Dislocations In Palladium Research Articles

Hydrogen-induced defects and multiplication of dislocations in Palladium

In the present work positron lifetime spectroscopy was employed for investigation of hydrogen-induced defects in Pd. Well annealed polycrystalline Pd samples were electrochemically charged with hydrogen and the development of defects with increasing hydrogen concentration was investigated. At low concentrations (α-phase region, xH<0.017H/Pd) hydrogen loading introduced vacancies surrounded by hydrogen atoms and characterized by a positron lifetime of ≈200ps. When the hydrogen concentration exceeded 0.017H/Pd the α-phase transformed into the hydrogen rich α′-phase. This generated dislocations characterized by a positron lifetime of ≈170ps. Dislocations can accommodate a large volume mismatch between the α and the α′-phase. Hardness testing revealed that absorbed hydrogen made Pd harder. In the α-phase region hardness increased due to solid solution hardening caused by dissolved hydrogen. Dislocations created by the α to α′-phase transition caused strain hardening which led to an additional increase of hardness.

Nanoscale hydride formation at dislocations in palladium:Ab initiotheory and inelastic neutron scattering measurements

Hydrogen arranges at dislocations in palladium to form nanoscale hydrides, changing the vibrational spectra. An ab initio hydrogen potential energy model versus Pd neighbor distances allows us to predict the vibrational excitations for H from absolute zero up to room temperature adjacent to a partial dislocation and with strain. Using the equilibrium distribution of hydrogen with temperature, we predict excitation spectra to explain new incoherent inelastic neutron-scattering measurements. At 0K, dislocation cores trap H to form nanometer-sized hydrides, while increased temperature dissolves the hydrides and disperses H throughout bulk Pd.

Interaction of hydrogen and deuterium with dislocations in palladium as observed by small angle neutron scattering

Small angle neutron scattering (SANS) measurements on Pd samples containing dislocations with a density of a few 10 11 cm −2 reveal an additional intensity for a scattering vector of 0.02 Å −1 to 0.2 Å −1 after loading with hydrogen (H) or deuterium (D). The corresponding net cross section is inversely proportional to the scattering vector as expected for line type scattering objects with a superimposed exponential decrease stemming from scattering within the Guinier-regime. This experimental finding is in accordance with a model where extended segregation of H or D within the dilated regions of edge dislocations occurs. In a first order approximation this corresponds to a precipitation of cylindrically shaped hydrides along the dislocation line and can be treated quantitatively yielding radii in agreement with SANS data. Whereas gas volumetric measurements at the same total concentration reveal no difference for the amount of H- and D-segregation, there is a pronounced effective difference in SANS intensities which cannot be explained by the different scattering lengths alone. However, the different sign of the latter quantity in combinations with an expected volume expansion within the hydride/deuteride region provides a reasonable explanation of the intensity difference observed. Knowing the amount of segregated H or D from gas volumetry and the dislocation density from electron microscopy the SANS results can be explained in a self consistent way.

Small angle neutron scattering of hydrogen segregation at dislocations in palladium

The effect of hydrogen on the surface morphology and nanomechanical properties of Ni-based Alloy 725 under solution-annealed (SA) and precipitation-hardened (API) conditions was thoroughly studied. The investigation involved in situ nanoindentation testing, microscopy characterization, statistical analysis, and numerical simulation approaches. The results showed the distinctive effects of hydrogen on the pop-in and hardness in the SA and API samples. For the SA sample, hydrogen mainly dissolved as solid solute in the matrix, causing enhanced lattice friction on the dislocation motion and increasing the internal stress via lattice expansion. Thus, an enhanced hardness, a reduced pop-in width/load ratio, and numerous surface steps were detected in the presence of hydrogen. For the API sample, the strengthening γ′′ phases were the stress concentrators, and the dislocations nucleated heterogeneously, demonstrating indistinctive pop-in phenomena. Furthermore, the precipitates in the API sample affected the trapping behavior of hydrogen, thereby resulting in the hardness change, which reflected the competition between solution hardening in the matrix and vacancy softening mechanism in precipitates.

Kharakteristiki gidridopodobnykh segregatsii vodoroda na dislokatsiyakh v palladii

Kharakteristiki gidridopodobnykh segregatsii vodoroda na dislokatsiyakh v palladii

The use of small angle neutron scattering in the study of hydrogen trapping at defects in metals

Small angle neutron scattering (SANS) techniques have been used to investigate the trapping of hydrogen and deuterium on dislocations in palladium. Calculations of the expected form of this scattering are presented. It is shown that the different scattering lengths of H and D, respectively negative and positive, can be exploited to prove explicitly that the hydrogen is being trapped at the feature that is causing the SANS in the metal sample. In particular, at an edge dislocation, we can expect the H/D to be trapped in the region of lattice dilation below the edge. Thus, because the scattering length of palladium is positive, deuterium trapping will compensate for the reduction in the scattering density in the host lattice and hence will reduce the intensity of the SANS progressively as D is added, until the local scattering density deviation becomes positive. At this point, the overall SANS cross-section passes through a minimum. On the other hand, increasing H trapping will continuously increase the SANS intensity. The model for the expected scattering is described and measurements of the SANS intensities from dislocations in palladium are presented for a series of D concentrations. A minimum is observed in the SANS at about 1% atomic concentration, in reasonable agreement with the theory.

