This study investigates the microstructure-dependent hydrogen transport properties and hydrogen embrittlement (HE) resistance of dual-phase (DP) ferritic-martensitic low alloy steels (LASs) for ...hydrogen-related applications. Ferritic-pearlitic and fully martensitic microstructures were also included as control conditions. Our results demonstrate that tempering DP-LAS reduces hardness and enhances carbide precipitation, leading to increased hydrogen diffusion coefficients and reduced hydrogen trapping. This effect is believed to be primarily associated with dislocation annihilation. Additionally, higher martensite content decreases the hydrogen diffusion coefficient in both as-quenched and tempered conditions. In slow strain rate tests, DP-LASs samples with approximately 50% tempered martensite exhibited the highest HE resistance. These findings offer new insights into the microstructure design of DP-LASs for hydrogen applications, particularly in the tempered condition.
•Tempering increased the hydrogen apparent diffusion coefficient and reduced trapping.•Negligible effect of the tempered-induced carbides on hydrogen trapping.•Hydrogen apparent diffusion coefficient decreased with increased martensite content in as-quenched and tempered conditions.•Hydrogen diffusion was primarily controlled by the amount of martensite rather than the martensite carbon content.•Dual-phase microstructure composed of 50.6% martensite had the highest hydrogen embrittlement resistance.
Hydrogen grain boundary (GB) trapping is widely accepted as the main cause for hydrogen induced intergranular failure. Several studies were conducted to unveil the role of GBs on hydrogen transport; ...however, a clear understanding is yet to be attained. This is due to the limitations of the state-of-the-art experimental procedures for such highly kinetic processes. In this study, we aim at providing a deeper understanding of hydrogen-GB interactions using full-field representative volume element (RVE). The phase-field method is chosen for generating RVEs, since it is an appropriate numerical tool to represent GBs. A novel fully-kinetic formulation for hydrogen diffusion and GB trapping is presented, which is compatible with the phase-field based RVEs. GB diffusivity (Dgb) and trap-binding energy (Egb) were used as parameters to understand the interactions between diffusion and GB trapping. Uptake and permeation simulations were performed with constant and gradient occupancy boundary conditions respectively. In both cases, increasing Egb, increased the hydrogen GB occupancy. The permeation simulations showed that the hydrogen flux along the GBs increased with increasing both, Dgb and, surprisingly, Egb. Since trapping increases the hydrogen occupancy along GBs, it also increases the occupancy gradients, resulting in a higher flux. This led to the conclusion that, in the case of an external occupancy gradient, GB trapping and diffusion cooperate, rather than compete, to increase the hydrogen flux. On the other hand, the decisive factor for the retention of hydrogen at the GBs in permeation simulations was Dgb rather than Egb.
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•A new full-field model for H diffusion and GB trapping is presented.•The model shows the kinetic interactions of GB diffusivity and trapping.•The build-up of H due to trapping increases the permeation flux.•GB diffusion and trapping constructively interact, increasing H flux.
Advanced high strength steels (HSSs), such as dual phase steels, are widely used in the automotive industry due to their excellent combination of strength and ductility. In certain applications, they ...might be exposed to hydrogen (H) which is known to be detrimental for the deformation. H embrittlement (HE) is still not fully understood. It might drastically reduce the energy absorbed in a crash event and limits the use of HSSs in car bodies. Although H diffusion is a highly time dependent phenomenon, so far, the combined effect of dynamic strain rates and electrochemical H pre-charging has not been studied. Therefore, a reproducible methodology has been developed. Tensile specimens were electrochemically H pre-charged and immediately tested in a split Hopkinson tensile bar setup. To distinguish between the effect of strain rate and HE, static tests have been conducted using the same procedure. Results show that the HE resistance decreased due to higher H amounts in the sample for all strain rates. The HE increased when slower strain rates were applied due to higher probability of H to diffuse to regions of stress concentration ahead of a crack tip and as such accelerating failure. At the highest strain rate considered (900 s
-1
), the material still lost about 10% of its ductility.
Hydrogen interactions with different microstructural defects were analysed in ultra-low carbon steel, with specific focus on the influence of carbon distribution. For this purpose, the steel was cold ...rolled and subjected to various annealing treatments, obtaining microstructures ranging from as cold rolled, over recovered up to fully recrystallized. Optical microscopy, transmission electron microscopy and hardness measurements were used to obtain information on the grain boundary structure and dislocation density. Positron annihilation spectroscopy measurements revealed metastable open volume defects related to both dislocations and vacancy clusters. The carbon distribution was characterized by internal friction experiments. Hydrogen interactions were studied by thermal desorption spectroscopy and internal friction measurements of samples electrochemically pre-charged with hydrogen.
The most dominant contribution in hydrogen trapping in the cold rolled material is provided by dislocations. However, their contribution is strongly reduced after annealing at temperatures in the range between 300 K and 600 K due to dissolution of metastable kink-pairs and small carbon-vacancy clusters. Dissolution of such clusters provides fresh supply of carbon to dislocations, which reduces the dislocation trapping capacity for hydrogen due to carbon-hydrogen repulsion. The presence of carbon also reduces the vacancy mobility, allowing clustering and growth during cold rolling resulting in strong hydrogen trapping sites. Binding energies at dislocations were obtained from thermal desorption spectroscopy and internal friction measurements and compared to various models. The small discrepancy in the activation energy is argued to originate from the quantum effect. Hydrogen release from vacancy clusters is determined by the energy required for complete cluster dissolution.
