Vacancy-hydrogen Clusters Research Articles

Atomistic insights into stress corrosion cracking of α-Fe in supercritical water: The coupling effect of hydrogen embrittlement and intergranular corrosion

Mechanistically understanding stress corrosion cracking (SCC) of Fe-based alloy under extremely high-temperature and high-pressure conditions is a key challenge for designing and optimizing structural materials of the supercritical water (SCW) reactors. The mechanism of SCC of α-Fe in supercritical water has been investigated by reactive force-field molecular dynamics simulations, and it is obtained that SCC is caused by the coupling effect of hydrogen embrittlement and intergranular corrosion. Hydrogen atoms in metals induce vacancy formation when the loaded stress is larger than the threshold stress, resulting in forming stable vacancy-hydrogen clusters. These stable vacancies pin grain boundary (GB) and act as stress concentrators under sufficient tensile stress, leading to the generation of more vacancies. A large number of vacancies in GB promote intergranular corrosion which directly results in materials failure.

Existing States of Co-Deposited Hydrogen in Electrolessly Deposited Copper Films from EDTA Complex Bath

The existing states of co-deposited hydrogen in copper films electrolessly deposited from an ethylenediaminetetraacetatic acid complex bath were investigated using thermal desorption spectroscopy. Small desorption peaks at 500–700 K and large desorption peaks at 900–1300 K were observed in the as-deposited copper film, and these were attributed to the break-up of vacancy-hydrogen clusters and the desorption of molecular hydrogen from nanovoids, respectively. Moreover, an unknown desorption peak appeared at around 390 K. The total amount of desorbed hydrogen from the copper film in atomic ratio was 1.2 × 10−2, most of which was non-diffusible hydrogen desorbed above 800 K. A large number of nanovoids distributed in the grains were observed in the copper film. After 7 days, no significant change in hydrogen desorption above 500 K was observed, while the unknown desorption peak around 390 K disappeared. The as-deposited copper film showed lattice expansion and compressive residual stress, which were reduced after 7 days. Therefore, we attributed the unknown desorption peak appearing at around 390 K to the desorption of hydrogen atoms trapped by regular interstitial sites in the copper lattice.

Segregation of hydrogen and vacancies at the Σ5(210)[001] symmetric tilt grain boundary in Ni and influence on cohesion

The segregation of hydrogen and vacancies at the Σ5(210)[001] symmetric tilt grain boundary (GB) was studied by atomic scale simulations in Ni. First, the hydrogen segregation energies and hydrogen–hydrogen pair interaction energies were calculated on every interstitial site of the GB. The vacancy–hydrogen clusters’ formation energies were also determined on the most favorable site. All these calculations were done using the density functional theory. Second, based on these elementary energies, a free energy functional was built to determine the concentration of segregated hydrogen and of vacancy-hydrogen clusters, as a function of the bulk hydrogen concentration and the temperature. It was found that two configurations exits in typical conditions where embrittlement is observed experimentally: H segregation only, with up to 3 hydrogen atom per structural unit or 50% occupancy by VH5 clusters (1 cluster every two structural unit). The cohesive stress and ideal work of fracture were evaluated by fracturing the GB with different degrees of hydrogen and vacancy segregation. H segregation alone (no vacancy) decreased the work of fracture by 25%. A significantly larger decrease of cohesion was obtained when considering vacancy-hydrogen clusters. A maximum drop of the cohesive stress, of a magnitude of 40%, was obtained when every structural unit was hosting a VH4 cluster. Finally, these data were transformed into cohesive stress models. They were used to evaluate the degree of localization of the shear displacement at the crack tip. The conclusion is that, even if cohesion is very significantly decreased, shear localization is still effective, meaning that dislocation emission should occur at the expense of crack propagation. The comparison with other grain boundaries in the literature shows that the GB studied is almost an ideal sink and therefore is very favorable for the formation of equilibrium VH n . It represents more an upper bound of the effect. Therefore, extra ingredients should be considered to explain the embrittlement observed experimentally.

