Critical intergranular cavity density for a transition from intrinsic ductility to brittleness

The influence of nanoscale cavities on the mechanical properties of a grain boundary is studied in a face centered cubic crystal. This first paper presents the raw data: the critical loads for emitting dislocations at a ductile crack tip and the critical loads for brittle propagation for characteristic void configurations. A first level of analysis is proposed: Pictures of the damaged zone illustrate the void growth mechanism and the time evolution of the void concentration profiles during propagation shows, in detail, the competition between growth and shear localization (Fig.1). The use of configurational forces, to constrain the emergence of localized shear zones, enables studying dynamical cracks. It is shown that a critical void size of 5 vacancies and a density of one void every 8 ao along the tilt axis are necessary to observe sustained brittle cracking at the expense of plastic blunting and crack arrest

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 -0.41 eV 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

Hydrogen influence on diffusion in nickel from first-principles calculations

We propose a method to evaluate the diffusion coefficient of vacancy-hydrogen clusters (VHn) 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 VHn 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, VHn clusters always induce a speedup of metal diffusion. The implications concerning H induced damage, in particular in regards to Ni-Cr oxidation, are discussed

Site stability and pipe diffusion of hydrogen under localised shear in aluminium

International audienceThis paper studies the effect of a plastic shear on the tetrahedral vs. octahedral site stability for hydrogen, in aluminium. Based on Density Functional Theory calculations, it is shown that the tetrahedral site remains the most stable site. It transforms into the octahedral site of the local hexagonal compact structure of the intrinsic stacking fault. The imperfect stacking is slightly attractive with respect to a regular lattice site. It is also shown that the shearing process involves a significant decrease of the energetic barrier for hydrogen jumps, at half the value of the Shockley partial Burgers vector, but not in the intrinsic stacking fault. These jumps involve a displacement component perpendicular to the shearing direction which favours an enhancement of hydrogen diffusion along edge dislocation cores (pipe diffusion). The magnitude of the boost in the jump rate in the direction of the dislocation line, according to Transition State Theory and taking into account the zero point energy correction, is of the order of a factor 50, at room temperature. First Passage Time Analysis is used to evaluate the effect on diffusion which is significant, by only at the nanoscale. Indeed, the common dislocation densities are too small for these effects (trapping, or pipe diffusion) to have a signature at the macroscopic level. The observed drop of the effective diffusion coefficient could therefore be attributed to the production of debris during plastic straining, as proposed in the literature

Influence of trap connectivity on H diffusion: Vacancy trapping

A model is given for the effective diffusion of interstitial solutes in the presence of traps. It goes beyond Oriani's by taking into account, in a simple way, the connectivity between interstitial trap sites. It shows, in particular, that the typical dimension of a network of trap sites, connected by low barriers, appears squared in the diffusion coefficient. Therefore, a large precipitate can be inefficient if it offers a fast diffusion path, even if each individual trap site is deep. The model is illustrated in the case of hydrogen trapping at vacancies in Ni, using ab initio calculations for migration barriers and Kinetic Monte Carlo for validation. Trapping/detrapping kinetic parameters for “Thermal Desorption Spectra” analysis are also given

Atomistic modelling of hydrogen segregation to the Σ9{2 2 1}[1 1 0] symmetric tilt grain boundary in Al

The paper establishes a quantitative link between the bulk hydrogen content and the local concentration in the core of the Σ9{2 2 1}[1 1 0] symmetric tilt grain boundary in Al. A detailed map of approximate segregation energies is obtained by combined semi-empirical and ab initio calculations. Even if the density of trap sites and the binding to the core are large, it is shown that segregation alone is not expected to lead to a significant loss of cohesion below 1000 ppm bulk concentration. Other mechanisms should be involved, like H-induced structural changes, to explain the experimentally observed failure of the interfaces at low H concentration. An example of such mechanism is reported

Segregation of hydrogen to defects in nickel using first-principles calculations: The case of self-interstitials and cavities

A detailed first-principles study of the interactions of hydrogen with different point defects in Ni is presented. In particular we discuss the trapping of multiple hydrogen atoms in monovacancies, divacancies and at the self-interstitial (dumbbell). We show that, contrary to the previous theoretical works, the dumbbell cannot trap H atoms. In the case of a single vacancy, the segregation energy is found approximately equal to -0.26-0.26 eV, in excellent agreement with implantation anneal experiments and thermal desorption spectra in the literature. This segregation energy is obtained for the relaxed octahedral (labeled O1O1) and tetrahedral (T1T1) positions inside the vacancy, with a slight site preference for O1O1. Outside the vacancy, the binding energy becomes lower than 20 meV after the second shell of octahedral sites (O2O2). The H2H2 molecules are never stable inside the small vacancy clusters. Therefore, VHn clusters show a maximum trapping capacity of six H atoms. In the case of the divacancy, the H segregation energy can be as low as −0.4 eV. This reconciles theory and experiments by attributing the deepest trap energies to multiple vacancies

Site stability and pipe diffusion of hydrogen under localised shear in aluminium

This paper studies the effect of a plastic shear on the tetrahedral vs. octahedral site stability for hydrogen, in aluminium. Based on Density Functional Theory calculations, it is shown that the tetrahedral site remains the most stable site. It transforms into the octahedral site of the local hexagonal compact structure of the intrinsic stacking fault. The imperfect stacking is slightly attractive with respect to a regular lattice site. It is also shown that the shearing process involves a significant decrease of the energetic barrier for hydrogen jumps, at half the value of the Shockley partial Burgers vector, but not in the intrinsic stacking fault. These jumps involve a displacement component perpendicular to the shearing direction which favours an enhancement of hydrogen diffusion along edge dislocation cores (pipe diffusion). The magnitude of the boost in the jump rate in the direction of the dislocation line, according to Transition State Theory and taking into account the zero point energy correction, is of the order of a factor 50, at room temperature. First Passage Time Analysis is used to evaluate the effect on diffusion which is significant, by only at the nanoscale. Indeed, the common dislocation densities are too small for these effects (trapping, or pipe diffusion) to have a signature at the macroscopic level. The observed drop of the effective diffusion coefficient could therefore be attributed to the production of debris during plastic straining, as proposed in the literature

Dynamique moléculaire contrainte et calcul de k critiques: simulation multi-échelle de la rupture

Nous avons développé des équations du mouvement qui permettent de contraindre certains degrés de liberté en MD pour contrôler l'activité plastique (mouvement et émission de dislocations). Nous les utilisons pour calculer des facteurs d'intensité des contraintes critiques pour l'émission de dislocations depuis une pointe de fissure et pour la propagation fragile

Effect of sub-surface hydrogen on intrinsic crack tip plasticity in aluminium

The effects of sub-surface hydrogen and mixed mode loading on dislocation emission in aluminium are studied using a combination of techniques including crack simulations with an empirical interatomic potential, generalised stacking fault energy (GSF) calculations, with empirical interactions and Density Functional Theory, and the model by Rice which links the critical stress intensity factor to the unstable stacking energy. The crack orientation is {111}⟨112⟩ and the loading is composed of a moderate traction along ⟨111⟩ and a shear along ⟨112⟩, such that Shockley partials are emitted along the crack plane. The role of the relaxations around the H atoms and of the concentration of H in the glide plane, in the GSF calculation, is revealed by comparing Rice's model to the results of brute force simulations. The enhanced GSF is then calculated ab initio. The conclusion is a large decrease of the critical load to emit a dislocation, due to the displacement transverse to the glide direction. The effect of sub-surface hydrogen is negligible with respect to the mechanical one