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Despite their good corrosion resistance and optimal mechanical properties, duplex stainless steels are affected by hydrogen embrittlement. Therefore, understanding the hydrogen-defect interactions in ...these steels is crucial. This study uses internal friction to evaluate the interactions of hydrogen with microstructural defects. Analysis of the internal friction spectra of the steels subjected to straining and hydrogen charging, together with thorough microstructural characterization, gives new insights in hydrogen interactions with defects present in the different phases, i.e. ferrite (body centered cubic) and austenite (face centered cubic).
While no significant effect of tensile deformation can be observed by thermal desorption spectroscopy, the internal friction spectra show a clear influence of the presence of defects. Detailed analysis of these spectra reveals the interactions in the austenite as dominant, while no indications for hydrogen-dislocation interactions in ferrite are observed. This can be related to limited trapping in ferrite due to the austenite sink action or to limited dislocation formation in ferrite. Indications for hydrogen interactions with dislocations in the austenite are found, possibly suggesting enhanced dislocation mobility when surrounded by hydrogen. Moreover, a pronounced influence of hydrogen charging on the vacancy cluster related peaks is observed, indicating strong interactions between hydrogen and vacancy clusters in austenite. This can be put in contrast to behavior of pure ferritic steels, where dislocations provided the strongest hydrogen interactions. As these specific defects are of primary interest in the hydrogen embrittlement mechanisms, internal friction is concluded to provide important unique insights in hydrogen-defect interactions, even for complicated multiphase microstructures.
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In the present study, the effect of Niobium (Nb) on the hydrogen embrittlement resistance of Quenched and Partitioning (Q&P) steel is investigated. For this purpose, the hydrogen uptake level and its ...impact on the mechanical properties of a Nb-free and a 0.024 wt% Nb Q&P steel are thoroughly analysed. The hydrogen trapping capacity is evaluated via thermal desorption spectroscopy (TDS). In-depth analysis of the desorption kinetics at different heating rates allows identification and quantification of the available trapping sites. The hydrogen embrittlement sensitivity of both steels is characterized using static and dynamic tensile tests. The addition of Nb results in an increase of the hydrogen concentration by more than 25%. The larger hydrogen content in the Nb steel, as a result of the higher fraction of grain boundaries/interphases, gives rise to a more severe embrittlement of the Nb steel compared to the Nb-free one. In addition to the larger hydrogen fraction in the Nb Q&P steel, the larger retained austenite fraction of low stability is detrimental due to the larger fraction of high carbon martensite formed when straining. This results in higher susceptibility to hydrogen embrittlement of the Nb microalloyed steel due to the brittle character of the high carbon martensite that forms easily during straining. Under dynamic loading conditions, the hydrogen embrittlement of both steels is minimal, which is attributed to a reduced hydrogen diffusion and the suppression of the transformation induced plasticity (TRIP) effect due to adiabatic heating.
The effect of hydrogen on the micromechanical characteristics of pure grade 2 titanium was investigated by comparing prenotched microcantilever bending in air versus in-situ electrochemical hydrogen ...charging. The nanoscale interaction between hydrogen and the metal's defects was analysed in three different grain orientations, i.e. the top surface parallel to {101‾2}, {0001}, and {202‾1}. The subsequent high resolution post-mortem characterisation enabled to characterise the interplay between hydrogen in solid solution and the dislocations in the local deformation process. The results showed that hydrogen facilitated a localised plasticity mechanism, causing hydrogen assisted degradation at the applied low hydrogen concentration below the hydride formation threshold. Meanwhile, hydrogen provoked a reduction in defect formation energy, following the defactant concept, which was responsible for a softening effect on the yield stress and flow stress. Furthermore, due to the anisotropic characteristics of the α titanium hexagonal close packed crystal lattice, the susceptibility to hydrogen effects also depended on the grain orientation. This single grain in-situ micromechanical testing is complementing the existing macroscaled identification of hydrogen effects in α titanium.
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•Micromechanical in-situ hydrogen bending of prenotched grade 2 titanium microcantilevers.•Solute hydrogen induces mechanical softening.•Solute hydrogen reduces stacking fault formation energy and promotes slip planarity.•Solute hydrogen facilitates the localisation of plastic deformation.•Hcp anisotropy causes a grain orientation dependent susceptibility to hydrogen degradation.
The mechanism of hydrogen embrittlement in grade 2 titanium was studied by a combined micro and nanoscale approach. Insights into the hydrogen related deformation and degradation mechanisms were ...acquired via a unique characterisation procedure, correlating the topological observations on the fracture surface after tensile testing to the dislocation arrangement underneath. Focused ion beam lift-out was employed to extract a subsurface slice perpendicular to the fracture surface. High resolution characterisation provided information about the dislocation configuration and active hydrogen-assisted degradation mechanism. A high hydrogen content forms a brittle titanium hydride phase, failing via hydride cleavage. At a moderate hydrogen quantity, hydrogen in solid solution is responsible for a ductility decrease via the hydrogen-enhanced localised plasticity mechanism.
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•Investigation of hydrogen embrittlement in titanium at micro and nanoscale.•Distribution of dislocations reveals hydrogen dependent deformation mode.•Indications of degradation via hydrogen enhanced localised plasticity mechanism.•Material degradation via brittle hydride cleavage at high hydrogen concentrations.
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