摘要
贮氚材料中,由于氚核自然衰变产生的He不溶于材料基体,He原子容易通过聚集而成泡,最终引起He的释放,从而破坏了晶体的结构,导致材料宏观性能的下降,并诱发材料使用性能的下降。因此,建立一套表征材料固氦属性方法来优选性能优异的贮氚材料是推进核聚变技术应用的关键之一。要表征材料固氦属性,就必须要弄清楚材料中He的释放机理,这就要求对材料中He的存在形态及其演化行为进行系统而深入的研究。利用实验手段很难在原子尺度跟踪He原子或He泡的扩散和演变过程,也不能直接观察或据此推测其扩散和演变机制,也就难以完整描述材料中He的扩散聚集和He泡形成及演变规律。计算机模拟是一种能有效研究材料中He行为的方法,能弥补实验的缺陷,从微观尺度探测He的演化行为。
本文选取几种典型的金属及合金(如Pd,Zr,Ti和ZrV2等)作为研究对象,结合分子动力学方法和改进分析型嵌入原子模型研究了晶体中He的各种行为。结合实验结果,建立了一套有效的量化评价方法来表征材料的固氦属性,为He问题的研究打下了基础,并为贮氚材料的选材提供了理论依据。
通过对几种典型金属及合金中单个间隙He原子热扩散性质的模拟研究,发现He原子的扩散性质与实验测得的材料固氦能力有关联,即He原子在材料中的扩散越弱,材料的固氦属性就越强。因此,通过计算材料中He原子的间隙扩散性质,可以初步评估材料的固氦属性。
通过对原子进行跟踪,发现材料中He泡的形核是金属原子的离位和He与空位结合成团共同作用的结果。在不存在离位损伤的情况下,间隙He原子通过与自间隙发射所产生的空位结合形成一系列He-空位团簇,并通过不断吸收附近的He原子或空位,以及通过团簇间的相互合并,排挤出团簇内部的金属原子,最终形成He泡。体系温度的升高可在一定程度加速He泡的形核过程。He泡形成后,将通过吸收He原子等逐渐长大。通过对晶体Pd和LaNi5中He泡合并长大过程的模拟,发现泡间距离是He泡合并发生的关键因子。在晶体内,存在某一临界距离,当He泡间的距离小于临界距离时,He泡间能通过相互作用最终合并,促进He泡的长大。若He泡间的距离大于临界距离,He泡间合并难以发生。温度的升高能有效加速He泡合并,且He泡合并临界距离随系统温度的升高而增加。
在金属晶体近表面区域存在一个“逃逸区”,一旦He泡进入这个区域,He泡迅速向晶体表面膨胀直至破裂,He原子通过不断排挤金属原子形成一条通向表面的通道,He泡中大量的He原子沿着这条通道扩散至材料表面,最终释放,仅有少量He原子滞留在晶体内。He的释放是一个瞬时过程,大量的He原子通过在表面形成的孔洞如火山喷发似的释放出来。通过对He释放行为进行微观机制分析,发现He泡释放“逃逸区”的厚度与材料表面张力强度密切相关。随着材料中He浓度的不断升高,金属及合金中He的释放大致经历初期释放和加速释放两个阶段。在He初期释放阶段,仅有少量表面区域的He原子及小He泡或团簇从晶体表面中释放。随着浓度的增加,晶体内部He泡相互连通并逐渐合并,从而形成相互连通的通道网络。当He浓度超过临界释放浓度时,He释放突然加速,晶体内大量的He原子通过连通网络从表面最终释放。这些过程与金属及合金中He原子及He团簇的扩散和迁移密切相关,基于此建立了He加速释放临界浓度与He原子扩散系数之间的量化关系以预测材料的固氦属性。
通过对金属Pd纳米丝的研究,发现He泡的存在对纳米丝力学性质有着重要影响。He泡通过有效抑制金属原子的相对运动,从而抑制纳米丝中滑移面的出现,造成纳米丝延展性能的降低。随着He泡尺寸的增加,抑制作用更明显,从而加速纳米丝的断裂。He释放产生大量的孔洞等缺陷,对材料力学性质也有重要作用。研究表明在拉伸作用下Pd纳米丝首先发生孔洞的填充。随着应变的增加,纳米丝中孔洞尺寸不同对其力学性能的影响不同。若孔洞较小,纳米丝的拉伸行为与完整Pd纳米丝基本相同；孔洞增加到一定尺寸后,能有效抑制层错的滑移,从而加速纳米丝的断裂。对于不同结构金属纳米丝,其形变机制是不同的。V纳米丝在拉伸作用下,主要以孪晶形式发生形变。通过研究含孔洞V纳米丝的拉伸性质,发现孔洞并不发生原子填充。孔洞通过抑制纳米丝中孪晶形变的发生和扩展,加速了纳米丝的断裂。随着孔洞尺寸的增加,孔洞附近区域在拉伸过程中由孪晶形变逐渐向结构无序化转变。
通过对以上内容的研究,对材料中He行为有了比较深入的了解,为评估材料固氦属性和优选性能优异的贮氚材料提供了理论依据。
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.
引文
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