Hydrogen, while a promising sustainable energy carrier, presents challenges such as the embrittlement of materials due to its ability to penetrate and weaken their crystal structures. Here γ’-Fe4N nitride layers, formed on iron through a cost-effective gas nitriding, are investigated as an effective hydrogen permeation barrier. The relatively short process carried out at 570 °C consisted of pre-nitriding in an atmosphere with higher nitriding potential, followed by treatment in a nitriding potential of 0.0016 Pa−1/2 to obtain a pure γ’ layer. A combination of screening methods, including atom probe tomography, density functional theory calculations, and hydrogen permeation analysis, revealed that the nitride layer reduces hydrogen diffusion (steady-state hydrogen flux 3.21 x 10−8 mol/m2·s) by a factor of 20 compared to pure iron, at room temperature. This reduction is achieved by creating energetically unfavorable states due to stronger hydrogen-binding at the surface and high energy barriers for diffusion. The findings demonstrate the potential of γ’-Fe4N as a cost-efficient and easy-to-process solution to protect metallic materials exposed to hydrogen at low temperatures, with great advantages for large-scale applications.@en

Medium manganese (Mn) lightweight steel has gained significant attention in the last decade due to its excellent mechanical properties and low mass density. This type of high-strength steel usually shows a complex microstructure composed of banded δ-ferrite and α-ferrite-austenite aggregates along the rolling direction. The mechanical response of such banded microstructure under different loading directions is crucial for understanding the forming properties of such steels. In this study, we focus on the anisotropic deformation behavior of a medium-Mn lightweight steel, employing various in-situ characterization techniques including synchrotron high-energy X-ray diffraction and high-resolution microscopic digital image correlation to study the evolution of stress/strain in different phases upon loading. We observe that the sample loaded along the rolling direction (parallel to the banding direction) exhibits a notably higher strain hardening capability compared to specimens loaded along the transverse direction. Such difference is due to the different strain distribution patterns that is dependent on the intrinsic mechanical properties of individual phases as well as on the orientation of the layered microstructure relative to the loading direction. This factor results in different kinetics of strain-induced martensitic transformation (i.e., varying transformation-induced plasticity effect) in different tensile directions, which explains the observed different tensile responses. Our study provides important insights into the future design of similar alloys, particularly for improved forming properties.

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Understanding hydrogen-metal interactions is critical for developing refractory complex concentrated alloys (CCAs), applicable to the hydrogen economy. In this study, we revealed a hydrogen-assisted spinodal decomposition phenomenon at the nanoscale in an equiatomic TiNbZrHfTa CCA upon its exposure to H₂ at 500 °C. Such a decomposition pathway was characterized by a periodic compositional modulation with an up-hill diffusion behavior of the principal metallic elements, particularly Zr, over an extended treatment period (from 0.5 h to 2 h) in an H₂ atmosphere, probed by three-dimensional atom probe tomography. Consequently, the decomposed alloy consisted of a needle-shaped phase enriched in Zr and Ti and a phase enriched in Nb and Ta. Crystallographically, the spinodal features aligned preferentially along 〈001〉 directions of the matrix phase to minimize elastic strain energy. To better understand the role of hydrogen in spinodal decomposition, a statistical thermodynamic model was further developed by incorporating hydrogen to predict the phase stability of the TiNbZrHfTa-H system. This analysis suggested that hydrogen destabilizes the single solid-solution phase by expanding the spinodal region. Such nanoscale spinodal decomposition enhanced the hardness and anti-abrasive properties of the investigated alloy. Thus, this study not only provides fundamental insights into the effect of hydrogen on phase stability, but also demonstrates a novel alloy design strategy by introducing hydrogen as an interstitial alloying element to tailor the microstructure.

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Refractory medium/high-entropy alloys (M/HEAs) are emerging as promising alternative materials for hydrogen storage and hydrogen combustion engines due to their favorable thermodynamic and kinetic conditions for hydrogen accommodation (for the former) and promising high-temperature mechanical properties (for the latter). A better understanding of hydrogen-metal interactions is necessary to advance the development of this material class, thus helping leverage hydrogen-based applications. Here we reveal the microstructural evolution of a TiNbZr MEA by in-situ synchrotron high-energy X-ray diffraction (HEXRD) during hydrogenation in pure H₂ gas at atmospheric pressure. At 500 °C, dissolved hydrogen atoms gradually expand the crystal lattice isotropically, and the body-centered cubic crystal remains stable up to a hydrogen concentration of ∼46.4 at.%. The thermodynamics of hydrogen accommodation associated with experimental observations in the crystal lattice is elucidated using density functional theory (DFT). The calculations suggest that tetrahedral interstitial sites are the thermodynamically favorable positions for hydrogen accommodation in both cases (i) for a single hydrogen in the special quasirandom structure supercell and (ii) at a high hydrogen concentration (∼45.4 at.%). In the latter case, hydrogen interstitials are randomly distributed on the tetrahedral sites. Upon cooling, it is observed that the body-centered cubic lattice transforms to a body-centered tetragonal structure. The DFT calculations show that this change is related to the ordering distribution of hydrogen interstitials within the TiNbZr lattice. By combining in-situ HEXRD experiments and DFT calculations, the study provides fundamental insights into hydrogen accommodation in the interstitial positions and its impact on the lattice symmetry in TiNbZr MEA.