Effects of alloying elements on vacancies and vacancy-hydrogen clusters at coherent twin boundaries in nickel alloys

The interactions between several typical alloying elements (i.e., Al, Cr, Mo, Nb and Ti) and hydrogen at coherent twin boundaries (CTB) in Ni alloys were systematically studied through first-principles calculations. It was found that solute atoms generally prefer to segregate at CTBs, with the segregation tendency prescribed by the size mismatch between the solute and host Ni atom. Moreover, there exist attractive interaction between certain solute atoms, potentially promoting solute co-segregation at CTBs. Though the solute presence does not favor hydrogen accumulation, it can considerably reduce the formation energies of vacancies and vacancy-hydrogen clusters at CTBs, which, if augmented by large-scale plastic deformation, may facilitate nanovoid nucleation to promote eventual crack initiation at CTBs. The present study demonstrated the critical role of alloying in hydrogen-induced crack initiation at CTBs, providing a new perspective towards understanding HE in Ni alloys.

The effect of irradiation-induced point defects on energetics and kinetics of hydrogen in 3C-SiC in a fusion environment

3C-SiC is a promising candidate for structural material of nuclear fusion reactors, and H, T, and D irradiation often causes undesired volume swelling, bubble formation, and degradation of the mechanical properties of the material. However, the underlying mechanisms of these processes are still not well understood. We thereby carried out systematical first-principles calculations to investigate the interaction of H with irradiation-induced point defects in 3C-SiC. Our results show that both self-interstitial atoms and vacancies can act as trap sites for H, which can effectively influence the retention of H and its isotopes in 3C-SiC. Self-interstitial C and Si atoms can trap up to six and five H atoms, respectively. A C vacancy can trap up to eight H atoms with two H2 molecules formed, while a Si vacancy can trap only four H atoms with no H2 molecule formation. The accumulation of H atoms in vacancy forming vacancy-hydrogen clusters may act as the nucleation site for bubbles or blisters in 3C-SiC. The accumulation of H in a vacancy can result in the instability of atoms around the vacancy, which may result in the growth of vacancy-hydrogen clusters to blisters or bubbles. Both Si and C vacancies can significantly slow down the diffusion of H, and energy barriers of H diffusion from the Si and C vacancies reach respectively up to 3.40 and 2.13 eV, which are much higher than that in bulk. These results explain why the calculated diffusion activation energy of H in perfect 3C-SiC is much smaller than experimental values. Our results are helpful for understanding the micro-mechanism of H retention and bubble formation experimentally observed in 3C-SiC.

H effects in Al-Mg, Al-Zn-Mg alloys and in Al: experiments, continuum and atomistic modeling.

Stress corrosion cracking of pure Al5%Mg is studied. It was shown by slow strain rate tests, constant force, and cyclic traction-traction tests in a simulated confined medium that α ligaments (precipitate-free) can be embrittled by hydrogen. Surface crack initiation in aged, annealed AA5083, after reversible H pre-charging, was modeled by a combination of digital image correlation and polycrystalline aggregate finite element simulation. It was shown, at the scale of the grain, that cracking occurs by a “decohesion” in between hard grains embedded in a deformation band. Cohesive zone modeling was used to quantify the possible impact of H weakened β precipitates on the global cohesion of a grain facet. It was shown that α embrittlement dominates. Finally, some basic aspects of hydrogen interactions with defects in Al are discussed from atomic scale simulations: the stability of vacancy-hydrogen clusters; their mobility, hydrogen trapping, intergranular H, and vacancy segregation; and their impact on cohesion.

