Elsevier

Journal of Alloys and Compounds

Volume 587, 25 February 2014, Pages 794-799
Journal of Alloys and Compounds

Hydrogen accommodation in α-iron and nickel

https://doi.org/10.1016/j.jallcom.2013.10.169Get rights and content

Highlights

  • The effect of H on the vacancy content in Ni and α-Fe is studied by ab intio method.

  • Vacancy formation energies were predicted and compared well with previous results.

  • In α-Fe the vacancy formation energy is lowered by the presence of single H atoms.

  • In contrast to α-Fe, only clustered H solutes reduce vacancy formation energy in Ni.

  • The increase in vacancy concentration has been compared to experimental observations.

Abstract

Ab initio calculations have been used to study the effects of hydrogen on vacancy concentrations in α-Fe and Ni. The presence of H interstitials aided vacancy formation in both metals but via two different mechanisms. In α-Fe, trapping of H by a vacancy is favourable. However binding of further hydrogen atoms was not predicted to proceed. The thermal equilibrium concentration of H interstitials in comparison to vacancies in α-Fe is many orders of magnitude higher over a wide temperature range. Excessive H interstitials in solid solution facilitate vacancy formation, lowering the required energy by 0.79 eV (down to 1.41 eV). In Ni, a single H interstitial is not expected to have an impact on the vacancy population, increasing the vacancy formation energy by 0.32 eV. Two bound H interstitials however are predicted to decrease the vacancy formation energy by 0.52 eV with expected arrangement along a 1 1 1 direction around the vacancy. The calculations show reasonable agreement with experimental data when comparing crystal lattice contractions of the pure metals and predicted melting temperature of the Me–H alloys.

Introduction

Hydrogen is a common impurity in most metals and alloys. It is introduced (intentionally or unintentionally) during production, fabrication, forming or finishing operation and/or exploitation. All steels can pick up hydrogen through rusting, electrochemical corrosion (at the anodic area), electroplating, pickling, soldering, brazing and welding (if the weld joint is contaminated with oil, rust, paint, moisture, etc.). Hydrogen in nickel is present as a result of manufacturing by electrodeposition or by direct reduction of NiO by H2.

Although alloying metals with hydrogen can be utilized to enhance their workability, see, for example [1], or for hydrogen storage,2 in most cases high hydrogen content has devastating effects on the service properties of metals [3], [4]. Furthermore, even a relatively low average concentration of hydrogen can lead to severe embrittlement if hydrogen is trapped by structural defects and its local concentration reaches a critical value. Hydrogen also contributes to delayed weld and/or heat affected zone cracking, blistering in plasma face materials [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], fuzz formation [16], [17] in fusion devices, etc.

Hydrogen degassing by thermal annealing is a conventional way of reversing mechanical properties of metals. In order to optimise thermal treatment of materials for best performance and to avoid recrystallisation, precipitation, coarsening, or other undesired changes in the microstructure, a deeper insight into the interaction of hydrogen with structural defects is necessary.

A series of investigations of Me–H systems under high pressures and elevated temperatures were carried out [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41]. XRD analysis of pure metal powders retained at high pressure and temperature in a sealed container with a hydrogen source revealed a reduction of the lattice parameters. The observed contraction of the materials under investigation was attributed to the facilitated formation of HxV,x=1,,6, vacancy-hydrogen clusters. Under an applied pressure of ∼5 GPa, the internal hydrogen concentration in the metals reached 50 at.% or more, i.e., the corresponding vacancy concentration in the experiments was ∼10 at.%. This is many orders of magnitude higher than the thermodynamic equilibrium concentration of vacancies in pure metals under the same conditions.

In this research we complement the experimental studies of NiH alloys [18], [20], [21], [22], [30], [31], and FeH alloys [35], [36] by evaluating the interaction of hydrogen atoms with vacancies using first principle calculations based on density functional theory. Our work is presented in the following manner: the calculation technique applied for studying the effect of hydrogen in α-iron and nickel is described in Section 2. The formation energies for HxV clusters in Ni and α-Fe are provided in the subsequent section. The discussion of the obtained results accompanied by a comparison with the available experimental data from [18], [20], [21], [22], [30], [31], [35], [36] in Section 4 is followed by the Summary.

Section snippets

Approach and Methodology

The ab initio, density functional theory (DFT) code VASP [42] was used to study the effects of hydrogen on the vacancy concentration and structure in body centred cubic (BCC) α-iron and face-centred cubic (FCC) nickel. Lattice enthalpies and volumes of the systems were calculated by carrying out geometry optimisation of supercells with no symmetry constraints, under constant pressure. Calculations for both α-Fe and Ni used the projector augmented wave (PAW) pseudopotentials [43] with the

Hydrogen in α-iron

Hydrogen solution into α-Fe has been studied and the effect of hydrogen on the defect population was subsequently investigated. Table 1 reports the calculated defect energies that are used to understand the defect behaviour in α-Fe containing H.

The vacancy formation energy can be calculated by considering the reactionFeFeVFe+Fe(s).

The binding energy of a single atom of Fe was calculated to be −4.85 eV resulting in a vacancy formation energy of 2.20 eV, similar to previously reported values of

Discussion

Typical studies of Me–H systems are carried out using multi-anvil presses where relatively uniform pressure in the range of 110GPa and temperature, up to 2000°C, can be produced. The experimental rigs allow in situ XRD measurements of the crystal structure properties of investigated samples. By applying X-ray diffraction, phase transitions in Fe powder subjected to high pressure at elevated temperatures were analysed and corresponding lattice parameters were evaluated [35]. It was established

Summary

The influence of hydrogen on vacancy formation in α-Fe and Ni was evaluated by first principle calculations. The obtained results match the experimental observations: the presence of hydrogen in solid solution reduces vacancy formation energy in both α-Fe and Ni, and contraction of the materials occurs. However the interaction mechanisms between vacancies and H solutes are different in the two metals. Trapping of single H interstitial in a vacancy is predicted in α-Fe. Further accommodation of

Acknowledgements

Fruitful discussions with D.Riley and G.Lumpkin are gratefully acknowledged. The calculations have been performed at the Multi-model Australian ScienceS Imagine and Visualisation Environment (MASSIVE), http://www.massive.org.au.

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