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The reduction of FeO (wüstite) to Fe represents the final and slowest step in the hydrogen-based direct reduction of iron ores for sustainable ironmaking. However, the atomistic-scale mechanisms and kinetics of this process remain poorly understood. Here, we employ ab initio meta-dynamics simulations to investigate reaction pathways and energy barriers for this redox process on FeO(1 0 0) and FeO(1 1 1)_O-terminated surfaces. Differences in surface configurations lead to variations in the number of H₂ molecules required, reaction pathways, and energy barriers. The FeO surface exhibits an autocatalytic effect, facilitating H₂ dissociation and reducing the energy barrier for breaking H₂ molecular bonds. Nevertheless, hydrogen dissociation and adsorption, forming O–H bonds, constitute the primary rate-limiting step. Following this, the Fe-O bond spontaneously breaks in the presence of individual H atoms. Increasing H₂ partial pressure enhances reaction efficiency by raising the density of reactive H₂ molecules, consistent with macroscopic observations. These insights advance the atomistic-scale understanding of hydrogen-based direct reduction, highlighting the influence of pressure and rate-limiting factors.

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Higher strength and higher ductility are desirable for structural materials. However, ultrastrong alloys inevitably show decreased strain-hardening capacity, limiting their uniform elongation. We present a supranano (<10 nanometers) and short-range ordering design for grain interiors and grain boundary regions, respectively, in fine-grained alloys based on vanadium, cobalt, and nickel, with additions of tungsten, copper, aluminum, and boron. The pronounced grain boundary-related strengthening and ductilization mechanism is realized through segregation of the short-range ordering near the grain boundary. Furthermore, the supranano ordering with a larger size has an enhanced pinning effect for dislocations and stacking faults, multiplied and accumulated in grain interiors during plastic deformation. These mechanisms promote continuously increased flow stress until fracture of the alloy at 10% strain with 2.6-gigapascal tensile stress.

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Steel production accounts for approximately 8% of all global CO2 emissions, with the primary steelmaking route using iron ores accounting for about 80% of those emissions, mainly due to the use of fossil-based reductants and fuel. Hydrogen-based reduction of iron oxide is an alternative for primary synthesis. However, to counteract global warming, decarbonization of the steel sector must proceed much faster than the ongoing transition kinetics in primary steelmaking. Insufficient supply of green hydrogen is a particular bottleneck. Realizing a higher fraction of secondary steelmaking thus is gaining momentum as a sustainable alternative to primary production. Steel production from scrap is well established for long products (rails, bars, wire), but there are two main challenges. First, there is not sufficient scrap available to satisfy market needs. Today, only one-third of global steel demand can be met by secondary metallurgy using scrap since many steel products have a lifetime of several decades. However, scrap availability will increase to about two-thirds of total demand by 2050 such that this sector will grow massively in the next decades. Second, scrap is often too contaminated to produce high-performance sheet steels. This is a serious obstacle because advanced products demand explicit low-tolerance specifications for safety-critical and high-strength steels, such as for electric vehicles, energy conversion and grids, high-speed trains, sustainable buildings, and infrastructure. Therefore, we review the metallurgical and microstructural challenges and opportunities for producing high-performance sheet steels via secondary synthesis. Focus is placed on the thermodynamic, kinetic, chemical, and microstructural fundamentals as well as the effects of scrap-related impurities on steel properties.@en

Ammonia is a promising alternative hydrogen carrier that can be utilized for the solid-state reduction of iron oxides for sustainable ironmaking due to its easy transportation and high energy density. The main challenge for its utilization on an industrial scale is to understand the reaction kinetics under different process conditions and the associated nitrogen incorporation in the reduced material that originates from ammonia decomposition. In this work, the effect of temperature on the reduction efficiency and nitride formation is investigated through phase, local chemistry, and gas evolution analysis. The effects of inherent reactions and diffusion on phase formation and chemistry evolution are discussed in relation to the reduction temperature. The work also discusses nitrogen incorporation into the material through both spontaneous and in-process nitriding, which fundamentally affects the structure and chemistry of the reduced material. Finally, the effect of nitrogen incorporation on the reoxidation tendency of the ammonia-based reduced material is investigated and compared with that of the hydrogen-based reduced counterpart. The results provide a fundamental understanding of the reduction and nitriding for iron oxides exposed to ammonia at temperatures from 500 to 800 °C, serving as a basis for exploitation and upscaling of ammonia-based direct reduction for future green steel production.

