Hydrogen accommodation in -iron and nickel
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 , 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 alloys [18], [20], [21], [22], [30], [31], and 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 -iron and nickel is described in Section 2. The formation energies for clusters in Ni and 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) -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 -Fe and Ni used the projector augmented wave (PAW) pseudopotentials [43] with the
Hydrogen in -iron
Hydrogen solution into -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 -Fe containing H.
The vacancy formation energy can be calculated by considering the reaction
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 and temperature, up to , 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 -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 -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 -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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2022, Acta MaterialiaCitation Excerpt :When applied to the TiZrNbHfTa system, the rule of mixtures provides a melting point prediction of 2522 K, much higher than our initial prediction of 1400 K. The uncertainty caused by the range of local environments does not account for this disparity in the melting point predictions and highlights a distinct difference in behaviour between this HEA and the simple monoatomic systems on which Fullarton et al. [34] based their model. The low vacancy formation energies that we report could be representative of phase changes or potentially issues regarding ordering, but this falls beyond the scope of this study and would need to be explored further in future work.
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2020, International Journal of Hydrogen EnergyCitation Excerpt :Nevertheless, Ni-based alloys are susceptible to HE [3,13–15], which impairs their excellent performance. In addition to the experimental studies, first-principles calculations are very suitable to study H behaviors in alloys fundamentally and have already been extensively utilized to investigate the HE of the Ni metal and Ni alloys, e.g., H on the Ni surface [16,17], H diffusion in Ni [18–20], and H in the vacancy of Ni [21–25], in which first-principles calculations can not only provide a deep understanding of the HE mechanism but also predict the susceptibility of metals and alloys toward HE. As is well known, H trapping and diffusion at the Ni grain boundary (GB) are crucial for HE phenomena, and GB serves as an area for accumulation of H atoms and diffusion of a rapid channel.
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