TY - JOUR A1 - Wang, Ning A1 - Hammerschmidt, Thomas A1 - Hickel, Tilmann A1 - Rogal, Jutta A1 - Drautz, Ralf T1 - Influence of spin fluctuations on structural phase transitions of iron N2 - The effect of spin fluctuations on the α(bcc)-γ(fcc)-δ(bcc) structural phase transitions in iron is investigated with a tight-binding (TB) model. The orthogonal d-valent TB model is combined with thermodynamic integration, spin-space averaging, and Hamiltonian Monte Carlo to compute the temperature-dependent free-energy difference between bcc and fcc iron. We demonstrate that the TB model captures experimentally observed phonon spectra of bcc iron at elevated temperatures. Our calculations show that spin fluctuations are crucial for both the α−γ and γ−δ phase transitions but they enter through different mechanisms. Spin fluctuations impact the α−γ phase transition mainly via the magnetic/electronic free-energy difference between bcc and fcc iron. The γ−δ phase transition, in contrast, is influenced by spin fluctuations only indirectly via the spin-lattice coupling. Combining the two mechanisms, we obtain both the α−γ and γ−δ phase transitions with our TB model. The calculated transition temperatures are in very good agreement with experimental values. KW - Structural phase transition KW - Magnetism KW - Spin-lattice coupling PY - 2023 DO - https://doi.org/10.1103/PhysRevB.107.104108 SN - 2469-9950 VL - 107 IS - 10 SP - 1 EP - 9 PB - American Physical Society (APS) AN - OPUS4-58790 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - Roy, Arkendu T1 - Hydroxide Based High Entropy MOF for Oxygen Evolution Reaction N2 - The energy crisis becomes more prominent in higher altitude countries like Germany, with higher annual energy demand. Thus, the generation of higher energy density fuel like hydrogen from renewable resources become the only way to solve the zero-emission energy system while avoiding the expensive batteries as an energy storage option. Therefore, water electrolysis cells to produce hydrogen and oxygen by storing of solar/wind energy in chemical bonds is a fruitful alternative for renewable and long-term energy generation. Thus, designing inexpensive water-splitting electrocatalyst material becomes a field of research of utmost importance. High entropy metal hydroxide organic frameworks (HE-MHOFs) are composed of high entropy hydroxide layer inside MOFs, resulting in a high degree of structural complexity and diversity than conventional MOFs. The concept of ‘high entropy’ refers to multiple types of metal ions (Metal = TM) in a near equimolar ratio in the same framework, creating a high degree of disorder and many possible structural configurations. Here, the HE-MHOF successfully synthesized by a conventional solvothermal process, crystallizes in the single phase with significant lattice distortion. A special-quasi-random (SQS) structure was simulated with equimolar compositions (~20% TM = Mn, Co, Ni, Cu and Zn) and a comparison was made with the ICP-AES (inductively coupled plasma atomic emission spectroscopy), XANES (X-ray absorption near edge structure) and XAFS (X-ray absorption fine structure) observations. Further, the elemental mapping (Energy dispersive Spectroscopy) of HE-MHOF shows the presence of all five different metals in the same crystallite to substantiate the ‘high entropy’ state of the MOF. The HE-MHOF offers improved thermal stability than mono-metallic MHOF and exhibit unique properties compared to traditional monometallic (Ni2+) MOF variants. Firstly, to elucidate the effect of the multimetallic system on the catalytic performance, we have performed density functional theory (DFT) calculations to investigate pre-redox cycles involved in the catalytic activation of HE (and Ni)-MOFs. HE-MHOF is further investigated as an electrocatalyst for oxygen evolution reaction (OER) due to its special high entropy hydroxide layered structure and electronic properties. Our DFT results have examined the traditional proton-coupled electron transfer (PCET) steps involving the single transition-metal site. The scrutiny of d-band centers and their behaviour in catalytic upgradation is investigated with density of states (DOS) analysis. It exhibits outstanding performance towards oxygen evolution reaction (OER) comparable