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Excellent properties (durability, wear and corrosion resistance) and long service life under extreme conditions are essential for the successful application of metallic materials in the energy sector. In particular, for future fusion applications, high Cr ferrous alloys (in our case Eurofer) are of great interest. Importantly, modified microstructure with higher dimensional stability improves corrosion and wear resistance properties. In this study, we successfully manipulate the desired type of microstructure, which could provide a solution to current challenges in such a high temperature, highly corrosive and highly irradiated environment, using a novel technique of cryogenic processing (CP). The research identifies the CP-driven changes not only to the microstructure, but also to the local chemistry and bonding state of the key alloying elements. The correlations and individual phenomena associated with CP have been evaluated using state-of-the-art techniques such as atom probe tomography and synchrotron-based in-situ scanning photoemission spectroscopy. This novel process and its novel microstructural manipulation opens up new possibilities for materials processing for future energy applications.

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Hydrogen-based direct reduction (HyDR) of iron ores has attracted immense attention and is considered a forerunner technology for sustainable ironmaking. It has a high potential to mitigate CO ₂ emissions in the steel industry, which accounts today for ~ 8–10% of all global CO ₂ emissions. Direct reduction produces highly porous sponge iron via natural-gas-based or gasified-coal-based reducing agents that contain hydrogen and organic molecules. Commercial technologies usually operate at elevated pressure, e.g., the MIDREX process at 2 bar and the HyL/Energiron process at 6–8 bar. However, the impact of H ₂ pressure on reduction kinetics and microstructure evolution of hematite pellets during hydrogen-based direct reduction has not been well understood. Here, we present a study about the influence of H ₂ pressure on the reduction kinetics of hematite pellets with pure H ₂ at 700 °C at various pressures, i.e., 1, 10, and 100 bar under static gas exposure, and 1.3 and 50 bar under dynamic gas exposure. The microstructure of the reduced pellets was characterized by combining X-ray diffraction and scanning electron microscopy equipped with electron backscatter diffraction. The results provide new insights into the critical role of H ₂ pressure in the hydrogen-based direct reduction process and establish a direction for future furnace design and process optimization. Graphical Abstract: (Figure presented.)

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Iron powder can be a sustainable alternative to fossil fuels in power supply due to its high energy density and abundance. Iron powder releases energy through exothermic oxidation (combustion), and stores back energy through its subsequent hydrogen-based reduction, establishing a circular loop for renewable energy supply. Hydrogen-based direct reduction is also gaining global momentum as possible future backbone technology for sustainable iron and steel production, with the aim to replace blast furnaces. Here, we investigate the microstructural formation mechanisms and reduction kinetics behind hydrogen-based direct reduction of combusted iron powder at moderate temperatures (400–500 °C) using thermogravimetry, ex-situ X-ray diffraction, scanning electron microscopy coupled with energy dispersive spectroscopy and electron backscatter diffraction, as well as in-situ high-energy X-ray diffraction. The influence of pre-oxidation treatment was studied by reducing both as-combusted iron powder (50 % magnetite and 50 % hematite) and the same powder after pre-oxidation (100 % hematite). A gas diffusion-limited reaction was obtained during the in-situ high-energy X-ray diffraction experiment, with successive hematite and magnetite reduction, and a strong increase in reduction kinetics with initial hematite content. Faster reduction kinetics were obtained during the thermogravimetry experiment, with simultaneous hematite and magnetite reduction. In this case, the reduction reaction was limited by a mix of phase boundary and nucleation and growth models, as analyzed by multi-step model fitting methods as well as by microstructural investigation. When not limited by gas diffusion, the pre-oxidation treatment showed almost no influence on the reduction time but a strong effect on the final microstructure of the reduced powder.

