COFUND PhD position - Corrosion / HYCOAST

Title of the thesis project: Influence of grain boundaries in high-pressure HYdrogen COASTal storage materials for green hydrogen

Scientific description of the research project

Hydrogen is seen as a clean, mobile energy carrier and source of fuel, as it has the great advantage of being present on the planet in an abundant amount. A genuine ‘hydrogen economy’ is henceforth envisioned. Much of the technology is established from the chemical industry using high-value materials, but cost pressure from investment and maintenance is becoming significant now that these technologies are rolled out at the gigawatt scale. One of the main challenges consists of the development of suitable, cost-effective, and durable materials for the transportation of hydrogen. Indeed, a major concern with hydrogen is that it is very detrimental to the durability of materials. This problem affects the structural materials in various industries, from subsea pipelines to aircraft and nuclear reactors. This severe degradation can manifest itself in several ways, such as a decrease in tensile elongation to cause fracture, or a decrease in the static load that can be supported by the metallic structure, for example. The effects of hydrogen on the ductility, toughness, and tensile strength are known to be significant, as the performance and lifetime of materials are drastically reduced in the presence of hydrogen. This outcome, known as Hydrogen Embrittlement (HE), was first described in 1875 by Johnson. This phenomenon results from a combination of different parameters related to the material’s characteristics, the source of hydrogen (internal or external), and the mechanical solicitations.

Nowadays, coastal areas are quickly becoming one of the main sources of green hydrogen due to their favorable potential for wind, tidal/wave, and solar power to drive water electrolysis. While complementary, these green power sources do show some intermittence, making buffering of the produced gaseous hydrogen in e.g. high-pressure tanks, necessary. These tanks and auxiliary equipment already exist; however, they are currently made from non-optimized materials, due to a lack of understanding in detail of the underlying failure mechanisms. Especially, the most important role of crystal interfaces, such as grain and grain boundaries, is not fundamentally understood.

It has been shown that the diffusion of hydrogen is low through the defects of the microstructures. But, at low temperatures, an accelerated diffusion of hydrogen occurring inside the microstructures points out the importance of investigating the effects of the intergranular phase.

However, despite the large number of investigations on this subject, the contribution of grain boundaries to the effective diffusivity and strength in polycrystals remains quantitatively unclear. The acceleration of hydrogen diffusion along grain boundaries has been reported in many studies. But, it has also been suggested that grain boundaries can reduce the overall hydrogen diffusivity under certain conditions. Its influence on the strength of specific grain boundaries has been much less investigated experimentally, instead being averaged over many grain boundaries in a polycrystal. Fundamentally, the diffusivity and strength are directly connected via the Borisov relationship. Since the effective diffusivity is measured in a heterogeneous aggregate with a mixture of microstructure features, it is difficult to separate the contribution of all the microstructural parameters in the diffusion process.

State of the art: It is difficult to find diffusivity data associated with each category of grain boundaries (GBs). Only some atomistic simulations have determined diffusion constants for specific GBs until Li’s experimental and numerical research, on four different bi-crystals labelled as special GBs. Triple junctions (TJs) have also been very little studied because of the complexity of experimental procedures; however, TJs occupy a significant portion of the intergranular region in nanocrystalline materials. These entities can form a high diffusivity percolating network throughout the volume of the material. Legrand investigated the effects of TJs on the effective diffusivity using 3-D simulated networks in his work. According to his simulation results, a significant increase in the effective diffusivity was observed compared to the case where the diffusivity of TJs is not examined. However, in his study, the segregation behavior of TJs was not included, and the TJs comprised diffusivity was chosen arbitrarily. In reality, the study of the effects of TJs is complicated due to a lack of experimental data on their diffusion property and segregation behavior. Moreover, additional difficulties come from the existence of a large variety of GBs and the complexity of their structure. Thus, the investigations and results of this proposal will help in understanding the fundamentals of hydrogen embrittlement and interaction with GBs. However, it covers an issue that has been poorly studied, and contradictory results have been published. While years of research have provided a lot of information on the hydrogen interaction with GBs, results are expected to increase the understanding of the hydrogen transport and trapping at different specific GBs and their impact on strength. This research will use state-of-the-art knowledge and state-of-the-art technology to develop an innovative approach to the understanding of the HE phenomenon through upstream investigation. It will help in designing the hydrogen embrittlement-resistant microstructures of the components exposed to hydrogen environments. The enhancement in their mechanical and corrosion resistance properties will reduce the financial impact on countries globally which are caused by different components failure and their maintenance. To synthesize a specific GB or TJ, the orientation of the crystals and the misorientation angle between those different crystals need to be controlled precisely, and experimental data available is rare.

