金属中空位-氢团簇电荷转移应力失稳机制及相关现象研究

Interaction between hydrogen and metals has become an extensively discussed issue because it defines materials performance in hydrogen storage and hydrogen-resistant applications. Among them, the hydrogen cluster growth around various defects (vacancies, dislocations, grain boundaries, etc) becomes the center of many related investigations. According to our initial investigations we have discovered that a mechanism involving charge transfer induced strain destabilization to interpret the hydrogen cluster growth around vacancies in bcc-type metals. It has been found that the hydrogen trapping is strongly coupled with the interstitial charge distributions making the sites with the highest pre-existed accumulated charges. When the dominant charge transfer to hydrogen cluster is stemmed from further metal atoms beyond the nearest-neighboring shell surrounding the vacancy, a sharp increase of the strain energies universally occur. Here, we have proposed this project to investigate in details the interaction between the metallic defects and hydrogen cluster formation, between interstitial charges and hydrogen atomic preferences in the interstitial sites of the lattice space mainly through first-principles calculations and necessary experimental characterization. In particular, we will systematically focus on whether or not this proposed mechanism also matches other types of metals and metal hydrides with such the concerns of the universal relationship between charge transfer and strain destabilization. Our results are expected to have important insight for design of metal-based hydrogen storage materials and hydrogen-resistant structural materials as well as the related phenomenon of hydrogen embrittlement which quite often occurs in metals.

由于氢对金属合金性能和使役行为有重要影响，因此金属与氢的相互作用吸引着广泛的注意，特别是氢与金属空位、相界和位错等缺陷之间的相互作用已经成为相关问题的研究核心。在研究BCC类型金属空位-氢团簇生长的结构稳定性和电子结构时，我们发现空位-氢团簇的电荷转移应力失稳现象。这种现象和已经用来解释空位-氢团簇稳定方式不同，它表明氢在金属中的位置与金属中间隙电荷的分布密切相关，空位-氢团簇生长与其周围临近金属原子能供给的电荷密切相关；当最临近金属所供应给氢簇的电荷出现匮乏，导致氢团簇生长停止，此时伴随着应力能的急剧增加。本项目以深入研究电荷转移与应力失稳之间的关系为出发点，揭示金属缺陷与氢团簇生成、金属间隙电荷与氢原子占位、电荷转移与应力失稳之间的必然联系，探索该现象是否也在其它类型的合金甚至金属氢化物中普遍存在，从而为从原子层次理解相关的现象（比如氢脆和抗氢/储合金开发）提供机理上的启示。

金属中氢原子的行为特点一直以来都是工程应用与基础研究等科研领域所关注的重点问题。氢原子被金属中的缺陷所捕获并于缺陷处大量聚集常常是导致材料力学性能退化的原因之一。虽然对金属中氢原子行为的研究已经进行的相当深入，但对于氢原子在缺陷周围的占位与氢-缺陷团簇生长机理的理解尚不清楚。基于此，本项目对一系列的体心及面心立方结构金属中氢原子的行为特点进行系统的计算和分析，主要结果是：研究了一系列体心立方结构金属（Fe、V、Cr、Nb、Mo、Ta及W）和面心立方结构金属（Ni、Cu及Al）中氢原子的团簇行为特点，发现在这些金属中，无论在其本征态或是缺陷（空位）态，氢原子毫无例外地倾向于固溶在电荷密度较高的间隙位置。“氢-空位”缺陷簇的长大并不是毫无规律可循的，它与周围金属原子能够提供的电荷密切相关。在此基础上提出了“电荷-应力”制约的一般化机制，并发现这一机制在体心和面心立方结构金属中是普遍存在的；研究了纯铁中一个及多个“氧-空位对”对氢原子的捕获，发现“氧-空位对”对氢原子的捕获数量持续稳定在4-5个左右，并且其周围的氢原子浓度并不会随着缺陷的聚集而进一步增大；研究了H原子在氧化物析出相Y2TiO5和Y2Ti2O7间隙以及在Y2Ti2O7/bcc-Fe界面形成团簇情况，分析发现这些H原子占位均容易固溶在电荷密度较高的间隙位置；而且在界面氢团簇长大过程中会吸收大量的H原子和空位。还分析了标准的第一原理计算方法在面心立方结构金属（如Au、Ag、Pt及Pd）的空位生成焓计算时与实验结果出现较大偏差的原因，提出该杂化密度泛函的计算结果与经Local Grüneisen Theory方法修正后得到的空位生成焓实验值更好地一致；进一步发现氢在TiSi表面稳定吸附与拓扑狄拉克节线表面态提供的局域电荷密切关联；经严格数学推导表明析氢反应中阴极材料表面的电子状态与溶液中氢离子的吸附和脱附过程状态紧密关联。这些结论也与在纯金属体心立方结构和面心立方结构金属中的现象相一致，“电荷-应力”制约的一般化机制得到了进一步验证。