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Grain boundaries in noble metal catalysts have been identified as critical sites for enhancing catalytic activity in electrochemical reactions such as the oxygen reduction reaction. However, conventional methods to modify grain boundary density often alter particle size, shape, and morphology, obscuring the specific role of grain boundaries in catalytic performance. This study addresses these challenges by employing gold nanoparticle assemblies to control grain boundary density through the manipulation of nanoparticle collision frequency during synthesis. We demonstrate a direct correlation between increased grain boundary density and enhanced two-electron oxygen reduction reaction activity, achieving a significant improvement in both specific and mass activity. Additionally, the gold nanoparticle assemblies with high grain boundary density exhibit remarkable electrochemical stability, attributed to boron segregation at the grain boundaries, which prevents structural degradation. This work provides a promising strategy for optimizing the activity, selectivity, and stability of noble metal catalysts through precise grain boundary engineering.

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For millennia, alloying has been the greatest gift from metallurgy to humankind: a process of mixing elements, propelling our society from the Bronze Age to the Space Age. Dealloying, by contrast, acts like a penalty: a corrosive counteracting process of selectively removing elements from alloys or compounds, degrading their structural integrity over time. We show that when these two opposite metallurgical processes meet in a reactive vapor environment, profound sustainable alloy design opportunities become accessible, enabling bulk nanostructured porous alloys directly from oxides, with zero carbon footprint. We introduce thermodynamically well-grounded treasure maps that turn the intuitive opposition between alloying and dealloying into harmony, facilitating a quantitative approach to navigate synthesis in such an immense design space. We demonstrate this alloy design paradigm by synthesizing nanostructured Fe-Ni-N porous martensitic alloys fully from oxides in a single solid-state process step and substantiating the critical kinetic processes responsible for the desired microstructure.@en

The ultrafine cellular structure promotes the extraordinary mechanical performance of metals manufactured by laser powder‐bed‐fusion (L‐PBF). An in‐depth understanding of the mechanisms governing the thermal stability of such structures is crucial for designing reliable L‐PBF components for high‐temperature applications. Here, characterizations and 3D discrete dislocation dynamics simulations are performed to comprehensively understand the evolution of cellular structures in 316L stainless steel during annealing. The dominance of screw‐type dislocation dipoles in the dislocation cells is reported. However, the majority of dislocations in sub‐grain boundaries (SGBs) are geometrically necessary dislocations (GNDs) with varying types. The disparity in dislocation types can be attributed to the variation in local stacking fault energy (SFE) arising from chemical heterogeneity. The presence of screw‐type dislocations facilitates the unpinning of dislocations from dislocation cells/SGBs, resulting in a high dislocation mobility. In contrast, the migration of SGBs with dominating edge‐type GNDs requires collaborative motion of dislocations, leading to a sluggish migration rate and an enhanced thermal stability. This work emphasizes the significant role of dislocation type in the thermal stability of cellular structures. Furthermore, it sheds light on how to locally tune dislocation structures with desired dislocation types by adjusting local chemistry‐dependent SFE and heat treatment.@en

NiTi-based composites possess great potential for concurrently improving both mechanical and functional properties. However, relying on traditional alloy design principles limits the design space and greatly hinders the advancement of high-performance NiTi-based composites. The concept of high-entropy alloys has expanded the compositional landscape, unveiling unique structural characteristics for alloy design and providing new prospects for addressing these limitations. Here, we report a compositionally complex NiTi-based composite that exhibits exceptional strength and ductility, along with remarkable recoverable strain. The composite, Ni ₄₀Ti ₄₀(NbMoTaW) ₂₀ (at.%), comprises a 78.0 % B2 NiTi matrix, a 19.2 % Nb-Mo-Ta-W-Ti-Ni compositionally complex body-centered cubic (BCC) phase, and a small amount of Ti ₂Ni. Notably, this composite demonstrates an engineering compressive strength of 3274 MPa, with a compressive fracture strain of 44.2 % and a maximum recoverable strain of 7.3 % (5.6 % elastic strain and 1.7 % inelastic recoverable strain). These outstanding mechanical properties result from the unique structural characteristics of the compositionally complex phase and the lattice strain matching induced by phase transitions. The substantial recoverable strain was obtained through the reversible B2⇌R⇌B19′ phase transition. This work not only innovates a new category of high-performance NiTi-based composites but also extends the applicability of the high-entropy concept.

