Hydrogenated vacancies lock dislocations in aluminium (original) (raw)
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Influence of dislocations on hydrogen diffusion and trapping in an Al-Zn-Mg aluminium alloy
Materials & Design, 2019
Cu-lean Al-Zn-Mg alloys are known to be susceptible to hydrogen embrittlement (HE), which currently limits their use in automotive industry. Several works suggested that the resulting loss of mechanical properties was related to hydrogen trapping in different metallurgical sites. The present work attempts to provide a better understanding of the hydrogen-dislocations interactions to evaluate their influence on the loss of mechanical properties of hydrogen-embrittled Al-Zn-Mg alloys. Pre-strained samples of 7046 aluminium alloy (AA7046) were therefore prepared in order to increase the density of motionless dislocations. Tensile samples, pre-strained or not, were then corroded in 0.6 M NaCl and mechanically tested to evaluate their HE susceptibility and the role of dislocations on hydrogen diffusion. Results highlighted a significant improvement of the HE resistance of the alloy with the increase in the density of motionless dislocations induced by the pre-strain step. This was attributed to preferential hydrogen trapping on motionless dislocations leading to a decrease in the hydrogen amount in the grain boundaries. The measurements of hydrogen penetration depth by Scanning Kelvin Probe Force Microscopy (SKPFM) for cathodically charged samples provided further evidence to support these assumptions.
Modeling Dislocation-Mediated Hydrogen Transport and Trapping in Face-Centered Cubic Metals
Journal of Engineering Materials and Technology, 2021
The diffusion of hydrogen in metals is of interest due to the deleterious influence of hydrogen on material ductility and fracture resistance. It is becoming increasingly clear that hydrogen transport couples significantly with dislocation activity. In this work, we use a coupled diffusion-crystal plasticity model to incorporate hydrogen transport associated with dislocation sweeping and pipe diffusion in addition to standard lattice diffusion. Moreover, we consider generation of vacancies via plastic deformation and stabilization of vacancies via trapping of hydrogen. The proposed hydrogen transport model is implemented in a physically based crystal viscoplasticity framework to model the interaction of dislocation substructure and hydrogen migration. In this study, focus is placed on hydrogen transport and trapping within the intense deformation field of a crack tip plastic zone. We discuss the implications of the model results in terms of constitutive relations that incorporate hy...
Hydrogen-dislocation interactions and their role in HELP mechanism of hydrogen embrittlement
Effects of hydrogen on elastic interactions in single dislocation dipole and dipole-like dislocation pile-up (DDP) were analysed. It was shown that there is no significant hydrogen shielding of elastic forces in the case of a single dislocation dipole, while hydrogen induces a notable effect on the elastic equilibrium of the DDP. Accumulation of hydrogen between adjacent dislocation pile-ups results in a reduction of external stresses which stabilises DDP. The tensile tests of hydrogen-charged austenitic stainless steel single crystals oriented for easy glide and observed hydrogen-induced strain localization are discussed in terms of such a dynamic softening. It is concluded that HELP takes place at the initial stages of plastic deformation.
Hydrogen-Enhanced Local Plasticity in Aluminum: An Ab Initio Study
Physical Review Letters, 2001
Dislocation core properties of Al with and without H impurities are studied using the Peierls-Nabarro model with parameters determined by ab initio calculations. We find that H not only facilitates dislocation emission from the crack tip but also enhances dislocation mobility dramatically, leading to macroscopically softening and thinning of the material ahead of the crack tip. We observe strong binding between H and dislocation cores, with the binding energy depending on dislocation character. This dependence can directly affect the mechanical properties of Al by inhibiting dislocation cross-slip and developing slip planarity.
Effect of hydrogen on dislocation nucleation in alloy 718
International Journal of Hydrogen Energy, 2017
In situ electrochemical nanoindentation has been used to study the effect of hydrogen on the nanomechanical response of Alloy 718. Observations show that hardness increase as a result of hydrogen charging. Also, the hydrogen charging gives a reduced pop-in load and pop-in width. This is related to a reduction in the energy needed for dislocation nucleation and the mobility of the dislocations in the presence of hydrogen. Two grains with different orientations has been tested here. The pop-in load and width obtained in the (101) orientation was more affected by the presence of hydrogen than those achieved in the (111) oriented grain.
