIAs,Vs are retained in the grain interior and gradually accumulated.At low temperature,the V is not activated and cannot undergo a long-range diffusion.In this case,the trapping effect of the GB on Vs is not reflected.Refining the grain,thus,can’t effectively alleviate the radiation damage inside the grain.At relatively high temperature and low dose rate,NC exhibit lower V accumulation concentration compared to coarse-grained(CG)systems,thus showing stronger radiation resistance.Meanwhile,GBs with a high V segregation energy can effectively capture Vs and inhibit their emission,which are expected to further promote the radiation resistance of NC materials.3.Vacancy accumulation mechanism at grain boundariesBy combining atomistic calculations,KMC and SCD methods,the interactions between the V and different GBs in α-Fe systems have been systematically explored.(1)The atomic processes of V emission and leakage were discovered in addition to the well-known trapping of the V by the GB.The occurrence of these processes depends on the V-GB binding energy and the distribution of V-trapping sites at the GB,in turn,depends on the GB character.Specifically,the general high/low-angle GB can efficiently capture the V.However,for the extremely low-angle GB,the V could pass through the bulk-like region lying between the dislocation cores.The leakage effect weakens the ability for such GB to capture and accommodate Vs.The twin boundary cannot efficiently trap the V due to the exceptionally small V-GB binding energy.The special coincidence site lattice(CSL)GB(e.g.,∑5(2 1 0)and ∑5(3 1 0))has an intermediate level of V-GB binding energy;correspondingly,the net quantity of Vs accumulated at the GB lies between the values for the twin boundary and general high/low-angle GB.(2)Under certain kinetic conditions,the V can diffuse within the GB and find a more stable site,which is termed as the V cruise.Through the combination of grain size,vacancy formation energy at the GB and migration energy barrier within the GB,we proposed the coupling equation between grain size and GB character.The V emission that originally occurred in a high energy level region could be inhibited by the V cruise along the GB as the equation is satisfied.This further brings about the uncertainty on the relation of the GB radiation response to the V-GB binding strength.4.The surface-structure dependence of accumulated radiation damageBy combining atomistic calculations,DE algorithm and KMC methods,the structure,energetics and kinetics of the Vn/SIAn on α-Fe surfaces of(100)and(110)have been investigated,which further reveal the surface-structure dependence of accumulated radiation damage.(1)Surfaces can act as sinks for SIAs/Vs with a preference for SIAs.Surfaces can provide the energetic driving force for SIAs/Vs on the surface to agglomerate,with SIAs/Vs accumulated on surface(110)exhibiting a stronger clustering tendency compared to those on surface(100).These surface SIAns/Vns can be basically divided into two types:compact configurations(type-1)and structures with branches at the periphery(type-2).Due to the difference of local atomic environments near clusters,the SIAn/Vn of type-1 corresponds to a lower SIA/V energy level compared to that of type-2.Small SIAns/Vns can contribute to the growth of large-size ones through dissociation and diffusion.While the stability of large SIAn/Vn exhibits a nonmonotonic trend with the increasing cluster size.(2)Surface(100)eliminates radiation damage in the grain interior mainly through surface-enhanced defect segregation;while for surface(110),the annihilation process involves the coupling of the V diffusion close to the surface with the SIA migration along the surface.Due to the differences in the energetics and kinetics of defects,the types of defects accumulated and survived on the surface are different,which makes the surfaces exhibit different morphologies under accumulative radiation.The above results reveal the accumulation mechanism of displacement damage within the parameter space composed of material characteristic parameters(interface character,grain size)and radiation parameters(temperature,dose rate,etc.),clarify the cross-scale image for the interaction between defects and interfaces under accumulative radiation,and provide some theoretical guidance for further optimizing the radiation resistance of PC materials based on interface engineering. |
