Hydrogen In Metallic Alloys Research Articles

Hydrogen in metallic alloys ─ embrittlement and enhanced plasticity: a review

Abstract The evolution of ideas concerning the nature of hydrogen embrittlement of engineering metallic materials is described based on a number of the proposed hypotheses and corresponding experiments. The main attention is paid to two of them, namely hydrogen-enhanced decohesion (HEDE) and hydrogen-enhanced localized plasticity (HELP). Recent attempts to interconnect the both models as HELP + HEDE and HELP-mediated HEDE ones are also estimated. A conclusion is made that HELP model is preferential for understanding the entire array of experimental data with a caveat that it is necessary to consider the chemical nature of hydrogen atoms and view them not only as point defects. Based on the studies of hydrogen effect on the atomic interactions in iron, nickel, titanium, and its alloys, it is shown that the electron approach to HELP phenomenon adequately describes two competitive features of hydrogen behavior in metals: increased brittleness and enhanced plasticity. Due to the increase in the concentration of free electrons, hydrogen decreases the elasticity moduli, which causes the crystal lattice to soften. For this reason, the formation of hydrogen atmospheres around the dislocations decreases the start stress of dislocation sources, as well as line tension of emitted dislocations, enhancing thereby their mobility, and weakens repulsion between dislocations in their pile-ups. The range of temperatures and strain rates in which hydrogen embrittlement occurs is controlled by the enthalpies of hydrogen atoms’ diffusion and their binding to dislocations. The resulting consequences for mechanical properties depend on the short-range atomic order, SRO, which inherently occurs in the metallic solid solutions and localizes plastic deformation both in the cases of short-range atomic ordering and of short-range atomic decomposition. Hydrogen enhances slip localization because of its different solubility in the submicrovolumes of short-range decomposed solid solutions. If SRO is absent or not remarkably formed, the hydrogen-increased concentration of free electrons results in enhanced plasticity. Available positive hydrogen effects on the plasticity of titanium β-alloys and austenitic steels are presented and interpreted.

Analysis of thermal desorption of hydrogen in metallic alloys

The degree of embrittlement of metallic alloys is sensitive to the concentration of absorbed hydrogen, with hydrogen traps (particularly at grain boundaries) playing an important role. Thermal desorption spectrometry (TDS) is widely used to measure the detrapping and diffusion behaviour of hydrogen in metallic alloys. However, it is problematic to obtain a consistent interpretation of TDS data from the literature, due to the large number of material parameters that influence the measurement, and this results in a wide range of quoted values for trapping parameters such as the number of trap types, trap binding energies and trap densities. In this paper, the governing partial differential equation for hydrogen diffusion with sink and source terms for a single trap is formulated in non-dimensional form, assuming local equilibrium between the hydrogen atoms at the lattice sites and the trap sites. An asymptotic analysis reveals two distinct regimes of diffusion behaviour in TDS tests. Kissinger-type behaviour is expected in a TDS test for low heating rates on an alloy with a low lattice activation energy. Contour maps of maximum hydrogen flux and the corresponding temperature are plotted using axes of trap density and trap binding energy by making use of the full numerical solution (and asymptotic solutions). These maps serve as a useful tool for an accurate and simple determination of the trap binding energy as well as the trap density.

Tensile behaviour of superelastic NiTi alloys charged with hydrogen under applied strain

ABSTRACTThe tensile behaviour of NiTi alloys is investigated after hydrogen charging during the austenite, half-transformation and martensite phases. The specimens are charged with different current densities and charging durations. During the tensile tests, the strain of the plateau transformation decreases due to hydrogen-induced residual martensite variants. This decrease becomes important when the charging happens during the martensite phase. Accordingly, the hydrogen ensures the stability of the phase in which the charging process occurs. Moreover, a heightening of transformation stress is noticed during the plateau. The transformation stress increases when the current density grows and the charging duration rises. This occurrence is caused by the interaction between the hydrogen and NiTi structures, where hydrogen delays the NiTi martensite transformation.This paper is part of a thematic issue on Hydrogen in Metallic Alloys

Microstructural and crystallographic features of hydrogen-related fracture in lath martensitic steels

ABSTRACTThis paper summarised the microstructural and crystallographic features of hydrogen-related fracture under tensile deformation in martensitic steels. The specimens with relatively low strength exhibited quasi-cleavage fracture. Crystallographic orientation analysis demonstrated that the quasi-cleavage fracture occurred on {011} planes. On the other hand, the macroscopic fracture surface morphologies of the specimens with relatively high strength appeared to be intergranular-like. However, nature of the fracture was somewhat different from a typical intergranular fracture, and the fracture surfaces consisted of facets parallel to {011} planes on a microscopic level. Based on the experimental results, we concluded that the crystallographic feature of {011} plane itself had an important role on the hydrogen-related fracture.This paper is part of a thematic issue on Hydrogen in Metallic Alloys

Recent progress in microstructural hydrogen mapping in steels: quantification, kinetic analysis, and multi-scale characterisation

ABSTRACTThis paper gives an overview of recent progress in microstructure-specific hydrogen mapping techniques. The challenging nature of mapping hydrogen with high spatial resolution, i.e. at the scale of finest microstructural features, led to the development of various methodologies: thermal desorption spectrometry, silver decoration, the hydrogen microprint technique, secondary ion mass spectroscopy, atom probe tomography, neutron radiography, and the scanning Kelvin probe. These techniques have different characteristics regarding spatial and temporal resolution associated with microstructure-sensitive hydrogen detection. Employing these techniques in a site-specific manner together with other microstructure probing methods enables multi-scale, quantitative, three-dimensional, high spatial, and kinetic resolution hydrogen mapping, depending on the specific multi-probe approaches used. Here, we present a brief overview of the specific characteristics of each method and the progress resulting from their combined application to the field of hydrogen embrittlement.This paper is part of a thematic issue on Hydrogen in Metallic Alloys

Hydrogen redistribution under stress-induced diffusion and corresponding fracture behaviour of a structural steel

ABSTRACTHydrogen redistribution under stress-induced hydrogen diffusion and corresponding fracture behaviour of a 960 MPa grade martensitic steel were studied. Slow strain rate tensile (SSRT) tests after hydrogen pre-charging were performed and the fracture surface was observed and analysed. The strain rate ranged from 10−6 to 10−4 s−1. In the pre-charged sample with a certain hydrogen content of 0.62 ppm, hydrogen distribution was homogeneous before the SSRT test. After tensile testing, brittle fracture features appeared in the centre of the fracture surface, while ductile features appeared in the surrounding area. Brittle region size increased with the strain rate slowing down in the range from 10−4 to 5 × 10−6 s−1, while it stabilised at the strain rate slower than 5 × 10−6 s−1. Relationship between the strain rate and the brittle region size was established and discussed based on the present data of hydrogen content in the material.This paper is part of a thematic issue on Hydrogen in Metallic Alloys

Effect of cathodic hydrogen-charging current density on the hydrogen diffusivity in nanostructured bainitic steels

ABSTRACTThe effect of cathodic hydrogen-charging current on the effective hydrogen diffusivity in nanostructured bainitic steels produced at transformation temperatures 200°C (BS200) and 350°C (BS350) was investigated and compared to that of mild steel. The effective hydrogen diffusivity at 10 mA cm−2 was the lowest for BS200, followed by BS350 and mild steel, due to the finer microstructure and higher dislocation density in the bainitic ferrite of BS200. Increase in the hydrogen-charging current density, i.e. 20 and 30 mA cm−2, increased the effective hydrogen diffusivity of mild steel by 37 and 135%, and BS350 by 49 and 150%, respectively. For BS200, the increase was not significant (2%) at 20 mA cm−2, but increased by 34% at 30 mA cm−2.This paper is part of a thematic issue on Hydrogen in Metallic Alloys

Theoretical evaluation of the role of crystal defects on local equilibrium and effective diffusivity of hydrogen in iron

ABSTRACTHydrogen diffusion and trapping in ferrite is evaluated by quantum mechanically informed kinetic Monte Carlo simulations in defective microstructures. We find that the lattice diffusivity is attenuated by two to four orders of magnitude due to the presence of dislocations. We also find that pipe diffusivity is vanishingly small along screw dislocations and demonstrate that dislocations do not provide fast diffusion pathways for hydrogen as is sometimes supposed. We make contact between our simulations and the predictions of Oriani's theory of ‘effective diffusivity’, and find that local equilibrium is maintained between lattice and trap sites. We also find that the predicted effective diffusivity is in agreement with our simulated results in cases where the distribution of traps is spatially homogeneous; in the trapping of hydrogen by dislocations where this condition is not met, the Oriani effective diffusivity is in agreement with the simulations to within a factor of two.This paper is part of a thematic issue on Hydrogen in Metallic Alloys

Improved resistance to hydrogen embrittlement by tailoring the stability of retained austenite

ABSTRACTThe susceptibility of hydrogen embrittlement (HE) in GCr15 bearing steel under two types of heat treatments: quenching and tempering (QT) and pre-quenching and austempering (PQA) were investigated. Results showed that PQA-treated specimen have higher mechanical stability of the retained austenite (RA) compared with QT-treated specimen. The experiment of indentation test and hydrogen bubbles suggested that PQA-treated specimen was less susceptible to hydrogen than the QT-treated sample. Subsequently, hydrogen permeation tests revealed significant differences in diffusion coefficient and the number of hydrogen trapping sites between QT and PQA specimens. It was demonstrated that PQA is an appropriate heat treatment to tailor the stability of the RA and enhances the resistance to HE of GCr15 bearing steel.This paper is part of a thematic issue on Hydrogen in Metallic Alloys

Hydrogen permeation through deformed and heat-treated Armco pure iron

ABSTRACTThe goal of this study is to obtain a deeper insight in the relation between hydrogen diffusion and hydrogen traps present in Armco pure iron. Cold deformation was applied to this material, which initially contained a limited amount of traps. The cold deformation was applied to increase the dislocation density and modify grain boundary characteristics. In this way, the hydrogen diffusivity decreased as the hydrogen trapping ability of the microstructure increased. A subsequent heat treatment allowed changing the density of microstructural defects again and consequently increased the hydrogen diffusion coefficient. In addition, studying blister formation showed that a higher degree of deformation caused more surface blisters, while recovery lowered the number of blisters. Electron backscatter diffraction characterisation provided the necessary input on the microstructural features and their evolution. Analysis of these samples allowed evaluating the correlation between hydrogen diffusion, blister formation and microstructural defects.This paper is part of a thematic issue on Hydrogen in Metallic Alloys

Analysis of electro-permeation of hydrogen in metallic alloys.

A reaction-diffusion type modelling framework is presented to analyse both electro-permeation (EP) and thermal desorption spectrometry (TDS) measurements of hydrogen in metallic alloys. It is assumed that the kinetics of hydrogen motion is governed by diffusion through the lattice, along with trapping/detrapping at specific sites such as dislocations, grain boundaries, etc. It is shown that the trapping and detrapping rates are typically much faster than the diffusion rate, and consequently a simplification of the governing equations suffices such that local equilibrium exists between lattice and trapped hydrogen. Using this local equilibrium assumption, we then present an asymptotic analysis of the governing kinetic equation for the EP test. This asymptotic analysis reveals that four regimes of behaviour exist, ranging from negligible trapping to the complete filling of deep traps. The analysis suggests that EP tests should be so-arranged that three regimes of behaviour are spanned, in order to extract the relevant material properties associated with hydrogen transport. The numerical solutions presented in this study support the asymptotic analysis. The hydrogen kinetics framework is also deployed to analyse both EP and TDS tests on the same martensitic steel. The EP measurements all lie in regime I and are thus insufficient to uniquely determine both the trap density and binding energy. Reasonable agreement is obtained between measurements and numerical predictions of TDS tests using parameters estimated from the EP tests. Further improvements in measurements are required to confirm the fidelity of this modelling approach.This article is part of the themed issue 'The challenges of hydrogen and metals'.

Prediction of hydrogen permeability in V–Al and V–Ni alloys

A semi-empirical methodology for predicting the permeability of hydrogen in metallic alloys is proposed by combining an atomistic simulation and a thermodynamic calculation. An atomistic simulation based on a modified embedded-atom method interatomic potential and a CALPHAD-type thermodynamic calculation technique was used to predict the diffusivity and solubility of hydrogen, respectively. The approach was applied to the prediction of the hydrogen permeability in V–Al and V–Ni alloys that are promising for non-Pd hydrogen separation membranes. The predicted permeability of hydrogen decreases, as Al or Ni concentration increases in the alloys. The predicted permeability is in quite good agreement with experimental data available in literature, successfully reproducing the overall trend for the effect of alloying elements, which enables an alloy design of metallic hydrogen permeable membranes.

Hydrogen in metallic alloys as an example of a lattice gas with random field and random bonds.

Evidence is presented that hydrogen dissolved in metallic alloys can be considered as a realization of a lattice gas with a random internal field and random bonds. The predictions of this model are in good agreement with experimental phase diagrams and results of diffuse X-ray scattering from microscopic density fluctuations of hydrogen in Nb(1-x)Mo(x) alloys. 22 references.