Microstructure And Irradiation Damage Of Zirconium And Its Alloys Based On Atomistic Simulation

Cladding materials of fuel element are widely used in nuclear industry due to its good overall performance.However,with the increasing economy,safety and reliability requirements of nuclear reactors,it urgently need to develop fuel elements with higher fuel burnup and longer life,and to design zirconium alloy cladding materials with more excellent overall performance.As a structural material,to enhance the comprehensive mechanical properties of zirconium alloys is one of the important indicators,mainly affected by the microstructure,especially the atomic arrangement.From the atomic scale point of view,the ease of starting the slip system directly affects its mechanical properties.Numerous studies have shown that the introduction of high-density surface defects can significantly improve the strength and ductility of the alloy,and solute atoms are one of the key influencing factors.It is well-known that a-Zr deformation needs to be coordinated by slippage and twinning at the same time due to less independent slip systems of HCP structure.It is easy to see that stacking faults and twins are the two most important types of plane defects in a-Zr.In order to provide theoretical guidance for the design and improvement of zirconium alloy cladding materials,this dissertation has carried out atomic scale simulation studies on the rules and mechanisms of the effect of solute atoms on these two kinds of plane defects,respectively.At the same time,in order to explore the mechanism of irradiation damage of zirconium alloy cladding materials,predict the effect of irradiation on its performance and provide the mathematical model and theoretical basis for the development of new zirconium alloy,the process of irradiation damage under different irradiation conditions for zirconium matrix are investigated.Designing new zirconium alloy:The generalized stacking fault energy(GSFE)of zirconium matrix with alloying elements(Sn,Fe,Cr and Nb)and nonmetallic solute atoms(H,N,O and C)and twin boundary energy(ETB)are computed by using first-principles calculations.Effects of these solute atoms on different slip systems such as basal {0001}<11-20>plane slip system,prismatic {10-10}<11-20>plane slip system and pyramidal {11-22}<11-23>slip system,and different twin interfaces such as {10-12} tension twin and {10-11} compression twin are investigated.Mechanism of irradiation damage for zirconium alloy:Based on the first-principles calculations,the structural stability of single vacancy with different vacancy concentration in pure zirconium,and zirconium based binary systems containing single vacancy or double vacancies are discussed.The irradiation damage process for production,development,and annealing of displacement cascades in a-Zr are investigated from molecular dynamics simulations under different conditions at T-300K,400K,and 500 K for energy of 2,6 and 10 keV with initial motion directions<0001>and<01-10>of primary knocked-on atoms(PKA)of irradiation.The main conclusions can be drawn in this work are as follows:? Differences in the atomic volumes and electronegativities between alloying elements and Zr substantially influence the GSFEs of Zr47Xi in various slip systems.Moreover,the potential to reduce ?us and ?is decreases in the order Fe>Cr>Nb>Sn in the basal {0001}<11-20>slip system and the pyramidal {11-22}<11-23>slip system,and Cr>Nb>Fe in the prismatic {10-10}<11-20>slip system.Therefore,the potential to improve the strength and ductility mechanical properties of Zr alloys decreases in the order Cr>Fe>Nb>Sn.? Four solutes such as H,N,O and C doped in zirconium matrix,unstable stacking fault energy(?us)and intrinsic stacking fault energy(?is)of the basal {0001}<10-10>plane slip system and the pyramidal {11-22}<11-23>plane slip system are reduced.The corresponding ductility parameters D are all less than 1,thus,all these four elements doped in zirconium matrix lead to the increase the brittleness of zirconium.? The larger the difference between the atomic radius of the alloying element and the zirconium atom,the smaller the {10-12} tensile twin segregation energy(ETBS1)and{10-11} compressive twin segregation energy(ETBS2).The order of ETBS1 and ETBS2 value is:Fe<Cr<Nb<Sn.This is consistent with the effect of charge distribution of four different solute atoms on twin segregation energy.Therefore,the order of the alloying elements that increase the strength by {10-12} tensile twinning energy and{10-11} compressive twin segregation are:Sn<Nb<Cr<Fe.? Atom H is stable at five different crowdion,four different octahedral,hexahedral,and two different tetrahedral interstitial sites near a {10-12} twin boundary.It means that H is easy to segregate from the {10-12} twin boundary to these interstitial sites.Atom C is easy to segregate from the {10-12} twin boundary to the first position of octahedral interstitial site.Atom N and O are easy to segregate from the {10-12} twin boundary to the fourth position of octahedral interstitial site.? The stable structures of 15 and 35 zirconium atoms with single vacancy have obtained by optimization procedure.The electronic density of an atom which has a vacancy as a first-nearest neighbor becomes stretched in the direction of neighboring atoms,whereas electronic density of those atoms,which have zirconium atoms,as first-nearest neighbor,remains symmetric.With an increase of zirconium atoms in pure a-Zr lead to the height of main peaks of total density of states increases.? Alloying elements of Sn and Nb doped in the second site,Cr doped in the first site is the stable site of supercells of pure a-Zr containing 142 zirconium atoms inside with one vacancy,respectively.However,two sites are both stable for Fe doped in supercells of a-Zr containing 142 zirconium atoms inside with one vacancy due to the similar value of Fermi levels.Alloying element of Sn doped in the third site,Fe doped in the first position,and Cr and Nb doped in the second site is the stable site of supercells of a-Zr containing 141 zirconium atoms inside with two vacancies,respectively.? When crystal temperature is T=300K,400K and 500K and initial direction of motion for PKA is<0001>the cascades volume is greater than for the initial direction<01-10>at same all other parameters.Moreover,the largest size of cascade is found after 0.4 ps,which regardless of crystal temperature,energy and initial direction of primary knock-on atom(PKA).The formation of crowdion is caused by channeling during cascade development.? The relaxation time of the cascade is a key parameter to describe the evolution of the micro-defect structure in the cascade anneal during the irradiation damage.The higher the energy of PKA,the more defects are generated and the larger the distance between defects,which leads to longer time of recombination.Therefore,the relaxation time of cascades increases with the increase of PKA energy.