The Stability,elasticity And Point Defect Energy Properties In Ta-Ti-V Medium-entropy Alloys From First-principles Calculations

The Ta-Ti-V medium-entropy alloys(MEAs)have excellent properties such as low neutron activity,good thermal stability,and good room temperature ductility.It has a broad application prospect in the future nuclear engineering.According to experimental results,a special quasi-random structure(SQS)model and first-principles calculations are used to calculate the structural stability and elasticity of ternary single-phase BCC Ta-Ti-V alloys with the compositions Ta₂₄Ti₁₄V₁₆,Ta₁₉Ti₁₇V₁₈,Ta₁₆Ti₂₁V₁₇,Ta₂₂Ti₂₃V₉ and Ta₁₈Ti₁₈V₁₈.The mixing energy of five compositions is positive,which indicating there are unstable at low temperature but stable at high temperature.Meanwhile,we predict the ability of elasticity in those five compositions,and the sequence of bulk modulus is Ta₂₄Ti₁₄V₁₆>Ta₁₉Ti₁₇V₁₈>Ta₁₈Ti₁₈V₁₈>Ta₁₆Ti₂₁V₁₇>Ta₂₂Ti₂₃V₉,and ductility as Ta₁₉Ti₁₇V₁₈>Ta₁₈Ti₁₈V₁₈>Ta₂₂Ti₂₃V₉>Ta₂₄Ti₁₄V₁₆>Ta₁₆Ti₂₁V₁₇.In addition,the influence of element content on elastic properties shows that Ta content will increase bulk modulus of the alloys,while Ti content decrease bulk modulus.Higher V and Ti content are beneficial to ductility,especially V content.Higher Ta content will sharply reduce the ductility of the al oys.Under the irradiation of high-energy neutrons,reactor materials will produce a large number of defects,such as vacanc ies,interstitial atoms,dislocation loops,etc.,which will greatly affect the performance of materials in their service.The vacancy formation energy and migration energy and the<111>dumbbell pair formation energy of equiatomic Ta Ti V MEA are calculated in this paper,and the results can help to evaluate its irradiation resistance.Due to the chemical complexity caused by random occupation of atoms,the defect energies are different in different lattice positions,that is,they exhibit distribution characteristics.Compared with the constituent pure metals,the resistance of Ta Ti V alloy to the formation of vacancies is decreased,and the V-site vacancy is the most easiest to form.According to the first nearest neighbors of vacancy defects,the local dependency of vacancy formation energy is obtained.With the increase of Ta atoms around a vacancy,the formation energy will increase,that is to say,vacancies are tend a Ta-poor environment.In addition,the formation energy of Ti vacancies increases obviously with the increase of the number of V atoms.In general,the diffusivity of vacancy defects will increase in the alloy compared with pure metals.Due to the low migration energy of Ta and Ti vacancies in Ta Ti V alloy,vacancy defects tend to exchange with nearby Ta and Ti atoms,resulting in a decrease in the number of Ta and Ti elements around vacancy-related defects(such as cavities),while the number of V element is in the majority.The optimization results show that not all dumbbells are in the<111>direction,indicating that the diffusion of some interstitials in Ta Ti V MEA should go through a rotation-translation process,and the overall diffusion rate could slow down.Meanwhile,the diffusion of V interstitials is the fastest compare to the other two elements.Meanwhile,the formation energy of each interstitial in equiatomic Ta Ti V alloy is lower than those in their corresponding pure metals,suggesting the interstitial defects are easier to form,and result in a sluggish diffusion of interstitials.Due to a decreased diffusion energy barrier of vacancies,the distribution of vacancy migration energy and interstitial migration energy can overlap in the alloy,which is conducive to the recombination of vacancy and interstitial defects,thus improving the material irradiation resistance.On the other hand,because V has the smallest atomic size,V-V and Ti-V dumbbell interstitials generally have lower energy formation than Ti-Ti and Ta-Ta dumbbell interstitials.Due to the good combination between the interstitials and the V atom,the diffusion mechanism of interstitials is mainly realized by V atom's diffusion.O ur research results provide a theoretical basis for the development of Ta-Ti-V.medium-entropy aloy.