to the experimental findings, which is also comparable to state-of-the-art OER catalysts based on precious metals such as iridium oxide and platinum carbon. We have considered different electrolyte solutions to elucidate the fast kinetics oxygen evolution reaction in the presence of various external nucleophilic anions. In hydroxyl-based MOFs, electrolytes' influence can substantially enhance catalytic activities. Here, with the help of DFT simulations, we have investigated the effect of the nucleophilicity of anion on each elementary reaction involved in the PCET mechanism of OER. T2 - EuroMOF 2023 CY - Granada, Spain DA - 24.09.2023 KW - Metal-organic Framework KW - High Entropy Materials KW - Electrocatalysis PY - 2023 AN - OPUS4-58971 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Zhao, H. A1 - Chakraborty, Poulami A1 - Ponge, D. A1 - Hickel, Tilmann A1 - Sun, B. A1 - Wu, C.-H. A1 - Gault, B. A1 - Raabe, D. T1 - Hydrogen trapping and embrittlement in high-strength Al alloys N2 - Ever more stringent regulations on greenhouse gas emissions from transportation motivate efforts to revisit materials used for vehicles. High-strength aluminium alloys often used in aircrafts could help reduce the weight of automobiles, but are susceptible to environmental degradation. Hydrogen ‘embrittlement’ is often indicated as the main culprit; however, the exact mechanisms underpinning failure are not precisely known: atomic-scale analysis of H inside an alloy remains a challenge, and this prevents deploying alloy design strategies to enhance the durability of the materials. Here we performed near-atomic-scale analysis of H trapped in second-phase particles and at grain boundaries in a high-strength 7xxx Al alloy. We used these observations to guide atomistic ab initio calculations, which show that the co-segregation of alloying elements and H favours grain boundary decohesion, and the strong partitioning of H into the second-phase particles removes solute H from the matrix, hence preventing H embrittlement. Our insights further advance the mechanistic understanding of H-assisted embrittlement in Al alloys, emphasizing the role of H traps in minimizing cracking and guiding new alloy design. KW - Atomistic models KW - Hydrogen KW - Metals and alloys KW - Mechanical properties PY - 2022 UR - https://nbn-resolving.org/urn:nbn:de:kobv:b43-543631 DO - https://doi.org/10.1038/s41586-021-04343-z SN - 1476-4687 VL - 602 IS - 7897 SP - 437 EP - 441 PB - Nature Publ. Group CY - London AN - OPUS4-54363 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - Hickel, Tilmann T1 - Hydrogen redistribution during electrochemical charging and mechanical testing: insights from a combined ab initio and finite element method N2 - In advanced high strength steels the mechanisms of hydrogen embrittlement are expected to be strongly connected to the amount of retained austenite (RA) contained in the microstructure and its transformation upon deformation. We have developed a multi-scale model for the hydrogen redistribution during hydrogen charging and tensile loading within martensitic/austenitic microstructure. Ab initio simulations have been used to resolve the complex energy profile of hydrogen in the martensite/austenite interface. Representative microstructures with different amounts of RA have been converted form experimental SEM-EBSD measurements. Simulations with the finite element method (FEM) have been used to analyse H concentration profiles within the microstructure. The simulations confirm an accumulation of H within the austenitic phase during charging, which undergoes a phase transformation under applied mechanical load during tensile testing and releases accumulated H into the martensite matrix. As a result, there is a pronounced H segregation to the microstructure regions subjected to high hydrostatic tensile stresses and plastic strains, which has a strong impact on the embrittlement behaviour. T2 - DPG Frühjahrstagung Sektion SKM CY - Berlin, Germany DA - 17.03.2024 KW - Hydrogen KW - Steel KW - Structural transformation KW - Ab initio simulation KW - Multiscale simulation PY - 2024 AN - OPUS4-62726 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - Hickel, Tilmann T1 - Hydrogen enhances cross-slip of dislocations in the