Atomitic Simulation Of The Behaviors Of Helium In Metals

Because of the extremely low solubility of helium in metals,helium atoms generated mostly by(n,α) reactions of neutrons with matrix nuclei during neutron irradiation tend to be trapped at defects that create excess volume within the crystal such as vacancies,dislocations and grain boundaries.It has been observed that helium easily precipitates into clusters or bubbles.The formation of helium bubbles in materials can lead to void swelling and produces high temperature intergranular embrittlement, surface roughening and blistering.These effects would significantly degrade the mechanical properties of materials,therefore,the production and evolution of He atoms in metals are studied with particular concern.In this paper,a systematic investigation of the atomitic behavior of helium inα-Fe has been carried out by molecular dynamics(MD) methods and MOLDY code. Furthermore,ab initio calculations based on density functional theory have been performed to study the relative stability of single He defect in bcc and fcc metals and the dissolution and migration of helium,and the stability of small helium-vacancy clusters in aluminum.1.Displacement cascades are simulated by MD methods inα-Fe containing a concentration of substitutional He atoms varying from 1 to 5 at.%at irradiation temperatures of 100 K and 600 K.Furthermore,the effects of using different interatomic potentials in MD simulations of the formation of He-vacancy clusters within displacement cascades inα-Fe are investigated using two sets of potentials.The results are compared with those performed in pureα-Fe.There are distinct differences in the number and size of point defects and defect clusters within displacement cascades with and without substitutional helium atoms.We find that the effect of the irradiation temperature on Frenkel pairs(N_F) is small as the temperature increases from 100 K to 600 K and the total number of point defects increases with increasing He concentrations and primary knock-on atom(PKA) energies.Particularly, large numbers of helium-vacancy(He-V) dusters are generated directly in displacement cascades,and the sizes of these clusters are much larger than those observed for clusters in pureα-Fe,i.e.in the presence of substitutional He atoms large vacancy clusters can be nucleated directly in displacement cascades.Furthermore,the number and size of He-V clusters generally increase with increasing helium concentration and PKA energy. The number density of He-V clusters almost increases with increasing temperature, while the mean size of He-V clusters remains almost constant for the same He concentration and PKA energy.Although the effects of interatomic potentials on the nucleation of He-vacancy clusters induced by cascades are relatively small,the number and size of He-vacancy clusters produced are significantly different for the different potentials employed in this study.From these studies,it is dear that Fe-Fe potential has significant effects on the formation of He-V clusters within cascades,followed by Fe-He potential,but He-He potential has little influences on cascade-induced cluster formation.2.The interactions of displacement cascades with helium-vacancy clusters are investigated using molecular dynamics simulations.Initial He-V clusters consist of 10 and 20 vacancies with the He-to-vacancy(He/V) ratios ranging from 0.2 to 3 and the PKA energy,Ep,varying from 2 keV to 10 keV.The effects of a single displacement cascade on the stability of a He-V cluster depend on the He/V ratio and the PKA energy. When the initial He/V ratio is larger than 1,the size of He-V clusters increases,and the He/V ratio decreases with the PKA energy increasing,but when the initial He/V ratio is less than 1,the size of He-V clusters decreases,and the He/V ratio increases with the PKA energy increasing.The clusters with the He/V ratio of 1 are relatively stable, which is independent of the PKA energy.One of the striking results is that large He-vacancy clusters are more stable than small clusters for the same He/V ratio under mono-irradiation condition.During multiple 5 keV cascade events,the final size of He-V clusters only depends on the initial He/V ratios.It is of interest to notice that the number of vacancies in a He-V cluster is determined by the first cascade event,while subsequent cascade overlap has a significant effect on its stability.These results are discussed in terms of the internal pressure of He-V clusters,the mobility of He atoms,the number of vacancies produced by cascades and the He/V ratio.3.Molecular static methods are performed to study the interaction between He-V clusters and an a/2＜111＞{110} edge dislocation inα-Fe through calculating the binding energies of He-V clusters in and near the edge dislocation with empirical potentials.The results show that the interaction depends on the He/V ratio of the clusters.For the ratio equal to or larger than 1,He-V clusters are strongly trapped on the tension side of edge dislocations and repelled from the compression side of edge dislocations,however,for the ratio less than 1,He-V clusters are trapped within about 0.5 nm from the dislocation core on both the compression and tension sides.On the slip plane,the effects of dislocations on the formation of He-V clusters is very small for the distances more than about 1.3 nm from the dislocation core.4.The relative stability of single He defect in bee and fee metals is investigated using ab initio calculations based on density functional theory(DFT).The calculations reveal that for all metals involved,except for V and Nb,the substitutional position is the most stable.Furthermore,the tetrahedral site is energetically more favorable for the He interstitial than the octahedral site in the bee metals,but the relative stability of He defects in the fcc metals is not definitive.It is interesting to find that the magnetism of host atoms does not directly affect the relative stabilities of He interstitial sites.Ab initio calculations based on density functional theory is also performed to study the dissolution and migration of helium,and the stability of small helium-vacancy clusters He_nV_m(n,m=0 to 4) in aluminum.The results indicate that the octahedral configuration is more stable than the tetrahedral.Interstitial helium atoms are predicted to have attractive interactions and jump between two octahedral sites via an intermediate tetrahedral site with low migration energy of 0.10 eV.The results indicate that the divacancy and trivacancy clusters are not stable,but He atoms can increase the stability of vacancy clusters.The binding energies of an interstitial He atom and an isolated vacancy to a He-V cluster mainly depend on the He/V ratio of the clusters rather than the cluster size.