Hydrogen influence on diffusion in nickel from first-principles calculations

We propose a method to evaluate the diffusion coefficient of vacancy-hydrogen clusters (${\mathrm{VH}}_{n})$ in metals. The key is a good separation of time scales between H diffusion and the metal-vacancy exchange. The Ni-H system is investigated in details, using ab initio calculations, but the arguments can be transposed to other systems. It is shown that cluster diffusion can be treated as an uncorrelated random walk and that H is always in equilibrium before the vacancy-metal exchange. Then, the diffusion coefficient is a sum over jump paths of the equilibrium probability of being in a specific ${\mathrm{VH}}_{n}$ configuration times the corresponding activation terms. The influence of H on the energy barrier is well reproduced by effective pair interactions between the jumping Ni and the H atoms inside the vacancy. This model is motivated by an analysis of the electronic charge redistribution in key saddle configurations. The interaction is repulsive and decreases with distance. The model is used to find easy jump path, reduce the number of saddle searches, and provide an estimate of the error expected from this reduction. The application to the Ni-H system shows that vacancies are drastically slowed down by H. The effects of temperature and bulk H concentration are explored and the origin of the non-Arrhenius behavior is explained. At equilibrium, ${\mathrm{VH}}_{n}$ clusters always induce a speedup of metal diffusion. The implications concerning H induced damage, in particular in regards to Ni-Cr oxidation, are discussed.

Thermodynamics of hydrogen-induced superabundant vacancy in tungsten

We investigate superabundant vacancy formation induced by hydrogen in tungsten in terms of an equilibrium thermodynamic model to estimate hydrogen isotope retention in plasma facing materials. Vacancy-hydrogen cluster concentrations in the bulk tungsten are calculated as a function of the H concentration at finite temperature. A monovacancy in usual bcc transition metals is capable of accommodating six H atoms, while a maximum of 12 H atoms can be accommodated in a tungsten monovacancy, according to first-principle calculations. The present results provide thermodynamic profiles of vacancy-hydrogen clusters trapping more than six H atoms for the first time. In present work, configurational transitions of H atoms trapped in the monovacancy and activation energies for them are investigated by examining the transition paths in order to calculate configurational entropy. Vacancy-hydrogen clusters trapping more than six H atoms exist in thermodynamic equilibrium. However, the major vacancy-hydrogen clusters are composed of six H atoms in a wide range of temperature and H concentration.

Stability of vacancy-hydrogen clusters in nickel from first-principles calculations

The interactions of hydrogen (H) atoms with vacancies are investigated by means of ab initio calculations. The lowest segregation energies are −0.27 and −0.41eV at single vacancies and divacancies, respectively. These values are in excellent agreement with those corresponding to the two characteristic peaks of the thermal desorption spectra. The microscopic interpretation of the experimental data is therefore clarified. An energetic model is built from the ab initio data and used to study the influence of H bulk concentration and temperature on the concentration of vacancy-H clusters. Analytical expressions, validated by Monte Carlo simulations, are given. The mean vacancy occupation and the H-induced vacancy enrichment are calculated at two temperatures representative of H embrittlement experiments and stress corrosion cracking at high temperatures. The stability domain of VH6 clusters is found to significantly overlap with the experimental conditions for embrittlement. Therefore, vacancy clustering at high concentrations can be qualitatively discussed based on VH6–VH6 interactions that are found weakly repulsive. Consequences on H damage in Ni are discussed. The effect of metal vibrations on segregation and local hydride stability is qualitatively evaluated by off-lattice Monte Carlo simulations using a semi-empirical Ni–H potential. They are shown to shift local hydride stability towards higher H concentrations.

Influence of hydrogen on room temperature recrystallisation of electrodeposited Cu films: thermal desorption spectroscopy

The mechanism of recrystallisation observed at room temperature in electrodeposited Cu films has been examined in light of the enhancement of metal atom diffusion by hydrogen induced superabundant vacancies. Thermal desorption spectroscopy revealed that Cu films electrodeposited from acid sulphate bath containing some specific additives showed a pronounced peak, which was ascribed to the break-up of vacancy–hydrogen clusters. The amount of desorbed hydrogen was comparable to that of vacancy type clusters estimated in previous positron annihilation experiments. The grain size of Cu films increased as hydrogen desorption proceeded. Such grain growths were not observed in the films deposited from the baths without additives. These results indicate that the room temperature recrystallisation of electrodeposited Cu films is caused by hydrogen induced superabundant vacancies.

