A First-principles Study Of The Grain Boundary Properties Of Materials With First Wall Structures In Fusion Reactors

Fusion energy is an important way to solve the energy problem,and structural materials are considered as one of key issues in fusion reactor.During the engineering lifetime of fusion reactor,transmutation induced by high-energy neutron irradiation can generate a large amount of detrimental impurities,which can segregate at grain boundary(GB)and deteriorate mechanical property of the structural materials.In this work,based on density functional theory,we mainly focused on He segregation and diffusion at GB in vanadium alloy and reduced activation ferritic/martensitic(RAFM)steels,as well as the temperature-dependent GB alloy phase transition in various Fe-Cr alloys,shedding some light on improving mechanical property of structural materials.Vanadium alloy is promising candidates for structural materials in future fusion reactors due to its low induced activation and high thermal conductivity.In order to investigate the interaction between He atom and high-angle GBs of V,three representative GBs were examined:?3(112)GB with a compact structure,?3(111)and ?5(012)GBs with open structures.Our results reveal that ?3(112)owns higher resistance to He segregation in comparision with another two GBs,due to its compact structure.As for He diffusion across GB,i.e.,diffusion into GB region from interior of grains,He can easily immigrate into ?3(112)with a higher rate,while is difficult to jump into ?3(111)and ?5(012)GBs.For the effect of He on the theoretical tensile strength of GBs,the cohesive strength of V GBs is weakened significantly,especially for ?3(111)and ?5(012)GBs,since He atoms deplete considerably charge density of GB region and weaken interlayer bonding strength of GBs.Reduced-activation ferritic/martensitic(RAFM)steels have become the most promising structural materials in fusion reactor,due to their high strength,high temperature resistance and good irradiation resistance.The RAFM steels are based on Fe matrix,and thus we investigated the interaction between He and high-angle GBs of pure ?-Fe.Two representative grain boundaries were considered,a compact ?3(112)GB and an open ?3(111)GB.Our results reveal that both GBs can act as effective two-dimensional sinks for helium,in which the chemical interaction between He and Fe atoms of GB region dominates the segregation tendency.For He diffusion along GB,?3(112)and ?3(111)can be inhibitors and do not provide fast diffusion piles.For He diffusion across GB,?3(112)enhances He diffusion into the GB region,while ?3(111)blocks He diffusion.The mechanical interaction induced by the distorted GB structures in the vicinity of saddle points may be responsible for the high migration energy barriers of He diffusion.As the main component of low activation steel,Cr plays an important role in the mechanical properties of RAFM steels.Thus,we studied the effect of Cr on He segregation and diffusion properties at ?3(112)of pure Fe.Our results show that,as Cr concentration increases,the GB energy decreases initially to the minimum 0.42 J/m2 and then increases until to the maximum 0.49 J/m2.The segregated Cr can increase effectively He segregation energy and suppress He segregation at GB.In addition,Cr can increase and/or decrease the He migration energy barriers along GB,depending on the Cr concentration at GB.The He migration energy barrier(Emig)along full Cr GB is 0.06 eV,which is close to the Emig of bulk 0.08 eV.In order to examine GB properties in various Fe-Cr alloys,we studied on the ?3(112)GB in Fe-rich Fe1-xCrx as a function of chemical composition(0 ? x ? 0.2)and temperature(0 ? T?900).The equilibrium amount of Cr at the GB undergoes a sharp transition from slight enrichment in low-alloyed Fe-Cr(x ?0.06-0.08)to complete Cr saturation for 0.08 ? x ?0.15.The GB chemistry at room-temperature and below is characterized by miscibility gaps,destabilizing a Fe-Cr interfacial solid solution towards decomposition into Fe-rich and Cr-rich clusters.The Cr-rich clusters at the GB may serve as nucleation centers for secondary phases in Fe-Cr alloys.In summary,two structural materials in fusion reactor,i.e.,vanadium alloy and Fe-Cr alloy,were selected to study the He segregation and diffusion behavior at GBs and its effect on the mechanical properties of GBs.Finally,in order to explore the Fe-Cr binary random alloys,the GB alloy phase with temperature and Cr concentration of matrix was studied using the coherent potential approximation(CPA)method.Our work mainly focused on the interaction between GBs and He in structural materials of fusion reactor and the grain boundary alloy phase transition,and is useful for understanding the fundamental properties of structural materials.