First-principle Study On Structures And Properties Of U-Co Alloys

U-based material is a kind of important nuclear material with high density and high strength,showing broad prospect in the fields of weapons,aerospace,energy,medicine and so on.Especially,U-based metallic glasses(MGs)are expected to further widen the application of U-based materials as structural materials,so it is worthy to be investigated.However,the irradiation of U increases the risk involved in experiments,hence it is of much importance to study the basic properties and glass forming process of U-based alloys by computer simulation.On the basis of previous research,the structures and properties of U-Co solid solutions,melts and MGs were studied by first-principles calculation.Firstly,the structures of α-U,β-U and y-U were analyzed and weak covalent bonds were found between U atoms,on which based the discussion of the relationship between bonding characteristics and properties.The effects of Co doping on the stability and electronic properties of y-U alloys were researched.The results showed that Co atoms introduced strong covalent U-Co bonds,thus improving the stability of y-U which was highest when the Co atom was located in the tetrahedral gap of U lattice.However,forming energy indicated that the formation of interstitial solid solutions required higher energy.And density of states(DOS)showed that electron states in the higher energy region decreased when U atom was substituted by Co atom,the activity of the U-Co alloy was reduced and the corrosion resistance was increased.Secondly,the structures of three typical U-Co melts were researched.The equilibrium state configurations of UsoCoso,U67Co33 and U86Co14 were obtained by ab initio molecular dynamics(AIMD),and the topological short-range order(SRO)was then analyzed.Abnormal split was found for the first peak of U-U partial pair distribution function(PPDF),which indicated U-U atomic distances were mainly distributed in two separate regions.It was attributed to the strong chemical interaction between some U atoms due to f electrons delocalization.U was supposed to play two different roles in U-Co melts,namely,the topological U atom and the chemical U atom.And the behavior of Co was similar to that of a metalloid atom,around which a prism-like SRO was formed.And the best glass forming ability(GFA)was predicted to occur when the content of Co was between 23.79-32.58%based on former discussion.Finally,U-Co MGs were compared with U-Co crystalline alloys in topological and electronic structures.The effect of microstructures and bonding characteristics on corrosion resistance was discussed and their dependance on the constituent of MGs was clarified.It was found that the structures of MGs inherited that of corresponding melts,and the first peak of gU-U(r)showed more obvious split in MGs than in melts,indicating that chemical order around U atom became stronger with the increase of U concentration,which influenced chemical short-range order(CSRO)in MGs.And it was believed that melts,MGs and crystalline alloys are united in nature.In respect of structures,the more Co atoms existed in the system,the more Co atoms existed in the first shell around U atoms.Therefore,one U atom was separated from another and corrosion was prevented.When it came to bonding characteristics,however,increase of U content would result in more obvious 5f electrons delocalization,more stable U-U bonds and lower reactivity of U atoms,thus improving the corrosion resistance of U-Co MGs.