Acta Materialia, 2010
This paper summarizes recent work at the University of Illinois on the fundamental mechanisms of hydrogen embrittlement. Our approach combines experimental and theoretical methods. We describe the theoretical work on hydride formation and its application to hydrogen embrittlement of Ti alloys through the stress-induced hydride formation and cleavage mechanism, the localization of shear due to solute hydrogen, and finally, we present experimental evidence that favors the decohesion mechanism of hydrogen embrittlement in a β-Ti alloy.
ISIJ International
Explaining the hydrogen effect on dislocation mobility is crucial to revealing the mechanisms of hydrogen-related fracture phenomena. According to the general perspective, reducing the speed of dislocation can give enough time to hydrogen to catch up with the dislocation migration. In this research, we conducted molecular dynamics (MD) simulations to investigate the impact of hydrogen on the edge-dislocation motion in α-iron at various dislocation speeds and temperatures. It was discovered that, for all hydrogen concentrations evaluated in this paper, the hydrogen effect on dislocation transition from pinning to dragging occurs at a dislocation speed of around 0.1 m/s at 300 K. When the dislocation velocity is reduced to 0.01 m/s employing long timescale MD simulations over 1 μs, it is observed that hydrogen follows dislocation motion with small jumps in the dislocation core. The required stress to migrate the edge dislocation at a speed of 0.01 m/s was discovered to be 400 MPa, even at a lower hydrogen concentration, which was achieved in a gaseous hydrogen environment with lower pressure than atmospheric pressure. Although the dislocation still traps hydrogen at 500 K, as temperature increases, the impact of hydrogen on the shear stress required for dislocation glide becomes negligibly small. The required shear stress at lower dislocation speeds was predicted by employing the stress-dependent thermal activation model assuming the hydrogen diffusion rate-determining. The finding demonstrated that the edge dislocation should slow down until 1 mm/s order or less in the presence of hydrogen and suitable stress for α-iron.
Modeling hydrogen dragging by mobile dislocations in finite element simulations
International Journal of Hydrogen Energy, 2022
Finite element simulation modeling permits to predict hydrogen concentration for various initial boundary-values problems, but the results depend on the underlying transport mechanisms accounted for. Trapping process is a key factor in the apparent hydrogen diffusion, and the case of mobile traps as dislocations needs modification of the hydrogen transport equation usually considered in the literature. An extension of this model is proposed where hydrogen dragging by mobile traps is modeled by reaction-diffusion equations, involving trapping and detrapping kinetic, and is applied for evolving trap density with plastic strain. The consequences of trapped hydrogen mobility on diffusive hydrogen repartition in a reference Small Scale Yielding configuration are focused on, especially in term of acceleration of hydrogen transport. The potentiality of the model is illustrated by the modeling of the trapped hydrogen breakaway from fast moving dislocations.
Hydrogen Embrittlement of Aluminum: The Crucial Role of Vacancies
Physical Review Letters, 2005
We report first-principles calculations which demonstrate that vacancies can combine with hydrogen impurities in bulk aluminum and play a crucial role in the embrittlement of this prototypical ductile solid. Our studies of hydrogen-induced vacancy superabundant formation and vacancy clusterization in aluminum lead to the conclusion that a large number of H atoms (up to twelve) can be trapped at a single vacancy, which over-compensates the energy cost to form the defect. In the presence of trapped H atoms, three nearest-neighbor single vacancies which normally would repel each other, aggregate to form a trivacancy on the slip plane of Al, acting as embryos for microvoids and cracks and resulting in ductile rupture along the these planes.
Quantitative tests revealing hydrogenenhanced dislocation motion in α-iron
Nature Materials, 2023
Hydrogen embrittlement jeopardizes the use of high-strength steels in critical load-bearing applications. However, uncertainty regarding how hydrogen affects dislocation motion, owing to the lack of quantitative experimental evidence, hinders our understanding of hydrogen embrittlement. Here, by studying the well-controlled, cyclic, bow-out motions of individual screw dislocations in α-iron, we find that the critical stress for initiating dislocation motion in a 2 Pa electron-beam-excited H2 atmosphere is 27–43% lower than that in a vacuum environment, proving that hydrogen enhances screw dislocation motion. Moreover, we find that aside from vacuum degassing, cyclic loading and unloading facilitates the de-trapping of hydrogen, allowing the dislocation to regain its hydrogen-free behaviour. These findings at the individual dislocation level can inform hydrogen embrittlement modelling and guide the design of hydrogen-resistant steels.