Stabilization of Lattice Defects in HPT-Deformed Palladium Hydride

Recent investigations on palladium hydride (Pd-H) showed, for the first time, evidence of formation of vacancy-hydrogen (Vac-H) clusters during Severe Plastic Deformation (SPD) effected by High Pressure Torsion (HPT). Vacancy concentrations produced in Pd-H by this method are extraordinarily high. DSC-scans show that the thermal stability range of vacancies is extended by about 150K due to trapping of hydrogen leading to the formation of vacancy-hydrogen clusters. Recent experiments give evidence that the mobility of the H atoms and/or the vacancies is conditional for the formation of Vac-H clusters during HPT. Results furthermore indicate defect stabilization by hydrogen trapping not only for vacancy-type defects but also for dislocations and grain boundaries.

Superabundant vacancy formation in Nb–H alloys; resistometric studies

The formation of superabundant vacancies (SAVs; vacancy–hydrogen clusters) wasstudied in Nb–H alloys by means of resistivity measurements as a function oftemperature, pressure and H concentration. The formation energy of a vac–H cluster(0.3 ± 0.1 eV),which is 1/10 of the formation energy of a vacancy in Nb, is explained tentatively as being theconsequence of six H atoms trapped by a vacancy with the average binding energy of0.46 eV/H atom. The SAVs were introduced from the external surface, and transported into theinterior by direct bulk diffusion and/or by fast diffusion along dislocations. The activationvolumes for the formation and migration of vac–H clusters were determined to be 3.7 and5.3 Å3, respectively.

The Phase Diagram of Mo-H Alloys under High Hydrogen Pressures

The phase diagram of the Mo–H system was determined by in situ X-ray diffraction using synchrotron radiation up to 1200 � C and the hydrogen pressure to p(H2) = 5.3 GPa. Three phases were found to exist; a bcc phase (� ) of low hydrogen concentrations at low hydrogen pressures, and two high-pressure phases, an hcp () phase at lower temperatures and an fcc (� ) phase at higher temperatures, both having a nearly stoichiometeric composition of x = [H]/[Mo] � 1. A gradual lattice contraction observed in the fcc phase indicates the formation of superabundant Mo-atom vacancies (vacancy-hydrogen clusters).

Phase Diagram and Superabundant Vacancy Formation in Cr-H Alloys

X-ray diffraction measurements on the Cr–H system were made using synchrotron radiation at high hydrogen pressures and high temperatures, and the phase diagram was determined up to p(H2)=5.5 GPa and T\\lesssim1400 K. Three solid phases were found to exist; a bcc phase (α) of low hydrogen concentrations, x=[H]⁄[Cr]\\lesssim0.03 existing at low hydrogen pressures (\\lesssim4.4 GPa), and two high-pressure phases, an hcp (ε) phase at lower temperatures and an fcc (γ) phase at higher temperatures, both having high hydrogen concentrations x∼1. A drastic reduction of the melting point is caused by dissolution of hydrogen. A gradual lattice contraction observed in the fcc phase indicates the formation of superabundant Cr-atom vacancies (vacancy-hydrogen clusters). Thermal desorption measurements after recovery from high p(H2), T treatments revealed several desorption stages including those due to the release from vacancy-hydrogen clusters and from hydrogen-gas bubbles, and allowed determination of relevant trapping energies.

Superabundant vacancy formation in Ni–H alloys

X-ray diffraction measurements on the Ni–H system were made using synchrotron radiation at high hydrogen pressures p(H 2)=3∼5 GPa and high temperatures T≲1000°C. Gradual lattice contraction occurring over several hours at high temperatures revealed the formation of superabundant vacancies (vacancy-hydrogen clusters). Superlattice reflections due to ordered arrangements of Vac-H clusters were also observed. The concentration of Vac-H clusters ( x cl≅0.30), deduced from the magnitude of the lattice contraction, was very nearly independent of pressure and temperature, and indicates the maximum possible cluster concentration to be accommodated by the metal lattice. A simple enlightening description of the physics of superabundant vacancy formation is given in Appendix A.