vicinity of grain boundaries N2 - Extensive experimental observations indicate the presence of nano-voids and the increase of free volume along the grain boundaries in hydrogen contaminated metals. This rate-dependent phenomenon motivates theoretical investigations of the underlying mechanisms. Here, a hydrogen enhanced cross-slip (HECS) mechanism in the close vicinity of the grain boundaries is demonstrated by direct molecular dynamics simulations. To this end, the interaction of the screw dislocations with a variety of symmetric tilt grain boundaries in H-charged and H-free bicrystalline nickel specimens is examined. The presence of segregated hydrogen atoms at the grain boundaries induces a stress field in their vicinity, and thus the barrier for cross-slip of screw dislocations considerably decreases. The enhanced crossslip of dislocations facilitates the formation of jogs. These jogs can form vacancies during the glide process. This mechanism shows nano-scale evidence of enhanced vacancy formation and subsequent increase in the free volume along the grain boundaries in the presence of H. This increase of the free-volume along the grain boundary dmaages the material and induce further embrittlement in addition to the direct effect of hydrogen in decreasing the fracture energy. T2 - International hydrogen conference 2023 CY - Park City, Utah, USA DA - 17.09.2023 KW - Atomistic model KW - Hydrogen embrittlement KW - Dislocation cross slip KW - Hydrogen enhanced vacancy formation PY - 2023 AN - OPUS4-58850 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - Hickel, Tilmann T1 - High-throughput segregation analysis at ferritic Fe grain boundaries: from first principles N2 - Understanding segregation to grain boundaries, and eventually their defect phase diagrams, is critical to enable rational grain boundary engineering for alloys design. Here, we discuss the progress and challenges one may face when generating such a database of defect-solute interactions, in the context of studying of grain boundary segregation of solutes and impurities in Fe GBs. To compute the large number of relevant defect-solute interactions we have performed high-throughput ab initio calculations using efficient and highly automated workflows using pyiron. We study has been performed across a representative set of coincident-site-lattice (CSL) type tilt GBs. Based on the large ab-initio datasets, we extract and analyse the chemical and structural trends observed in the solute segregation behaviour across the periodic table. The features which are most important in evaluating site segregation are presented and discussed. T2 - DPG Frühjahrstagung der Sektion Kondensierte Materie CY - Dresden, Germany DA - 26.03.2023 KW - Ab initio simulations KW - Defects KW - Phase diagrams PY - 2023 AN - OPUS4-58844 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Darvishi Kamachali, Reza A1 - Wallis, Theophilus A1 - Ikeda, Yuki A1 - Saikia, U. A1 - Ahmadian, A. A1 - Liebscher, C. A1 - Hickel, Tilmann A1 - Maaß, Robert T1 - Giant segregation transition as origin of liquid metal embrittlement in the Fe-Zn system N2 - A giant Zn segregation transition is revealed using CALPHAD-integrated density-based modeling of segregation into Fe grain boundaries (GBs). The results show that above a threshold of only a few atomic percent Zn in the alloy, a substantial amount of up to 60 at.% Zn can segregate to the GB. We found that the amount of segregation abruptly increases with decreasing temperature, while the Zn content in the alloy required for triggering the segregation transition decreases. Direct evidence of the Zn segregation transition is obtained using high-resolution scanning transmission electron microscopy. Base on the model, we trace the origin of the segregation transition back to the low cohesive energy of Zn and a miscibility gap in Fe-Zn GB, arising from the magnetic ordering effect, which is confirmed by ab-initio calculations. We also show that the massive Zn segregation resulting from the segregation transition greatly assists with liquid wetting and reduces the work of separation along the GB. The current predictions suggest that control over Zn segregation, by both alloy design and optimizing the galvanization and welding processes, may offer