Helium Behavior In Some Typical Metals And Alloys:the Atomic Simulations

Metals and alloys used to store tritium are subjected to extensive interaction with helium atoms produced by transmutation reactions from energetic tritium fusion neutrons. Owing to the extremely low solubility, helium atoms tend to congregate together and form helium nano-bubbles, which leads to the significantly degradation of materials’mechanical properties. The release of helium atoms will decrease the purity of tritium and also influence the plasma performance in fusion environment. Therefore, the theoretic and experimental investigations on the behaviors of helium are the important issues for the tritium-storage materials. In experiments, it successfully determined the evolution processes of helium bubbles in macroscopic, and provided important insights into helium behaviors. However, the atomic-level behaviors, including the diffusion of helium atoms or nano-bubbles etc. is unknown, owing to the limits of experimental instruments and methods. Computation simulations can be considered as the important methods to study the behavior of helium.In our work, we have implemented molecular dynamics (MDs) simulations and modified analytic embedded atom method (MAEAM) to investigate the behaviors of helium in some typical metals and alloys (Pd, Zr, Ti, and ZrV2, etc.). Combined with the experimental results, an effective method is established to evaluate the helium-retention capacity of the materials quantitatively, which provided a feasible instruction for the selection of excellent tritium-storage materials.The diffusion of helium atoms is the fundamental behavior in materials, and we firstly studied the thermal diffusion of a helium atom in metals and alloys. The results presented that helium atom diffusion is difficult to arise for the material with the excellent retention capacity. Thus it can be considered as an effective method to evaluate the helium retention capacity of materials via the helium atom diffusion behavior.Then helium atoms would congregate together and form some helium bubbles through the diffusion of helium atoms. In Pd and LaNi5crystals without defects, it is found that it firstly emits some self-interstitial atoms (SIA) and forms some vacancies in materials by the accumulation of helium atoms. Then helium clusters are formed by the binding of helium atoms and vacancies, and they further to grow by absorbing some isolated helium atoms and vacancies, or by the interaction of clusters. More and more metallic atoms are pushed out from the clusters, and finally the helium bubbles are formed until no any other metallic atoms in the clusters. Consequently, it is concluded that the formation of helium bubbles is resulted from the emission of metal atoms and the binding of helium atoms and vacancies. It is also found that it facilitates the nucleation of helium bubble with the increase of ambient temperature. We also simulated the growth of helium bubble in the crystals of Pd and LaNi5, it is shown that the strong interaction between the bubbles with a short distance induces the cracks of bubbles and the coalescence of them, facilitating the growth of helium bubbles. The coalescence of helium bubbles are much correlative with the distance between bubbles. There exists a critical distance between each other. Only when distance between bubbles is less than the critical value, the coalescence arises; whereas bubbles are far away from each other, it is much difficult to coalescence. It is also found that the critical-distance is increased as the increase of ambient temperature, implying that high temperature facilitates the coalescence of bubbles.Molecular dynamic simulations are also implemented to investigate the escape of helium atoms from a helium-filled nano-bubble near the surface of crystalline palladium. Significant deformation and cracking near the helium bubble occur initially, and then a channel forms between the bubble and the surface, providing a pathway for helium atoms to propagate toward the surface. The helium atoms erupt from the bubble in an instantaneous and volcano-like process, which leads to surface deformation consisting of cavity formation on the surface, along with modification and atomic rearrangement at the periphery of the cavity. The present simulation results show that, near the palladium surface, there is a helium-bubble-free zone, or denuded zone, with a typical thickness of about3.0nm. Combined with experimental measurements and continuum-scale evolutionary model predictions, the present atomic simulations demonstrate that the thickness of the denuded zone, which contains a low concentration of helium atoms, is somewhat larger than the diameter of the helium bubbles in the metal tritide. The present studies also determine the relationship of the tensile strength and thickness of metal films, and well provide a reasonable explanation for the release of helium nano-bubble. In practical, the materials are filled lots of helium bubbles, and thus it is much necessary to study their release behavior. In materials, helium release experiences early release and accelerated release stages. At the stage of early helium release, only the near-surface helium atoms or bubbles would be released. With the increase of helium concentration, helium bubbles in the interior will link-up and coalescence to produce a network or path linkage towards the materials’surface. Then accelerated helium release occurs as the helium concentration exceeds the critical value, and lots of helium atoms are released as the network is fully established in the materials owing to the inter-bubble fracture. These phenomena result from the diffusion or migration of helium atoms and clusters in materials. The relationship between helium diffusion and critical release concentration is proposed to predict and evaluate the helium retention capacity for the materials.He bubble degrades the mechanical properties of materials.The tensile behavior of Pd nanowires containing a He bubble or void is investigated using MD simulation. It is demonstrated that helium bubble reduces the ductility of wire and facilitates its rupture. Increasing the size of He bubble or decreasing wire’s cross section width accelerates the rupture of nanwire. He bubble induces the fracture of nanowire at the vicinity of bubble and impedes the relative glide of Pd atoms. There exists a critical value of effective cross section width (～0.56) for Pd nanowires. The stacking fault is difficult to form and the plane-gliding is inhibited as less than the critical width of effective cross section. The formation of voids owing to the release of He also change the materials’ mechanical properties. The void is firstly filled during the tensile process. It is known that the size of void is much important on the mechanical behavior of nanowrie. It is much similar with the pure Pd wire as the void is much small as the increasing of tensile strain. As exceeding a critical size, it inhibits the gliding of plane, and accelerates the rupture of wire. During tensile process, the deformation of nanowires is different for the metals with different structure. It is shown that V nanowire deforms in the form of twinning. For V nanowire containing a void, no void-filled-process occurs during tensile process. The void inhibits the nucleation and spread of twinning in V nanowires, and facilitates the rupture of nanowires. BCC-atoms are disordered ones in form of phase transformation rather than twinning deformation, as the increase of void size.Based on the above investigation, it is much clear for the helium behavior in the metals and alloys, providing a feasible and theoretical instruction to evaluate the helium retention properties.