Modeling of hydrogen-assisted ductile crack propagation in metals and alloys

This paper presents a finite element study of the hydrogen effect on ductile crack propagation in metals and alloys by linking effects at the microstructural level (i.e., void growth and coalescence) to effects at the macro-level (i.e., bulk material deformation around a macroscopic crack). The purpose is to devise a mechanics methodology to simulate the conditions under which hydrogen enhanced plasticity induces fracture that macroscopically appears to be brittle. The hydrogen effect on enhanced dislocation mobility is described by a phenomenological constitutive relation in which the local flow stress is taken as a decreasing function of the hydrogen concentration which is determined in equilibrium with local stress and plastic strain. Crack propagation is modeled by cohesive elements whose traction separation law is determined through void cell calculations that address the hydrogen effect on void growth and coalescence. Numerical results for the A533B pressure vessel steel indicate that hydrogen, by accelerating void growth and coalescence, promotes crack propagation by linking simultaneously a finite number of voids with the crack tip. This “multiple-void” fracture mechanism knocks down the initiation fracture toughness of the material and diminishes the tearing resistance to crack propagation.     

The effects of dislocations on the trapping and transport of hydrogen in 3.3 Ni - 1.6 Cr steel during plastic deformation

In order to make a clear distinction between hydrogen trapping and transport by dislocations, it is necessary to take the apparent diffusivity, D_app, of hydrogen obtained from specimens involving approximately comparable dislocation structures and to compare them critically. The specimens in question should undergo plastic deformation differences only within a small range of strain to produce similar dislocation structures with different dislocation densities. The unstrained specimen, the 3.5 %-prior strained specimen and the 3.5 %-prior strained specimen, followed by additional straining at different strain rates have been prepared from 3.3 Ni - 1.6 Cr steel. The present static and dynamic hydrogen permeation experiments permitted us to measure the D_app of hydrogen and correlate the results with the trapping and transport effects.     

A mechanism of void growth in the region of the plastic zone ahead of a crack-tip

The mechanism of void growth in the region of the plastic zone ahead of a crack tip proposed earlier is further developed. As a result of partial annihilation of lattice dislocations representing the plastic zone with elastic dislocations representing the void, ledge steps are formed on the void surface causing void growth. A detailed dislocation model of the mechanism of void growth is illustrated using both the discrete dislocation analysis and the continuous distribution method. The latter is particularly developed to provide the scaling methods, so that the results can be readily obtained for different sizes of the crack and the void as well as for different positions of the void with respect to the crack. Application of these results to void growth in the region of the plastic zone ahead of a crack tip subjected to alternating stresses and creep is also suggested.     

Interaction of a transonic dislocation with subsonic dislocation and point defect clusters

The moving speeds of all observed dislocations in crystals are subsonic. There has been a view in the literature that the speed of subsonic dislocations can not be accelerated above the speed of sound because the energy required would be infinitely large. Recent molecular dynamics (MD) simulation had shown that it is possible to generate dislocations with an initial moving speed higher than the velocity of sound in solids. This raises a question: what will happen when a supersonic dislocation meets other defects along its moving path? This work reports the results of MD simulation on the interaction of a transonic dislocation with other subsonic dislocations as well as with point defect clusters. The results show that a vacancy cluster such as a void has an insignificant slow-down effect on the transonic dislocation, while a subsonic dislocation slows down the transonic dislocation to subsonic one. In some cases, the subsonic dislocation (or a subsonic part of a transonic dislocation) can overcome the traditional sound barrier.     

Atomistic modeling of plastic deformation in B2-FeAl/Al nanolayered composites

In this work, the deformation response of the B2-FeAl/Al intermetallic composites, as a model material system for nanolayered composites comprised of intermetallic interfaces, has been explored. We use atomistic simulations to study the deformation mechanisms and the interface misfit dislocation structure of B2-FeAl/Al nanolayered composites. It is shown that two sets of dislocations are contained in the interface misfit dislocation network and are correlated with the initial dislocation nucleation from the interfaces. The effects of layer thickness on the uniaxial deformation response of the B2-FeAl/Al multilayers are investigated. We observed that under compressive loading the smaller proportion of the FeAl layers leads to the lower overall flow stress. Under tensile loading, the void formation mechanism is investigated, suggesting the interface structure and the dislocation activities in the FeAl layers playing a significant role to trigger the strain localization which leads to void nucleation commencing at the interface. It is also found that the deformation behavior in the “weak” Fe/Cu interface behaves substantially different than that of the “strong” FeAl/Al interface. The atomistic modeling study of the nanolayered composites here underpinned the mechanical response of “strong” intermetallic interface material systems. There is no void nucleation during the entire plastic deformations in the Fe/Cu simulations, which is attributed to much higher dislocation density, more slip systems activated, and relative uniformly distributed dislocation traces in the Fe phase of the Fe/Cu multilayers.

     

Molecular dynamics simulations of dislocation interaction with voids in nickel

A high density of voids is expected to form in irradiated face centered cubic metals, which can have a negative impact on the ductility and cause an increasing strength. Molecular dynamics simulations of the interaction between gliding dissociated edge dislocations and voids in nickel have been performed to investigate the effect of the void size, the corresponding detachment mechanism, and dynamic effects of the dislocation on the obstacle strength. As expected, the void strength is observed to increase with increasing void size. The dislocation interaction and detachment process are determined by the applied shear stress, the repulsive interaction between partial dislocations and the image interaction between the partial dislocations and the void surface. For voids with a diameter smaller than 2 nm, the repulsive stress between the partials dominates, resulting in the detachment of the leading partial from the void while the trailing partial remains pinned. Consequently, the detachment process and obstacle strength are controlled by the trailing partial. For voids with a diameter larger than 2 nm, the attraction between the dissociated dislocations and the void dominates causing the detachment process and void strength to be influenced by both partials individually. This transition in detachment process at a void diameter of 2 nm is consistent with other research, and this transition is shown to be dependent on the void separation distance along the dislocation line and the dissociation distance between the partials, thus the stacking fault energy. Finally, by comparing the quasi-static and dynamic simulation results, an estimate for the static detachment stress is proposed in terms of the dynamic detachment stress and the dislocation velocity after detachment.     

The control of catalytic poisoning and stress corrosion cracking

When a structural metal is stressed in a hydrogen environment, the metal may crack at stress levels much lower than its normal strength. This embrittlement is caused by hydrogen atoms or ions rather than hydrogen molecules. The catalytic dissociation of hydrogen molecules into atoms or ions takes place at dislocation sites on a metal surface. The termini of dislocations on a metal surface are sites of localized high energy. There are “poisons” which will retard or even stop catalytic processes. The toxicity of a poison depends on the binding energy of the foreign atom to a dislocation terminus.Cracked specimens made of 4340 steel were tested in hydrogen gas and its mixture with SO₂, CS₂, CO₂, N₂ and Ar. Both SO₂ and CS₂ are very “toxic” and can stop a running crack. The toxicity of CO₂ is moderate. n₂ and Ar have no noticeable effect on a running crack.     

Void formation and morphology in NiAl

Void formation in stoichiometric NiAl was studied through controlled heat treatments and microstructural characterization through transmission electron microscopy. Voids were observed to form at temperatures as low as 400°C, but were noted to dissolve during annealing at 900°C. Two distinct void shapes, cuboidal and rhombic dodecahedral, were observed, often at the same annealing temperature. At higher temperatures (800°C) extensive dislocation climb, rather than void formation, was noted. The relative incidence of void formation and dislocation climb can be related to the mobility of vacancies at each annealing temperature. The shape of void type is described in terms of their relative surface energy and mode of nucleation.     

Formation of D–V<Subscript>Zn</Subscript> complex defects and possible p-type conductivity of ZnO nanoparticle via hydrogen adsorption

The hydrogen adsorption on surfaces and on defect sites of ZnO nanoparticles (NPs) has been studied by using Raman and Fourier transform infrared spectroscopic methods. The presence of hydrogen at defect sites bound to zinc vacancy with different coordinations has been confirmed. To further identify the existence of isolated V_Zn and H–V_Zn complexes in the ZnO NPs, coincidence Doppler broadening (CDB) spectroscopic studies have been performed with respect to the CDB spectra of a 99.9999% pure Al single crystal. The broad momentum dip ρ_L showed between 15–17?×?10^?3 m₀c suggests the trapping of positrons with the core electrons of 3p Zn. However, positron annihilation takes place between ρ_L 20–25?×?10^?3 m₀c and this may occur with an electron belonging to OH bonds (V_Zn–H_i–O). Here the lattice hydrogen H⁺ ion acts as a compensating centre, and it can bind with the V_Zn around the dislocation and stacking faults (SFs) core, which may produce the acceptor-type complex defect for p-type conductivity. Finally, the existence of SFs and dislocation defects, including edges and steps, was confirmed by transmission electron microscopy.     

Formability,fracture and void coalescence analysis of a cryorolled Al–Mg–Si alloy

This paper deals with the combined analysis of the forming and the fracture limit diagrams and the void coalescence for AA 6061 sheets that were rolled up to a 50% reduction at two different temperatures, viz. room temperature and cryogenic temperature. Rolled sheets were examined for their microstructure, tensile properties, formability, and void coalescence. The tensile properties and the formability of the sheet metals were correlated with the fractography features and a void-parameter analysis. The cryorolled (CR) samples exhibited better mechanical properties than the room temperature rolled (RTR) samples. Lower values of ‘n’ were observed for both of the rolled conditions than for the base material due to the large dislocation densities in the rolled sheets. Further, the void sizes were observed higher in the CR sample than in the RTR samples. It is believed that the heterogeneous slip in the CR condition contributed to a higher necking percentage and larger void sizes.     

Dislocation resonance in superconducting tin

Ultrasonic attenuation in the superconducting state of strained samples of single crystals of tin shows a strong dependence on the dislocation density. The dislocation density is estimated with the help of a suitable etch pit method. The etch pits are square in shape and exhibit symmetric dislocation lines normal to the (100) planes. The number of etch pits reveals a direct dependence on the physical state of the specimen. The dislocation density estimated by this etch pit method can be related to the extra attenuation Δα_s shows a resonant behaviour and the resonance frequency is dependent on the dislocation density. It is also observed that the functional relationship between the resonance frequency and the dislocation density is different in different regions of the dislocation density.     

Effect of hydrogen on deformation behavior of Alloy 725 revealed by in-situ bi-crystalline micropillar compression test

Nickel-based Alloy 725 bi-crystalline micropillars with different types of grain boundaries(GBs)were compressed in hydrogen-free and in-situ hydrogen-charged conditions to investigate the hydrogen effect on the deformation behavior of the selected GBs.In the presence of hydrogen,the compressive stresses on the micropillars increase regardless of the GB type.It was proposed that this hydrogen-induced hardening behavior is the synergistic effect of hydrogen-enhanced dislocation multiplication and interactions,the pinning effect of hydrogen on dislocation motion,and hydrogen-enhanced lattice friction.Transmission electron backscatter diffraction(t-EBSD)results demonstrate that both low-angle GBs and high-angle GBs can effectively suppress dislocation transmission through the GBs,resulting in dislocations pile up along the GBs in the hydrogen-charged condition.In contrast,this behavior was not observed in the micropillars with twin boundaries.     

OVERLOAD EFFECTS IN ENVIRONMENTAL HYDROGEN INDUCED FRACTURE

A study has been made of the effect of short periods of overloading on the environmental hydrogen induced fracture (HIF) life of 0.42% C, 0.87% Cr, 0.2% Mo steel tested in a 0.5 mol/L H₂SO₄ solution under continuously hydrogen charging conditions. Experimental results showed that when the overloading was applied during the early or middle stage of the test, the HIF life was longer than that obtained at constant stress; however, if the overloading was applied during the later stages, a shortened HIF life was obtained. It is important to note that the processes of HIF (including hydrogen absorption, transportation and accumulation, crack initiation and propagation) depend not only on the electrochemical condition, but also on both stress-strain state and stress history. In view of the above considerations, effects of plasticity induced closure, residual compression stress, dislocation shielding and overload damage, which control HIF life, are discussed.     

Large-scale simulations of crack–void and void–void plasticity in metallic fcc crystals under high strain rates

We have simulated the failure of three-dimensional fcc solids containing voids under mode one tension using molecular dynamics, simple interatomic potentials and a system comprising 15 million atoms. When a linear brittle crack front approaches a void, the void acts to impede the progress of the front by causing dislocation emission, thereby rendering the system ductile. When two voids are alone in the system, failure is via ductility with, first, dislocation loops being emitted from the void surfaces and, then, these loops interacting with one another to form.     

Analysis of fracture limit curves and void coalescence in high strength interstitial free steel sheets formed under different stress conditions

Void formation, which is a statistical event, depends on inhomogeneities present in the microstructure. The analysis on void nucleation, their growth and coalescence during the fracture of high strength interstitial free steel sheets of different thicknesses is presented in this article. The analysis shows that the criterion of void coalescence depends on the d-factor, which is the ratio of relative spacing of the ligaments (δd) present between the two consecutive voids to the radius of the voids. The computation of hydrostatic stress (σ_m), the dominant factor in depicting the evolution of void nucleation, growth and coalescence and the dimensional analysis of three different types of voids namely oblate, prolate and spherical type, have been carried out. The ratio of the length to the width (L/W) of the oblate or prolate voids at fracture is correlated with the mechanical properties, microstructure, strains at fracture, Mohr’s circle shear strains and Triaxiality factors. The Lode angle (θ) is determined and correlated with the stress triaxiality factor (T), ratio of mean stress (σ_m) to effective stress (σ_e). In addition, the Void area fraction (V _a), which is the ratio of void area to the representative area, is determined and correlated with the strain triaxiality factor (T_o).     

Hydrogen embrittlement studies of aged and retrogressed-reaged Al–Zn–Mg alloys

The hydrogen embrittlement (HE) of Al–4.35Zn–1.4Mg–0.059Zr alloys in different artificial aging tempers and after retrogression and reaging (RRA) treatments has been investigated by tensile testing hydrogen precharged specimens. The influence of RRA and hydrogen charging on the dislocation structure was studied by TEM. The under-aged temper was the most susceptible while the over-aged temper was the most resistant to HE. The RRA treatment improved the HE resistance of all the tempers. This has been attributed to the reduction in dislocation density upon retrogression and reaging. Flat fractography features near the surface of the hydrogen charged specimen have been correlated to the depth of hydrogen penetration. The hydrogen dislocation interaction and hydride cracking mechanism of HE have been addressed.     

Hydrogenation characteristics of the proton conducting oxide-hydrogen storage alloy composite

Hydrogen absorption-desorption characteristics of a proton conducting oxide, SrCe_0.95Yb_0.05O_3−δ, were investigated using an electrochemical technique for the first time. It is suggested that the oxide can electrochemically repeat hydrogen absorption and desorption like hydrogen storage alloys. Subsequently, we attempted to improve activation properties of a hydrogen storage alloy, MmNi_3.72Co_0.60Mn_0.45Al_0.32, by mixing the proton conducting oxide. As a result, the oxide-alloy composite was found to enhance the activation performance of anode in Ni-metal hydride battery. The proton conducting oxide lying on the surface corrosion layer of alloy particles plays a role as a new path for hydrogen atoms to go into the alloy.     

Fatigue behavior and dislocation substructures for 6063 aluminum alloy under nonproportional loadings

In this paper, the fatigue behavior and dislocation substructures of 6063 aluminum alloy were studied under several nonproportional path loadings, which were circle, ellipse, rectangle and square paths. After fatigue test the micro-structure especially the dislocation substructures of the failure materials was carefully observed with the transmission electron microscope (TEM) method. Under the same 93 MPa equivalent stress amplitude loading, the alloy has the shortest life and the most severe cyclic additional hardening with circle path loading among all the loading paths. This attributes to the complicated dislocation substructures and severe stress concentration of the alloy during the cycling process. While under the ellipse path loading, the alloy has a comparably long life and light cyclic additional hardening. The deformation of the alloy and the morphology of the dislocation substructures determine the fatigue behavior of 6063 alloy under the same equivalent stress amplitude loading. Under the circle path loading, the fatigue life decreases while the cyclic strain increases as the loading stress amplitude increases from 47 MPa to 163 MPa. The dislocation evolution of 6063 alloy during the cycling process under circle path loading was examined with TEM. It was found that the dislocation merges with each other and changes from single lines to crossed bands. The movability of dislocation reduces and the stress concentration degree rises during the cycling process.     

Distinctive features of operation of an internal combustion engine running on hydrogen-containing fuels

Experimental investigations have been carried out on an internal combustion engine with hydrogen added to the hydrocarbon fuel, i.e., gasoline. The possibility of improving the energy and environmental indices in the case of hydrogen feed to the engine’s air path has been shown. It has been established that increase in the fraction of hydrogen in the fuel mixture causes the operating process of the engine to improve, with the result that the flow rate of gasoline as a function of the H₂ fraction decreases by nearly 70%. Considerable reduction in the content of CO, CO₂, and CH (of approximately 5–60% depending on the amount of the added H₂) is observed. However, adding hydrogen to the fuel-air mixture leads to an increase in the content of nitric oxides in the combustion products because of the growth in the velocity of propagation of the flame and increase in the combustion temperature.