In this project, the applicants will leverage their complementary expertise in model material synthesis, characterization, and simulation (LaSIE), as well as high-pressure materials testing and atomic-scale analysis (FAU), to gain fundamental insights into the role of grain boundaries (GBs) as critical failure points in hydrogen-facing materials under pressure. Nickel (Ni) is selected as the model material due to its well-characterized behavior and suitability for fundamental studies. The applicants will synthesize Ni single crystals (SX) and bicrystals (BX) at the macroscale, enabling the isolation and analysis of individual grain boundaries and the comparison to boundary-free systems. The central objective of the proposal is to assess and predict grain boundary networks and microstructures that exhibit reduced sensitivity to hydrogen embrittlement through grain boundary engineering. To achieve this, various configurations of Ni SX and BX systems will be investigated, encompassing a wide range of grain boundary energies, vacancy concentrations, and excess volumes. The influence of hydrogen on these grain boundaries will be characterized through key parameters including hydrogen diffusivity, trapping/segregation energies, and cohesive energies—spanning multiple length and time scales. Ultimately, this data will inform the design of a hydrogen embrittlement-resistant microstructure, featuring optimized grain boundary networks.

To reach the previously stated goals, the project has established 4 technical and non-technical specific objectives (SO), interconnected with the project's results (R), key performance indicators (KPI) and work packages (WP):

SO1: Fabricate the samples with zero or one grain boundaries. 

To fill an important gap concerning the effect of GB, it is necessary to have access to experimental knowledge on samples of different, well-defined single GBs. Today, it is impossible to acquire such samples in the market. The LaSIE Laboratory recently purchased a CZ furnace, which enables the manufacture of such samples during this project. Results: Manufacturing and characterisation of a representative variety of GBs encountered in nickel (R1.1);. Inventory of GB in nickel with a coverage of at least 70% of all the identified, most likely encountered (R1.2); Definition of optimized building procedure for each GB. KPI: 6 different special GB, 4 different Random GB.

Characterization using X-ray diffraction, Electron Back Scatter Diffraction (EBSD) (LASIE), and High-Resolution Transmission Electron Microscopy (HR-TEM) (FAU).  

Related WPs: WPs1.

SO2: Develop a database of material characterization testing on representative grain boundaries in nickel. 

There is a strong need to document the effect of GB on the diffusion of hydrogen in metals, using a harmonized building protocol which will be established in the framework of this project. These will be exposed to electrochemical (LaSIE) and high-pressure (FAU) hydrogen in the form of protium and deuterium. These sensitized states will be tested in tensile loading at different scales, centimeter-sized specimens (LaSIE) and using meso-sized specimens (1mm diameter) (FAU) to achieve a controlled stress state and avoid finite size effects. They will also be analyzed for their hydrogen concentration through ultra-high vacuum thermal desorption spectrometry (TDS) in a special ultra-high sensitivity Al/Ti TDS system, allowing us to analyze the difference in uptake between grain boundary-containing BX and grain boundary-free SX specimens (FAU). This will be combined with cryogenic atom probe tomography in a special Ti atom probe with no spurious hydrogen contamination, to analyze the equilibrium grain boundary concentration of hydrogen in BX (PF). HEXRD at the Hereon-operated beamlines at the DESY synchrotron facility in Hamburg will be used to analyse crystal and grain boundary anisotropy from hydrogen take-up and resulting stresses. Results: (R1.2); Recommendation on methodological validation of experimental testing procedures for the assessment of H2-material compatibility and testing procedure for hydrogen diffusion (R2.2); Dataset of characterized relevant macroscopic properties of GB in relation to H (R2.2). 

