Theoretical Investigation On The Irradiation Resistance Improvement Of Nickel-based Alloys Via Microstructure Modification

As one kind of clean energy,nuclear energy which has advantages of good economy,high security,zero release of toxic gases and so on has been paid more and more attention.Molten Salt Reactor(MSR)is one of the six Generation IV nuclear systems.Thorium-based molten salt reactor(TMSR)has been studied in our institute,and the nickel-based alloys are the most promising structural materials of TMSR.Nickel-based alloys have been identified as promising structural materials in GenIV nuclear reactors due to their good high-temperature creep resistance,excellent corrosion resistance to molten salts.However,the irradiation-induced void swelling and helium embrittlement in nickel-based alloys,mainly caused by the two-step transmutation reactions in strong neutron irradiation environment,would result in mechanical property deterioration and eventually be detrimental to their service performance.Hence,improving the irradiation resistance of nickel-based alloys is one of the key challenges in the development of TMSR.It is feasible to adjust the microstructure by improving the existing alloy composition and synthetic processes,which is one of main directions for optimizing the irradiation resistance of nickel-based alloys.The stability of microstructures in nickel-based alloys and the mechanism of microstructures improving the irradiation resistance are explored with the first principle calculations,which would provide theoretical basis for understanding the irradiation behavior of nickel-based alloys and guidance for designing new alloys.The detail contents and conclusions are in the following.The substitutional 3d,4d and 5d transition metal solute effect on the interstitial carbon in nickel-based alloys are systematically investigated by using density functional theory(DFT).The nearest neighbor carbon exhibits repulsive behaviors with most transition metals except Cr,which shows much attractive interaction with carbon.The carbon-metal atom interaction and its micro-mechanism are mainly governed by the local strain and chemical bonding.The introduced vacancy changes its local strain and causes local charge density redistribution,resulting in the changes of binding energy.The effects of interstitial C/N/O on the helium behavior such as solution,diffusion,and aggregation in nickel are systematically investigated,which is one of emphases in this paper.The calculation results show that the presence of interstitial C/N/O changes the prior occupation of is adjacent helium in nickel,which is related to local strain caused by the insertion of C,N or O and the chemical bonding between He and C/N/O.With the existence of C/N/O,the diffusion energy barriers for helium increase a lot when helium migrates outwards(far away from C/N/O),indicating that C/N/O can hinder the diffusion of helium in nickel.The interstitial C,N and O can attract smaller clusters and repels larger ones,suggesting they have the potential to suppress the aggregation of helium and the growth of helium bubbles.The behaviors of TiC and TiC-Ni interface adsorbing interstitial helium are studied to explore the ability of dispersed TiC nanoparticles improving the irradiation resistance in nickel.It is found that the nucleation of TiC cluster is an exothermic process in nickel.The absorption effect of TiC on interstial helium is much stronger than that of carbon or titanium in nickel,and the trapping radii of TiC is also bigger than that of carbon or titanium in nickel.The ability and radii of TiC cluster trapping helium enhance with the enlarging size of cluster.In the end,it is discovered that TiC-Ni interface is the most stable structure for helium adsorbing among the TiC bulk,Ni bulk,Ni grain boundaries and TiC-Ni interface,meaning TiC-Ni interface can prevent helium diffusing toward grain boundaries.