First-principles Study Of The Interaction Between Fe And Li

Liquid lithium is regarded as one of the most promising plasma facing materials for future fusion devices.However,with strong chemical activity,Li exhibits undesirable corrosive properties when contacting with structural stainless steel.Experimental observations show that the Li corrosion will change the microscopic structure and degrade the macroscopic mechanical properties of the stainless steel which challenges the safety of fusion devices.However,up to now,the mechanism of Li corrosion still needs further investigations.Therefore,in order to understand the microscopic mechanism of the Li corrosion,we should perform some theoretical simulations to study the microscopic process of the corrosion of Li in stainless steel.In this paper,first-principles calculations have been performed to investigate the dissolution and diffusion behaviors of Li in bcc-Fe.We discover that Li atom favors being in the substitutional site on the premise that vacancies exist.Substitutional Li atom can attract the vacancy migrating in the Fe matrix and diffuse via vacancy mechanism.The energy barrier for a substitutional Li atom to exchange with a nearest vacancy is much lower than the Fe migration energy barrier.The diffusion coefficient for Li is larger than the self-diffusion coefficient by one to three orders of magnitude.Thus,the diffusion of substitutional Li atom via vacancy mechanism should be much faster than Fe self-diffusion.Substitutional Li atom can also attract the moving self-interstitial atoms in bcc-Fe and form the mixed dumbbell<111>_Fe-Li,which is the most stable configuration for interstitial Li atom in bcc-Fe.Based on our calculations,the interstitial Li atom can migrate between the adjacent<111>_Fe-Lisite via interstitial mechanism.The migration energy is evaluated to be 0.063 e V.In comparison with the diffusion of substitutional Li atom via vacancy mechanism,interstitial Li atom can move much faster via interstitial mechanism.The dissolution behaviors of Li in bcc-Fe can be modified by the external load.Except for a slight increase within a small compressive hydrostatic or uniaxial load ranges,the formation energy of substitutional Li atom decreases with the implement of hydrostatic,shear and uniaxial loads.Based on our calculations,the formation energy of interstitial Li atom will decrease with increasing the tensile hydrostatic load but increase with increasing the compressive hydrostatic load,<111>_Fe-Liis always the lowest energy site.The shear load reduces the formation energy of interstitial Li atom within the tensile（110）surface and（27）11 1（29）_s^II becomes the lowest energy site.With increasing the tensile uniaxial load,the formation energy of interstitial Li atom decreases and（27）11 1（29）_U^I is always the lowest energy site.With increasing the compressive uniaxial load,the formation energy of interstitial Li atom increases and TIS_U^Ibecomes the lowest energy site.Furthermore,the linear elastic theory can be employed to explain the load-dependent dissolution properties of Li atom in bcc-Fe.The diffusion behaviors of Li in bcc-Fe can also be modified by the external load.Based on our calculations,the migration barrier for substitutional Li atom will decrease with increasing the tensile hydrostatic load but increase with increasing the compressive hydrostatic load.Under shear load,substitutional Li atom prefers to migrate along the Short Path.With increasing shear load,the migration barrier for the Short Path decreases.Both compressive and tensile uniaxial loads are found to increase the migration barrier for solute Li.However,the magnitude of the increase is lower than that for hydrostatic load and shear load.The migration barrier for interstitial Li atom will decrease with increasing the tensile hydrostatic load but increase with increasing the compressive hydrostatic load.Interstitial Li atom can migrate between the adjacent<111>_Fe-Lisite via interstitial mechanism.Shear load restrains the interstitial Li atom from migrating out of the tensile（110）plane and renders the diffusion process two dimensional.At large shear load,the Li migration is enhanced.Besides,interstitial Li atom energetically prefers to migration along the direction perpendicular and parallel to the applied uniaxial load under compressive and tensile loads,respectively.Interestingly,the preferred migration paths are both within the tensile plane and the Li diffusivity is enhanced under both signs of uniaxial loads.