Stabilization of Lattice Defects in HPT-Deformed Palladium Hydride (original) (raw)
Related papers
2015
Atomistic calculations were carried out to investigate the mechanical properties of Pd crystals as a combined function of structural defects, hydrogen concentration and high temperature. These factors are found to individually induce degradation in the mechanical strength of Pd in a monotonous manner. In addition, defects such as vacancies and grain boundaries could provide a driving force for hydrogen segregation, thus enhance the tendency for their trapping. The simulations show that hydrogen maintains the highest localization at grain boundaries at ambient temperatures. This finding correlates well with the experimental observation that hydrogen embrittlement is more frequently observed around room temperature. The strength-limiting mechanism of mechanical failures induced by hydrogen is also discussed, which supports the hydrogen-enhanced localized plasticity theorem.
Fundamental Study of Hydrogen Segregation at Vacancy and Grain Boundary in Palladium
2015
We have studied the fundamental process of hydrogen binding at interstitial, vacancy and grain boundary (GB) in palladium crystals using Density-Functional Theory. It showed that hydrogen prefers to occupy the octahedral interstitial site in Pd matrix, however a stable H-vacancy complex with most H occupations would contain up to eight hydrogen atoms surrounding the vacancy at tetrahedral sites. Furthermore, H presence assists the pairing or formation of nearby vacancies, which in agreement with previous suggestions by both experiment and theory investigation. Also, this observation could imply about a hydrogen embrittlement (HE) mechanism through the connections of microvoid and cracks. The segregation of hydrogen at grain boundary, nevertheless, has shown a different effect. High H accumulation results in grain boundary extension, which is related the HE mechanism of grain decohesion observed by experiments.
Hydrogen Embrittlement in Pd Crystals: Critical Hydrogen Binding at Vacancy and Grain Boundary
We studied the fundamental process of hydrogen binding and embrittlement at interstitial, vacancy and grain boundary (GB) in palladium crystals using Density-Functional Theory. Hydrogen prefers to occupy octahedral interstitial in Pd bulk, however the stable H-vacancy complex with most H occupations will contain eight hydrogen surrounding the vacancy at its tetrahedral sites. Furthermore, H presence assists the pairing or formation of vacancies, which in agreement with other experimental and theoretical studies. Also, this observation implies about an hydrogen embrittle-ment (HE) mechanism through the connections of microvoid and cracks. Segregation of hydrogen at grain boundary, however, results in another way of possible ruptures. At GB, H-Pd bond length is the same as that in tetrahedral interstitial site and H atoms prefer locations of threefold bonding with Pd. High H accumulation results in grain boundary extension, which supports HE mechanism of grain decohesion observed by experiments. This is the first time high H occupation at grain boundary was studied and the critical H concentration at GB was reported, by means of first-principles calculations.
Dynamic stability of palladium hydride: An ab initio study
International Journal of Hydrogen Energy, 2011
We present results of our ab initio studies of electronic and dynamic properties of ideal palladium hydride PdH and its vacancy ordered defect phase Pd 3 VacH 4 with L1 2 crystal structure proposed theoretically and found experimentally. Quantum and electronic properties of these hydrides, such as phonon dispersion relations and the vacancy formation enthalpies have been studied. Dynamic stability of the defect phase Pd 3 VacH 4 with respect to different site occupation of hydrogen atoms at the equilibrium state and under pressure was analyzed. It was shown that positions of hydrogen atoms in the defect phase strongly affect its stability and may be a reason for further phase transitions in the defect phase.
Hydrogenated vacancies lock dislocations in aluminium
Nature Communications, 2016
Due to its high diffusivity, hydrogen is often considered a weak inhibitor or even a promoter of dislocation movements in metals and alloys. By quantitative mechanical tests in an environmental transmission electron microscope, here we demonstrate that after exposing aluminium to hydrogen, mobile dislocations can lose mobility, with activating stress more than doubled. On degassing, the locked dislocations can be reactivated under cyclic loading to move in a stick-slip manner. However, relocking the dislocations thereafter requires a surprisingly long waiting time of ∼103 s, much longer than that expected from hydrogen interstitial diffusion. Both the observed slow relocking and strong locking strength can be attributed to superabundant hydrogenated vacancies, verified by our atomistic calculations. Vacancies therefore could be a key plastic flow localization agent as well as damage agent in hydrogen environment.
Modification of Plastic Strain Localization Induced by Hydrogen Absorption
Advances in Materials Sciences, 2008
In order to highlight hydrogen effects on the plasticity, the slip morphology after straining (under tension up to 4% of plastic strain in ambient air) of hydrogenated (at 135 wt.ppm) and non-hydrogenated 316L stainless steel polycrystals was compared. A statistical analysis of both slip band spacings (SBS) and slip band heights (SBH) was performed using atomic force microscopy. Tensile tests were performed at low strain rate, specimens being previously charged at controlled hydrogen concentration. The plastic strain field heterogeneity in polycrystals was taken into account thanks to numerical simulation of crystalline plasticity. On each grain, the calculated plastic shear was correlated with the distribution of SBS and the average number of emerging dislocations per slip band. In comparison with uncharged specimen and for an equivalent cumulated plastic strain, the hydrogenated specimen shows an increase of the slip band spacing (SBS) and of emerging dislocations. This result confirms a plastic localization induced by absorbed hydrogen.
In situ X-ray diffraction analysis of the Pd–H system at high pressure
Journal of Alloys and Compounds, 2005
In the present work palladium thin foils have been hydrogenated at high pressure. In situ monochromatic X-ray diffraction performed under 5 GPa of hydrogen pressure has allowed to follow the formation of hydrogen-rich Pd-H phases as well as the presence of ordered superabundant vacancy phases. The occurrence of hydrogen-rich Pd-H phases as well as the presence of ordered superabundant-vacancy phase is discussed. Sme discrepancies with the early experiments of Fukai are observed. In particular, our results suggest the occurrence of a superstoichiometric defect hydride related to the ␥ phase. A structural model is proposed and needs to be confirmed by using neutron diffraction.
The Effect of Included Hydrogen on the Motion Parameters of Edge Dislocations in Palladium Membranes
Protection of Metals, 2001
The effect of included hydrogen on the mechanical and transport properties of palladium membranes is discussed. The dissolution of hydrogen in palladium changes the properties of the metal and can result in its hydrogen embrittlement and cracking, which depends on the motion parameters of dislocations. During the slipping motion of edge dislocations, hydrogen atoms affect the Peierls activation barrier. During the creeping motion, the included hydrogen affect the diffusivities of interstitial metal atoms. On the basis of the atomic model, the concentration dependence of the shear modulus, as well as the self-diffusivities and mass transfer coefficients of the included hydrogen and palladium atoms, is analyzed. The rates of elementary jumps of included atoms between interstitial sites are calculated according to the transition state model for imperfect reaction systems. An increase in the concentration of included hydrogen at the change in the phase state of the membrane under nonequilibrium conditions is shown to reduce the mobility of edge dislocations and promote the accumulation of internal stress.