Unified mechanism for hydrogen trapping at metal vacancies

Abstract

Interaction between hydrogen (H) and metals is central to many materials problems of scientific and technological importance. Chief among them is the development of H storage and H-resistant materials. H segregation or trapping at lattice defects, including vacancies, dislocations, grain boundaries, etc, plays a crucial role in determining the properties of these materials. Here, through first-principles simulations, we propose a unified mechanism involving charge transfer induced strain destabilization to understand H segregation behavior at vacancies. We discover that H prefers to occupy interstitials with high pre-existing charge densities and the availability of such interstitials sets the limit on H trapping capacity at a vacancy. Once the maximum H capacity is reached, the dominant charge donors switch from the nearest-neighbor (NN) to the next-nearest-neighbor (NNN) metal atoms. Accompanying with this long-range charge transfer, a significant reorganization energy would occur, leading to instability of the H-vacancy complex. The physical picture unveiled here appears universal across the BCC series and is believed to be relevant to other metals/defects as well. The insight gained from this study is expected to have important implications for the design of H storage and H-resistant materials. Interaction between H and lattice defects underlies diverse materials phenomena [1][2][3][4], including H storage [3,5], H embrittlement [6][7][8], metallic H membranes [9], nuclear fusion reactors [10,11] and H-assisted superabundant vacancy formation in metals [1,12,13], etc, to name a few. Crucial to all these problems is trapping of H at the lattice defects, such as vacancies, voids, dislocations, grain boundaries and cracks [14][15][16][17][18][19][20][21][22]. In particular, H trapping at vacancies has attracted the most attention thanks to the facts that (1) vacancies are easier to study than other defects but have profound influences on materials properties; (2) vacancies hold many surprises and mysteries yet to be explored; (3) the insight gained from vacancies can be applied to other defects as well.It is found that for BCC and FCC metals, up to six H atoms can be trapped by a monovacancy in general because H prefers to occupy the six octahedral (O) interstitials surrounding the vacancy [1,16,17]. However, there are exceptions -the maximum H capacity can go up to 10 for Molybdenum (Mo) [16,17,23] and 12 for Tungsten (W) and Aluminum (Al) [14]. Different interpretations of the available experimental results have also been put forward: (i) The competition between metal-H hybridization and the Coulombic repulsion determines the position and number of H atoms at the vacancy in BCC-Fe [24]; (ii) Comparing to Fe, the greater H trapping capacity in Al is attributed to a larger lattice constant and more delocalized electronic states [14]; (iii) In W, it is found that the vacancy provides an isosurface of optimal charge density that facilities the formation of H bubbles [25]. Clearly, an important question to ask is ...

Predictive model of hydrogen trapping and bubbling in nanovoids in bcc metals

“…Multiscale simulations provide a way to circumvent such limitation to reveal those missing details [25][26][27] . At the fundamental level, ab initio calculations based on density functional theory (DFT) have been widely used to study the atomistic behaviors of H in vacancies [28][29][30][31][32][33][34][35][36][37][38][39][40][41][42] , showing that individual H atoms preferentially reside around tetrahedral interstitial sites or octahedral interstitial sites 28-34, 36, 37 on vacancy surfaces as adatoms, and further accumulation of H will quickly saturate the vacancy due to repulsion between H adatoms [28][29][30][31][32][33][34][35][36][37][38] . However, most of previous DFT studies focused only on H in monovacancies 28-35, 39, 40 , rendering a knowledge gap for understanding H in general nanovoids.…”

Unusual hydrogen implanted gold with lattice contraction at increased hydrogen content

“…Obviously, dilute concentrations require large supercells for modeling. The available theoretical studies of the interaction of hydrogen with metal surfaces usually take into account hydrogen in tetrahedral ( 43m) or octahedral (m 3m) site or both [37][38][39][40][41]51,52 . These studies implement either slabs (i.e.…”

Взаимное влияние водорода и вакансий в alpha-цирконии на энергетику их взаимодействия с металлом

“…Исследования твердого раствора ниобий−водород методами электронно-позитронной ан-нигиляции, дифракции рентгеновского излучения и про-свечивающей электронной микроскопии [3,4] показали, что растворение водорода способствует образованию ва-кансий с последующим формированием сложных комп-лексов водород-вакансия, структура которых зависит как от концентрации водорода в металле, так и от спосо-ба его введения. Теоретические исследования металлов с кубической структурой [5][6][7], а также ГПУ металлов Ti [8] и Zr [9] выявили, что наличие водорода и вакансий в решетке металлов приводит к формированию различ-ных комплексов водород−вакансия. Очевидно, что на-копление этих комплексов может явиться дополнитель-ным фактором водородного охрупчивания материалов.…”

“…Деградация механических свойств конструкционных материалов, обусловленная водородным охрупчивани-ем [1], стимулирует интенсивные исследования влияния водорода на атомную и электронную структуру метал-лов и сплавов [2][3][4][5][6][7][8][9]. Исследования твердого раствора ниобий−водород методами электронно-позитронной ан-нигиляции, дифракции рентгеновского излучения и про-свечивающей электронной микроскопии [3,4] показали, что растворение водорода способствует образованию ва-кансий с последующим формированием сложных комп-лексов водород-вакансия, структура которых зависит как от концентрации водорода в металле, так и от спосо-ба его введения.…”