Helium Bubble Stability Under High-energy Cascade Effect. Alpha-iron

IntemationalThe effects of irradiation on the first wall materials of a fusion reactor are being extensively studied internationally.Because of the extremely low solubility of helium in metals,helium atoms tend to be trapped at defects and form He-V 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 bubbles in metals are studied with particular concerns.In this thesis,the stability of nm-sized He bubbles inα-Fe have been investigated using molecular dynamics simulations.The interactions of high-energy displacement cascades with helium bubbles and the stability of helium bubble have been studied.The bubbles with initial volumes of 212 and 636(?)³ are simulated,and the helium-to vacancy(He/V) ratio in the bubbles varies from 0.5 to 3.Primary knock-on atom(PKA) energy is up to 40 keV.The change of volume and He/V ratio are studied,and the effects of initial He/V ratio and PKA energy on the stability of He bubbles due to displacement cascades are analyzed.Furthermore,the effects of different interatomic potentials on the changes of He bubbles induced by cascades are considered.The first set of potentials:Ackland,Wilson-Johnson and Beck potentials were used to describe the interactions of Fe-Fe,Fe-He and He-He,respectively;the second set of potentials:G. J Ackland,T.Seletskaia and R.A.Aziz potentials were used to describe the interactions of Fe-Fe,Fe-He and He-He,respectively.The results show that the change of volume and He/V ratio due to displacement cascades depend much on the He/V ratio and the recoil energy,slightly on the bubble size.For an initial He/V ratio of 3,the cacscades lead to increase in the volume of He bubble and decrease in the He/V ratio.For an initial He/V ratio of 0.5,the interactions of cascades with the He bubbles result in the decrease in the volume of He bubble and the increase in the He/V ratio.For an initial He/V ratio of 1,the stability of the bubbles slightly depends on the PKA energy.Furthermore,the results of the effect of He bubbles with He/V ratio of 3 on cascades clearly demonstrate that the size of self-interstitial atom(SIA) clusters increases with the bubble size for the same PKA energy,but their total numbers decrease.For the same bubble,the number of small SIA clusters increases with the PKA energies,but the size remains almost constant.However,big difference between the two sets of potentials can be observed.The results are discussed trying to identify the characteristics of the potential that siglificantly affect the interaction between He bubble and cascades.The change of the number of vacancies in He bubble after cascade impacting with the second set of potentials is more significant than with the first set of potentials.Furthermore,the probability of He atoms escaping from the mother He bubble decreases with increasing PKA energy for the two sets of potentials,but the probability with the second set of potentials is larger than with the first set of potentials.When the initial He/V ratio is 3, the number of SIAs in-cascade clustering is potential dependent.The number and the size of SIA clusters with the second set of potentials after cascades are larger than with the first set of potentials for the same PKA energy,which may be attributed to more subcascades formed in the thermal spike phase with the second potentials,since subcascade overlap seems to favour the formation of larger cluster.All of the above indicate that with the first set of potentials the helium bubble is more stable than with the second set of potentials.