Atomistic simulations and Peierls–Nabarro analysis of the Shockley partial dislocations in palladium

A detailed study of Shockley partials in Pd is presented, using a full-scale atomistic simulation of the structure and energetics of the dislocation motion. The interatomic potential is derived by fitting to data from first-principles electron-structure calculations. The outcome from the atomistic simulation is compared with the corresponding results using the Peierls–Nabarro (PN) model. The required input to the PN model is determined from the interatomic potential used in the atomistic simulation in order to make the comparison as decisive as possible. We find that the core, obtained in the atomistic simulation, is much wider and extends over a larger region compared with the PN model. A modification of the PN model is suggested to make it more consistent with the atomistic description. Using the modified version of the PN model improves the core structure considerably. The magnitude of the Peierls barrier from the atomistic simulation is in reasonable agreement with both experimental data and the corresponding values obtained using the PN model.

A 2D NMR study of deuterium trapping by dislocations in palladium

Deuteron spin-lattice and spin-spin relaxation rates have been measured in annealed and cold-rolled foil samples of palladium deuteride PdDapproximately=0.6 in the temperature range 100-390 K using pulsed NMR at 14.8 MHz. The CPMG sequence spin-spin decay in samples containing different densities confirms the coexistence of two relaxation rates, R2a<R2b ( approximately=11 and approximately=170 s-1 at room temperature for instance), both of which are much larger than R1 (<or=0.42 s-1). R2a describes relaxation of deuterons in normal interstitial sites whereas R2b is thought to originate from deuterons trapped in dislocation core regions. For T<or=160 K, R1 and R2a are attributed to a diffusion-modulated, deuterium-vacancy-induced, quadrupolar interaction; for T>or=160 K, R2a is dominated by quadrupolar interactions of diffusing deuterons with long-range electric field gradients associated with dislocation strain fields. The temperature dependence of R2b yields an activation energy of approximately=0.024 eV for deuterium diffusion within the core regions where a quadrupolar coupling constant of at least 4 kHz is measured. In spite of this low activation energy, diffusion in the cores is relatively slow since nearly all sites in core regions are filled. It is estimated from the signal amplitudes that trapping can cause almost all dilated deuterium sites to be filled out to a radius of 30 AA around dislocation cores for a dislocation density approximately 1012 cm-2.

Dislocations in palladium

Dislocations in palladium

Interaction of hydrogen with dislocations in palladium—I. Activity and diffusivity and their phenomenological interpretation

Activity and diffusivity of hydrogen in deformed and annealed palladium has been measured by an electrochemical technique for different degrees of deformation (73, 50, 15, and 6%), at 295 and 322 K and over a wide range of hydrogen concentration from about 1 to 10 4 at.ppm. Deviations from the ideal solution behavior are drastic at the low concentration side where the solubility enhancement is as high as 1.2·10 6 for 73% deformation and 1 at.ppm. From a plot of the free excess enthalpy the ratio of the dislocation densities in samples of different deformation is obtained. The diffusivity decreases with decreasing hydrogen concentration but at very low concentrations of some at.ppm an enhancement of the diffusion is observed. The high concentration results can be calculated from the activity measurements assuming that the activity gradient is the driving force for diffusion, but the increase of diffusivity at low concentrations disagrees with these calculations and is explained by dislocation pipe diffusion. Evidence for this transport mechanism is also given by an unusual diffusion behaviour during current pulse measurements at low concentrations. At 295 K the dislocation pipe diffusivity is about ten times as much as the bulk value in annealed samples.

Interaction of hydrogen with dislocations in palladium—II. Interpretation of activity results by a fermi-dirac distribution

Measurements of the activity of hydrogen in deformed palladium have revealed a strong interaction with dislocations. Therefore, a Fermi-Dirac distribution was applied and the distribution function could be replaced by a step function similar to the electrons in metals. The “Fermi energy of hydrogen” is equal to the interaction energy and contains a contribution of −18kJ/mole H which was attributed to the formation of a hydride close to the dislocation core. The elastic contribution was calculated from the stress field of an edge dislocation and its dependence on the reciprocal distance from the dislocation core corresponds to a dependence on the reciprocal square root of concentration in agreement with experimental findings. Deviations from this behavior at small distances were explained by a failure of the continuum mechanical calculations of the stress field yielding a cut-off radius of one Burgers vector. The application of a step function is not allowed for high concentrations and numerical calculations were made showing that the hydride phase grows to a diameter of about 25 Å at concentrations of some thousand at.ppm H. Consistent results were evaluated at high and low concentrations for different degrees of deformation and for two temperatures.