Molecular Dynamics Simulation Of Helium Segregation And Its Damage Behavior In ? Phase Of Fe-Cr Alloy

First wall material of the fusion reactor will suffer from harsh service conditions(e.g.,high temperature,high pressure and high flux neutron irradiation),and these will lead to helium(He)cumulative effects and atomic displacements from radiation cascades,which are of great importance to safe and stable operation of the fusion reactor.He atoms can easily migrate and form He bubbles with other defects(such as vacancies,dislocations and grain boundaries)owing to low migration energy,which are responsible for severe degradation of structural materials(e.g.,swelling,hardening,embrittlement etc.).But at present,the microcosmic mechanism of these phenomena is not very clear,it is difficult to deeply further understand He diffusion and segregation behaviors,as well as the interaction between He bubbles and material structure defects through the experimental method.Molecular dynamics(MD)is a simulation technology to study multi-atomic or molecular systems at the atomic scale,and it has incomparable advantages in revealing the microscopic physical mechanism of materials.The research mainly focuses on Fe-Cr alloy with body centered cubic structure,and irradiation defects(including point,line,body defects and macroscopic crack)are main lines in this paper.Molecular dynamics simulation was used to investigate the point defects diffusion and equilibrium relaxation evolution behavior,formation and evolution of dislocation(ring)structures,He bubbles nucleation and growth mechanism,and He bubbles interaction with the dislocations.Effect of material structure defects(precipitated phase,dislocations,voids and cracks),temperature and composition on He atoms diffusion and segregation behavior in Fe-Cr alloy were also investigated.All these results revealed He bubbles equilibrium evolution,segregation behavior and their effects on plastic deformation mechanism at the atomic scale.The main conclusions can be summarized as follows:(1)Fe-14Cr electron irradiation models were established by imbedding Frenkel pairs,and point defects diffusion and evolution behaviors were investigated.The results showed that,during the relaxiation process,Fe and its substitutional element(such as Cr)atoms could diffuse in short distance,and the evolution of point defects only promoted the formation of vacancy clusters and dislocation loops.The ordered enrichment of substitutional element and intermediate phase(Cr-rich a' phase)could not observed in the entire process.With the increase of Frenkel pairs concentration,Fe and Cr atoms diffusion increased,and the possibility of dislocation loops and vacancy clusters nucleation also increased.With the increase of temperature,the probability of Frenkel pairs recombination increased until the recombination was complete,the number of vacancy clusters and dislocation loops decreased.Especially,at 1200 K,complete recombination of Frenkel pairs occurred at 2.6 ns.Cr interaction with interstitial atoms could form<1 1 1>Fe-Cr,Cr-Cr crowdion,which resisted the recombination of Frenkel pairs and increased dislocation density.(2)Edge dislocation models with different dislocation densities and spatial configurations were built in Fe-Cr alloy,and He bubble interaction with dislocations was investigated.The microcosmic mechanism that low temperature could inhibit He bubble growth,and high Cr contentration could promote He bubble nucleation,were also elucidated.The results showed that,He diffusion behavior could be divided into two mechanisms due to He concentration:one was long-range diffusion mechanism at low concentration of He atoms(<0.1 at.%),and the other was short-range diffusion mechanism at high concentration of He atoms(>0.1 at.%).Higher temperature and lower Cr concentration could increase He atom diffusion rate and dislocation mobility.Furthermore,it was found that He segregation on dislocations could adjust the spatial distribution of He bubbles,and larger He bubbles with discrete distribution could promote dislocations multiply and enhance deformability in the process of tensile deformation.Therefore,material irradiation brittleness could be improved by controlling dislocation distribution.(3)Voids and Cr-rich ?' phase with different number densities and sizes,and He bubbles with different He/V ratios were built in Fe-Cr alloy.Through tensile testing,according to the stress-strain response,plastic deformation mechanism of voids,He bubbles and Cr-rich ?' phase in Fe-Cr alloy were investigated and atomic scale mechanism of voids,He bubbles and Cr-rich ?' phase accelerating matrix fracture process were revealed.The results showed that,plastic deformation of Fe-Cr model with and without Cr-rich ?' phase could be divided into four stages:elastic deformation,fcc phase transition,hcp fault and fracture.Plastic deformation of Fe-Cr model with voids could be divided into three stages:elastic deformation,fcc phase transition,and fracture.The number density increase of voids and Cr-rich ?' phase could inhibite hcp fault generation.The diameter increase of Cr-rich ?' phase could induce fcc phase transition and hcp fault generation.When the He/V ratio of He bubble was different,the fracture mechanism of the He bubble model was different:with the increase of He/V ratio from 0.5 to 5,the fracture mechanism was changed from fcc phase transition mechanism to dislocation strengthening mechanism.(4)The effects of different crack orientations,irradiated He and substitution Cr concentration on crack propagation were clarified.The results showed that,(0 0 1)[0 10]crack growth behavior was suffering from elastic deformation,phase transformation,and cleavage fracture along the phase zone boundary.(1 2 1)[1 1 1]crack growth behavior was suffering from elastic deformation,twin transformation,and multiple cavity coalescence fracture due to stress concentration at the twin tip.He and Cr concentration had opposite influence on the crack growth behavior.The influence of He concentration could be divided into two respects:At low concentration(<0.9 at.%),He addition could only slow down the rate of phase transition and twin transition,and the fracture mechanism of two crack models could not be changed.At high concentration(>0.9 at.%),the fracture mechanism of two crack models were changed,and He clusters could evolved into cavities,multiple cavities coalescence and fracture in both of two crack models.However,the influence of Cr atoms on crack growth behavior in two crack models was independent of Cr concentration,Cr addition could increase the phase transition ratio and accelerate the twin transition rate,which could postpone the fracture of(0 0 1)[0 1 0]crack but induce the fracture of(1 2 1)[1 1 1]crack.