preventive strategies against liquid metal embrittlement. KW - CALPHAD KW - Microstructure Design KW - Grain boundary engineering KW - Steels KW - Density-based Model KW - Segregation Engineering PY - 2024 UR - https://nbn-resolving.org/urn:nbn:de:kobv:b43-584292 DO - https://doi.org/10.1016/j.scriptamat.2023.115758 SN - 1359-6462 SN - 1872-8456 VL - 238 SP - 1 EP - 5 PB - Elsevier CY - Amsterdam AN - OPUS4-58429 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - Hickel, Tilmann T1 - Fundamental insights into the mechanism of hydrogen embrittlement N2 - The talks starts with a study on H trapping and embrittlement in high-strength Al alloys. Then the phase stability and transformation between different Fe-H phases is discussed, together with implications on crack nucleation. In the last part MD-MC simulation of hydride formation at crack tips in Al alloys are shown. T2 - Integrated Computational Materials, Process and Product Engineering Conference, IC-MPPE CY - Leoben, Austria DA - 06.06.2024 KW - Hydrogen KW - Ab initio thermodynamics KW - Multiscale simulation KW - Hydride formation KW - Cracks PY - 2024 AN - OPUS4-62727 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - Ávila Calderón, Luis T1 - Framework for Curation and Distribution of Reference Datasets on the Example of Creep Data of Ni- Based Superalloys N2 - This presentation introduces our current effort to develop a best practice framework for generating and distributing MSE reference datasets using creep data of Ni-based superalloys as an example, building on previous approaches. Such reference datasets can be used in the verification of testing equipment or to compare experimental or simulation results among different laboratories or collaborative centers. Our framework concept, for reference data, includes the generation of the data along with an agreement on the necessary metadata, considering the intended usage, and the integration into an easily accessible digital infrastructure to annotate and discover/distribute the datasets using a metadata schema accordingly aligned to a domain ontology. T2 - International Materials Science and Engineering Congress - MSE 2024 CY - Darmstadt, Germany DA - 24.09.2024 KW - Referenzdaten KW - Digitalisierung KW - Data schema KW - NFDI-MatWerk KW - Datenschema KW - FAIR PY - 2024 AN - OPUS4-61175 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Chakraborty, P. A1 - Mouton, I. A1 - Gault, B. A1 - Tehranchi, Ali A1 - Neugebauer, J. A1 - Hickel, Tilmann T1 - Effect of Sn on stacking fault energies in zirconium and its hydrides N2 - Hydrogen embrittlement in Zr-alloy fuel cladding is a primary safety concern for water-based nuclear reactors. Here we investigated the stabilization of planar defects within the forming hydrides by Sn, the primary alloying element of Zircaloy-4 used in the cladding. In order to explain the formation of hydrides and planar defects observed in our experiments, we performed atomic-scale ab initio calculations focusing on the solute interactions with generalized stacking faults in hcp 𝛼-Zr and fcc zirconium hydrides. Our calculations showed that an increase in Sn concentration leads to a stabilization of stacking faults in both the 𝛼-Zr and hydride phases. However, the solution enthalpy of Sn is lower in the 𝛼-Zr as compared to the other hydride phases, indicative of two competing processes of Sn depletion/enrichment at the Zr hydride/matrix interface. This is corroborated by experimental findings, where Sn is less soluble in hydrides and is mostly found trapped at interfaces and planar defects, indicative of stacking faults inside the hydride phases. Our systematic investigation enables us to understand the presence and distribution of solutes in the hydride phases, which provides a deeper insight into the microstructural evolution of such alloy's properties during its service lifetime. KW - Defects KW - First-principles calculations KW - Interface and surface thermodynamics KW - Microstructure KW - Hydrides KW - Structural properties PY - 2024 UR - https://nbn-resolving.org/urn:nbn:de:kobv:b43-618044 DO - https://doi.org/10.1103/PhysRevMaterials.8.033605 SN - 2475-9953 VL - 8 IS - 3 SP - 1 EP - 9 PB - APS AN - OPUS4-61804 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER -