Ab Initio Study Of He Point Defects In Fcc Au-Ag Alloys

As a radioactive metal, plutonium （Pu） is susceptible to be attacked by aging. WhenPu decays, the produced helium （He） may easily precipitate into clusters or bubbles,leading to degradation of the mechanical properties of Pu. Because of the activity andinstability of δ-Pu at room temperature, it is difficult to experimentally investigate theHe behavior in δ-Pu. However, it has been a significant development to search areplacement material of δ-Pu, which can be used to study the effects of radiationdamage on mechanical properties of δ-Pu. The gold （Au）-silver （Ag） alloys have beenchosen to replace δ-Pu. In the present work, the behaviors of He defects in fcc Au-Agalloys are investigated with Ab initio calculations based on density functional theory.The present calculations have been performed with DFT implemented in the Vienna Abinitio Simulation Package （VASP） code, which focuses on the behavior of helium in theAu-Ag alloys. The present studies include the following aspects:（1） Relative stabilities of He in pure Au and Ag were investigated, which followsthe order: substitutional site （sub）>octahedral site （octa）>tetrahedral site （tetra）. Thesimilar relative stability of He defects in both Au and Ag may be attributed to thesimilar interaction between He defects and metals （Au and Ag） because of the similarvalence electronic structures of Au (5d¹⁰6s¹) and Ag (4d¹⁰5s¹).（2） The formation energies of the possible He defects in Au₃Ag₂alloy werecalculated and the relative stabilities of He atoms were investigated. The density ofstates and migrations of He atoms in Au₃Ag₂alloy were also investigated. Thecalculated data show that the positions occupied favorably by single He atom pointdefects in Au₃Ag₂alloy follows the sequence: sub>octa>tetra. The stabilities of Hedefects in Au₃Ag₂alloy mainly depend on the atomic arrangements of the nearestneighboring host metals. A He interstitial prefers to stay at a site with more Agneighboring atoms in Au-Ag alloys, while the favorable substitutional site prefers tohave more Au neighboring atoms.The hybridization between the transition metals dstates and He p states is responsible for the site-preference order of He defects inAu3Ag2alloy. The migration results show that He interstitials at the octa-1sites difficultly migrate into the near octa-1sites trough tetra-3at low temperature.（3） The formation energies of all the possible He defects in AuAg alloy werecalculated and the relative stabilities of He atoms were investigated. The density ofstates and migrations of He atoms in AuAg alloy were also investigated. The resultabout the favorable positions for single He atoms in AuAg alloy is the same as that inAu3Ag2alloy, i.e. sub>octa>tetra. The stabilities of He defects in the AuAg alloys alsomainly depend on the atomic arrangements of the nearest neighboring host metals. Theneighboring metal atom distribution of a favorable position for a He defect in Au-Agalloy is similar to that in Au₃Ag₂alloy. The hybridization between the transition metalsd states and He p states is responsible for the site-preference order of He defects inAuAg alloy. The migration results show that the He interstitials at the octa-2sitesdifficultly migrate into the near octa-2sites trough octa-1at low temperature. When Heis at the tetra position which has higher formation energy, it possibly migrates to neartetra position directly.Comparing the behavior of He defects in Au₃Ag₂alloy and AuAg alloy, it is ofinterest to note that the stabilities of He defects in the two alloys mainly depend on theatomic arrangements of the nearest neighboring host metals, also slightly depend on themass-density of Au-Ag alloys.