KPI: 10 different GB, including special and random GB, 54 tensile tests; 80 Electro-permeation tests (LASIE), 80 H2 high pressure charging ( FAU), and thermal desorption spectroscopy (TDS) (LASIE and FAU), APT (FAU), and HEXRD (Hereon) on each selected GB orientation.

Related WPs: WPs1-3.

SO3: Develop an atomistic numerical modelling approach for simulating and predicting hydrogen diffusion and trapping at the GB.

In this Sub-objective, the atomistic level investigation will be done to understand the fundamentals of hydrogen embrittlement of GBs and TJs. The bi- and tri-crystals fabricated and characterized in WP1 and WP2 will be modelled in the Molecular Dynamics code using LAMMPS software for understanding at the atomistic level. Challenge:  Modelling of GBs and TJs will be done by giving the same orientation as fabricated crystals characterized through EBSD. The concentration of hydrogen atoms will be inserted according to the experimental observations and pressure field, and the misfit volume near the GBs and TJs. Anisotropic stresses will be used from HEXRD analysis at Hereon. The model will simulate the situation at room temperature and pressure, and based on the measurement of the mean square displacement of hydrogen atoms concerning time, the diffusion coefficient will be derived for GBs and TJs.  Innovation: investigations of the segregation energy and migration energy of hydrogen atoms will be correlated with grain boundary energy, vacancy concentrations, dilatation, and hydrostatic elastic energy of GBs and TJs. This will help us to comprehend the experimental observation thoroughly and to draw some relevant conclusions.

Results: simulations of various amounts of GB encountered in nickel. (R3.1): building the different types of grain boundaries using LAMMPS (R3.2): calculating the diffusion coefficient of hydrogen along and across the GB. (R3.2): Linking the experimental and numerical work. KPI: 10 different GB assessed.  (LaSIE)

Related WPs: WP3

SO4: To facilitate the uptake and exploitation of the project results by the academic community, technology developers, and end-users. 

Communication is a key lever to ensure that the project effectively reaches specific audiences, promoting the project and its results, generating awareness and interest, and encouraging engagement with the project activities and results. The main strategic objective of the communication plan is to ensure that the project results are widely communicated to the target communities via appropriate means and translated into meaningful and tangible action. To streamline the communication activities and efforts, the Communication Strategy will be defined at the start of the project (M3) and will define: i) target audiences; ii) communication objectives towards each group; iii) communication tools and channels; iv) communication-related KPIs. The main message will focus on the impact and benefits of the project’s results. The results are also designed to reach all 8 target groups identified (academia, industry, material manufacturers, end users (DSOs, H2 providers), technology communities, media outlets, citizens, students (from schools to universities).  A tentative list of communication means (and KPIs) that have been selected due to their proven effectiveness in previous projects is presented below. 

• National and international conferences on hydrogen energy, hydrogen storage and hydrogen embrittlement in the coastal environment, etc.
• Scientific papers in specialized peer-reviewed journals.
• Technical journals: Free access to peer-reviewed articles will be ensured via the subject-based repositories of the candidate (Orcid ID and ResearchGate), supervisor, and lab team members, and via the website of La Rochelle.
• The project and significant findings, in an amenable form to the general public, will be presented on the website of La Rochelle University and FAU. Also, novel details on methodology and results will be included in the specialization courses in both universities.
• A minimum of 4 papers in peer-reviewed journals is to be expected. The group will present results at at least four international conferences across a wide spectrum. 
• The project website: presents the proposal, its scheme, and strategy, and will advertise the breakthrough results as they develop through diverse social media. It will also be exposed to the general public during annual science fairs such as “Fête de la Science” and through public lectures such as “Café sciences”. 
• Social Media: Promotion of progress of the project, a LinkedIn dedicated to the project, and an account on Twitter.
• COMMUNICATION TOOLKIT: A brochure and poster, in addition to the project identity kit for the consortium: logo, colour scheme, and presentation template. This toolkit includes the innovation factsheet, Available at M3.
• PRESS KIT & RELEASES: E-newsletters, leaflet presenting the overview of the project and interim/final results to stakeholders.

