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Sciencedirect.Com Sciencedirect international journal of hydrogen energy 39 (2014) 12221 e1 2 2 2 9 Available online at www.sciencedirect.com ScienceDirect journal homepage: www.elsevier.com/locate/he Atom probe tomography observation of hydrogen in high-Mn steel and silver charged via an electrolytic route * D. Haley , S.V. Merzlikin, P. Choi, D. Raabe Max-Planck-Institut fu ¨r Eisenforschung, Max-Planck-Stra ße 1, D-40237 Du ¨sseldorf, Germany article info abstract Article history: We investigate an electrolytic route for hydrogen charging of metals and its detection in Received 11 March 2014 Atom Probe Tomography (APT) experiments. We charge an austenitic Fe-30Mn-8Al-1.2C Received in revised form (wt.%) weight reduced high-Mn steel and subsequently demonstrate the detectability of 21 May 2014 deuterium in an APT experiment. The experiment is repeated with a deposited Ag film Accepted 26 May 2014 upon an APT tip of a high-Mn steel. It is shown that a detectable deuterium signal can be Available online 2 July 2014 seen in the high-Mn steel, and a D:H ratio of 0.84 can be reached in Ag films. Additionally, it was found that the predicted time constraint on detectability of D in APT was found to be Keywords: lower than predicted by bulk diffusion for the high-Mn steel. Atom probe Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights High-Mn steel reserved. Electrolytic charging microstructural design, further direct characterisation of the Introduction and motivation interaction between hydrogen and the microstructure is desirable. A greater microstructural knowledge, and thus The fundamental driving force for hydrogen-related materials design capability, would allow for enhancing the ductility and research is the embrittling nature of hydrogen, whether origi- toughness of these materials in the presence of hydrogen [7] . nating from a gaseous molecular form, or from hydrogen con- Localisation of hydrogen at the nanostructural level is a taining liquids [1] . As a species in operating environments, complex nano-analysis task, with few methods able to hydrogen is near-ubiquitous. Austentic steels, such as Al- simultaneously localise the position of hydrogen and quantify containing Mn-stabilised steels, have been proposed as an its concentration. Indeed, much knowledge about hydrogen in alternative material for environments where hydrogen the context of engineering materials has been garnered from embrittlement may be problematic [2,3] . Al-enriched material either simulation or bulk testing. Such applicable techniques has been demonstrated to have a higher embrittlement resis- include thermal desorption spectroscopy [8] , computational tance than Al-free materials [4] , which has been attributed to methods such as quantum-mechanical calculations [9] , or modification of stacking fault energies [5] , surface oxide for- more recently, micro-scale scanning methods that measure mation, stress reduction or change in shear modulus. The exact hydrogen absorption into a hydride forming layer via tip- mechanism is, however, the subject of ongoing discussion [6] . scanning [10] . In order to gain an understanding of the hydrogen For interpreting integrative experimental results, such as embrittling mechanism and thus allow for improved thermal desorption, or validating theoretical predictions of * Corresponding author . Tel.: þ49 (0) 211 67 92 152. E-mail address: [email protected] (D. Haley). http://dx.doi.org/10.1016/j.ijhydene.2014.05.169 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. 12222 international journal of hydrogen energy 39 (2014) 12221 e1 2 2 2 9 hydrogen trapping and embrittlement mechanisms a better performed for sufficiently high D-content within materials. knowledge of the actual position of hydrogen in the affected The highest observable D:H ratios with a visible 3D distribu- microstructure is required. Hence, in this work we develop an tion are reported by Gemma for VH (D:H >> 1) [13] , by Taka- approach to charge metals with hydrogen and map it subse- hashi for VC (D:H ~0.5) [12] and TiC (D:H ~0.066) [11] , and quently by using Atom Probe Tomography (APT). finally Takamizawa in D-implanted Si wafers with a value of e 1 Hydrogen detection in APT has been conducted previously 0.04 (D:H H2:H) [15] . Karenesky reports a D:H ratio of ~0.2 for by several authors [11 e15] , by modification of their APT sys- a 316 stainless steel, but no 3D distribution is given [14] . tems to accept gaseous deuterium. However, this route has In this work, this ratio is used as the primary measurement several limitations. Firstly, the method requires the modifi- of hydrogen signal quality within the material. The absolute cation of an APT instrument from current commercial con- magnitude of the mass ¼ 2 peak is a weaker metric than the figurations [16] to obtain spatially resolvable signals, and the ratio measure, D:H, for the effectiveness of hydrogen local- second limitation is the achievable loading fugacities. An isation, due to the non-negligible probability of ionisation of þ þ alternative method for introducing deuterium into a sample an H 2 molecule, which overlaps with the D peak. Having a was investigated by Takamizawa [15] , whereby deuterium large ratio allows for determining if a fluctuation in the was implanted by ion beam methods. One of the more curious mass ¼ 2 peak is simply a reflection of increased H generation applications for deuterium charged APT-like samples is in due to local specimen chemistry or surface crystallography, experimental compact neutron generators [39] , where very particularly important when examining low total D high deuterium evaporation rates were observed. concentrations. þ As hydrogen is largely considered a difficult to remove For experiments with a high unloaded H 2 peak ampli- contaminating species in APT designs, systems are not usu- tude, this background should be subtracted from mass ally configured to allow deliberate introduction of such gasses spectra of deuterium loaded samples. This is particularly into the vacuum system, making gaseous introduction important for laser assisted APT experiments, due to the impractical for many APT systems. Secondly, the theoretically increased molecular ion formation associated with laser achievable fugacities using electrolytic charging are several experiments [26] . Furthermore, it may be that the absolute orders of magnitude higher than are achievable with a low- H levels may vary with the presence of features in the pressure gas charging apparatus [17] , and thus significantly dataset, which could give the appearance of deuterium at higher hydrogen concentrations should be able to be intro- microstructural features if only observing D counts, rather duced into samples. Realisable concentrations can (with suf- than a localised ratio. ficiently fast quenching) be up to 0.5 (at. H)/(metal atom) [17] . Deuterium was considered an appropriate substitute for With higher hydrogen concentrations, the ability for APT hydrogen investigations, as necessitated by the omni- systems to detect deuterium segregation should be present H background. However, it is hypothetically possible commensurately improved, yielding a higher signal-to-noise that there is a different partitioning of deuterium than for ratio. hydrogen to trapping locations. From a practical perspective With the objective of investigating the practicality and this seems unlikely, as the most selective reactions for limitations of hydrogen measurement in APT when intro- deuterium enrichment ( i.e. the Girdler-Sulfide process for duced via an electrolytic pathway, we investigate deuterium heavy water enrichment), have D:H separation coefficients detection by APT within austenitic Fe eMn materials. of approximately 2.3 [27] . This implies that, at equilibrium, each hydrogen atom at a binding location is accompanied by 2.3 deuterium atoms for this reaction. It is unlikely that Current experimental results on H and D trapping in steel samples would exceed this selectivity by trapping in APT orders of magnitude, and lead to different binding sites for D and H, and such a result would be highly interesting in and Deep trapping of hydrogen at energetically favourable lattice of itself as a candidate for heavy water enrichment. defect positions is a well-known mechanism for hydrogen Obtaining such a result seems unlikely, given the scope of entrapment within materials, for which several theoretical this study. models exist [18] . These models, used to explain bulk mea- Thus, this work aims to investigate electrochemical surements [19] , predict the trapping of hydrogen to specific deuterium loading as a route for maximising the observable microstructural features within the bulk material. Possible deuterium content in APT experiments. This approach opens trapping sites could include grain boundaries [20] , dislocation new possibilities for APT experiments in two ways. Firstly, the cores [21] , nanovoids [22] , or point-defects such as vacancies increased signal allows for better detectability for hydrogen, [23] . However, to date, the capacity for direct real-space im- particularly to potential trapping defects, and secondly, the aging of hydrogen has been limited, in part due to difficulties method simplifies the experimental requirements for H in characterisation of light elements by microscopic means charging and detection, making this method available to [24] . Direct imaging of hydrogen at microstructural features almost any APT system. Specifically,
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