Simulation Study On Mechanism Of Radiation Resistance Of Nanoporous Tungsten

In the reactor,the bombardment of high-energy particle to structural materials creates a large number of point defects within the material,such as self-interstitial atoms(SIAs)and vacancies(Vs).The defect migration,aggregation and further evolution will change the microstructure of the material,resulting in a serious degrade in material properties.Surfaces and grain boundaries(GBs)as effective defect sinks can improve the radiation resistance of the material.In this paper,by using molecular dynamics(MD)simulation,molecular static(MS)calculation and Kinetic Monte Carlo(KMC)method,we inestigated the radiation resistance mechanism at across-scales in nanoporou W and nano-crystal W.The thesis is mainly divided into three parts.1.Program developmentAiming at the limitation of the time scale(nanosecond)in classical MD simulation,a KMC program was developed to simulate the behavior of radiation defects on the long time scale.Irradiation damage and the repair mechanisms by surfaces and grain boundaries in tungsten were investigated at the experimental temperature and time scale.2.Surface-dependent mechanism for healing radiation-damage in nanoporous WThe anti-irradiation mechanism of nanoporous tungsten was studied by MD,MS and KMC.The study found:(1)Basic processes of primary irradiation damage in nanoporous tungsten at 1000 K.The MD simulations of primary cascade collisions reveal the basic processes involved in the evolution of the defect structure,including the preferential segregation of the SIA,the recombination of the SIA and V,the surface sputtering and adsorption.(2)Energetic and kinetic properties of defects near and on the surface.1),There is a spontaneous trapping region of the SIA near the surface.The V in the bulk tends to migrate to the surface,but with an uphill-downhill energy landscape.2),There is a spontaneous annihilation area around the SIA on the surface.3),For the SIA and V on the surface,their annihilation is completed through the movement of the SIA to V as they are far away from each other and the motion of V as they are close enough.(3)The dynamic behavior of the defect near and on the surface at long timescale.AKMC simulations show that the V near surface is segregated in an approaching-reflection manner.The trajectories of the SIA and V on the surface are related to the surface structure.(4)Picture of healing irradiation damage in nanoporous tungsten.The SIA in the bulk preferential segregates to the surface.During segregation,it can also recombine the V on its path.After segregation,the SIA will be localized on the surface,and its subsequent motion depends on the surface structure.On the surface(1 0 0),the SIA is pinned on.The V near the surface,recombines directly with the SIA,or segregates in an approaching-reflection manner.Finally,the migration of the SIA to V induces annihilation.On the(110),the SIA moves rapidly and the V in the bulk is segregated.The coupling of both motions makes V-SIA annihilation.(5)Clustering and segregation of small vacancy clusters near(0 0 1)surface.It's found the competition between segregation and clustering of vacancies near the surface.Small vacancy clusters hava a high mobility comparable to that for the single vacancy.Mono-vacancies around the cluster prefer to bind with the cluster.V2 and V3 become unstable near the surface and get trapped by the surface by dissociation.Under a low radiation dose,surface(0 0 1)could act as a sink for small vacancy clusters.3.Dynamic healing picture for radiation damage in nanocrystal tungstenThe SIAn behavior related to GB local structure was studied by using multi-scale simulations.The study found:(1)Segregation and reflection of the SIAn near the GB.MD and MS calculations reveal that the SIAn is trapped by GB when approaching the locally loose region of GB,or is reflected back into the bulk when approaching the locally dense region.The capture and reflection of the SIAn near GB corresponds to energy landscapes with downhill and uphill-downhill features,respectively.Object KMC results show that interstitial reflection mechanism contributes to the annihilation of Vs in the bulk,especially at low temperatures.(2)Annihilation of the V via dynamic interstitial emission at GB.After the segregation of the SIA,MS calculations show that the SIAs can aggregate at the GB due to low diffusion barrier and large binding energy.Object KMC simulation indicates that the SIAs first move rapidly along the GB and aggregate into the SIA2.By coupling the movement of the SIA2 along the GB and the jump of the Vs towards the GB,the defect annihilation is achieved.