KPI: Workshops will explain new results, guidelines, and protocols, as well as other outcomes derived from the project. One seminar and one webinar will be organized, directed at the scientific, academic, and standardization communities related to the project’s innovations and research.

Related WP: WP4

The project will be divided into 4 work packages. The first WP will focus on the most important part of the project, which involves the crystal growth of the samples using the CZ furnace and their characterization to verify the crystal orientations using X-ray and EBSD in La Rochelle and APT in FAU, corresponding to SO1. 

WP2 corresponds to the full characterization of the samples concerning hydrogen charging and mechanical testing. In La Rochelle, the electrochemical charging and permeation, as well as the tensile testing, will be performed. While FAU will concentrate on gaseous charging of the samples, as well as micro-tensile testing. The HEXRD measurements are also part of this work package.

WP3 corresponds to the modelling at different scales: the atomic scale using molecular dynamics (MD) and the meso scale using FEM. The modelling work will enable us to question the experimental data. Indeed, all experimental data provide global information; the modelling of each microstructure will provide information on the effects of the different defects, such as vacancies, dislocation densities, and, for BX, the effects of each type of grain boundary. 

The combination of these 3 technical work packages will provide the scientific community with a wide library of grain boundary effects that are unique and important to the scientific community interested in hydrogen embrittlement. 

Thus, the communication in WP4 will have critical importance in providing all the results not only to the scientific community but also to the industry and the general public. This WP corresponds to SO4.

The Gantt chart of WPs, the tasks, the deliverables, as well as the milestones, is provided in Table 1.

List of deliverables:

D1.1: Manufacture of SX; D1.2: Manufacture of BX; D1.3: Manufacture of 5 BX

D2.1: Characterization of SX; D2.2 Characterization of BX

D3.1: MD modelling of SX and BX with H; D3.2: FEM modelling of polycrystalline microstructure.

D4.1: Poster in conference and scientific outreach in La Rochelle and in FAU; D4.2: 1st journal paper, oral conference and scientific outreach in FAU; D4.3: 2 journal papers, 1 conference outreach activities in La Rochelle.

List of Milestones:

M1: Manufacture of SX, Modelling of SX

M2: End of characterization of SX, manufacture of BX

M3: Manufacture of all the BX

Description of the Doctoral Candidate’s tasks:

Technical program

In La Rochelle : 

Crystal growth of a bi-crystal and a tri-crystal

Characterization (XRD, EBSD, TEM) of each grain and grain boundary (a minimum of 4 GBs) and a triple junction

Hydrogen charging (electrochemical), TDS 

Mechanical testing with and without H

Modelling application in FEM

In FAU Erlangen :

Gaseous hydrogen charging (cylindrical samples), TDS

Atom Probe

HRTEM at least one bi-crystal and one tri-crystal

Mechanical testing on cylindrical samples with and without H

Secondment in HEREON:

Synchrotron nano-tomography and diffraction imaging to capture 3D defect distributions.

Communication program

Participation in one domestic conference/year and one international conference

Writing of at least 2 journal papers as first author

3 outreach activities (ma these en 180s, et fête de la science, Long night of science at FAU)

Training Program

In agreement with both doctoral schools, La Rochelle and FAU Erlangen

Since its creation in 1993, La Rochelle Université has been on a path of differentiation.

Thirty years later, as the university landscape recomposes itself, it continues to assert an original proposition, based on a strong identity and bold projects, in a human-scale establishment located in an exceptional setting.

Anchored in a region with highly distinctive coastal features, La Rochelle Université has turned this singularity into a veritable signature, in the service of a new model. Its research it addresses
the societal challenges related to Smart Urban Coastal Sustainability (SmUCS).

The new recruit will join the Laboratoire des Sciences de l’Ingénieur pour l’Environnement (LaSIE).

Cotutelle: FAU Erlangen-Nuremberg, Germany. Department of Materials Science.

The PhD student applicant should have a master’s degree in Mechanical/Materials Science Engineering with an appetite for both experimental and numerical research. A high proficiency in English is expected, along with a strong capacity for teamwork, adaptability, and autonomy. The student should be aware that the project will take place at 2 universities, FAU and La Rochelle Université, and that a three-month stay at Helmholtz-Zentrum Hereon near Hamburg is planned.