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Metallurgical production traditionally involves three steps: extracting metals from ores, mixing them into alloys by liquid processing and thermomechanical processing to achieve the desired microstructures ^1,2. This sequential approach, practised since the Bronze Age, reaches its limit today because of the urgent demand for a sustainable economy ^2–5: almost 10% of all greenhouse gas emissions are because of the use of fossil reductants and high-temperature metallurgical processing. Here we present a H ₂-based redox synthesis and compaction approach that reforms traditional alloy-making by merging metal extraction, alloying and thermomechanical processing into one single solid-state operation. We propose a thermodynamically informed guideline and a general kinetic conception to dissolve the classical boundaries between extractive and physical metallurgy, unlocking tremendous sustainable bulk alloy design opportunities. We exemplify this approach for the case of Fe–Ni invar bulk alloys ^6,7, one of the most appealing ferrous materials but the dirtiest to produce: invar shows uniquely low thermal expansion ^6,8,9, enabling key applications spanning from precision instruments to cryogenic components ^10–13. Yet, it is notoriously eco-unfriendly, with Ni causing more than 10 times higher CO ₂ emission than Fe per kilogram production ^2,14, qualifying this alloy class as a perfect demonstrator case. Our sustainable method turns oxides directly into green alloys in bulk forms, with application-worthy properties, all obtained at temperatures far below the bulk melting point, while maintaining a zero CO ₂ footprint.

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When solid-state redox-driven phase transformations are associated with mass loss, vacancies are produced that develop into pores. These pores can influence the kinetics of certain redox and phase transformation steps. We investigated the structural and chemical mechanisms in and at pores in a combined experimental-theoretical study, using the reduction of iron oxide by hydrogen as a model system. The redox product (water) accumulates inside the pores and shifts the local equilibrium at the already reduced material back toward reoxidation into cubic Fe1-xO (where x refers to Fe deficiency, space group Fm3¯m). This effect helps us to understand the sluggish reduction of cubic Fe1-xO by hydrogen, a key process for future sustainable steelmaking.

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The reduction of magnetite-based iron ore fines in a hydrogen-induced fluidized bed becomes an attractive fossil-free ironmaking route. Our previous study showed that a prior oxidation treatment of magnetite was helpful to improve its fluidization and reduction behavior. However, the underlying oxidation mechanisms of magnetite ore fines remained unclear and required further investigations. In this study, two magnetite ore brands were analyzed viain situ high-temperature X-ray diffraction (HT-XRD) during oxidation, to investigate the thermal transformation of Fe ₃O ₄ to α-Fe ₂O ₃ at crystal scale. The lattice constants and crystallite sizes of both phases and oxidation degree were evaluated at different temperatures based on the HT-XRD patterns. The lattice constants of Fe ₃O ₄ and α-Fe ₂O ₃ increased with an increase in temperature due to the thermal expansion and can be successfully fitted with temperature by second-order polynomials. With Fe ₃O ₄ being oxidized into Fe ₂O ₃, the Fe ₂O ₃ crystallite grew and showed a certain growth habit. The Fe ₂O ₃ crystallite grew faster along the a/b axis than the c axis. The oxidation kinetics followed the parabolic law as shown by the sigmoid-shaped oxidation degree curve, suggesting that the solid diffusion of ions was the rate-limiting step.

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Out of the multitude of researched processing routes for sustainable ironmaking, hydrogen-based direct reduction and hydrogen plasma smelting reduction (HyPSR) are currently the most promising candidates for a successful industrial application. Both processes operate under gaseous atmospheres, which turn the partial and absolute pressure of hydrogen into a relevant process parameter. Here, we present first insights into the influence of total pressure and concentration of hydrogen on the reduction of hematite, focusing on the more pressure-sensitive route (HyPSR). The effect of pressure on the dissociation of H2 molecules into metastable H atoms or H+ ions,- and the overall hydrogen utilization is discussed using a thermodynamic approach. Validation experiments were conducted to testify the practical feasibility of adjusting these parameters. A decrease in the total pressure of the system from 900 mbar to 450 mbar resulted in an improved hydrogen utilization, thus suggesting that a larger population of H atoms can exist in the plasma arcs ignited under 450 mbar. An increase in the hydrogen concentration to 20 vol.% lead to undesired evaporation, likely because of a parallel increase in plasma temperature. Possibilities and challenges for exploiting these pressure-related phenomena for the industrial production of green steel are outlined and discussed.@en

The effect of hydrogen on the surface morphology and nanomechanical properties of Ni-based Alloy 725 under solution-annealed (SA) and precipitation-hardened (API) conditions was thoroughly studied. The investigation involved in situ nanoindentation testing, microscopy characterization, statistical analysis, and numerical simulation approaches. The results showed the distinctive effects of hydrogen on the pop-in and hardness in the SA and API samples. For the SA sample, hydrogen mainly dissolved as solid solute in the matrix, causing enhanced lattice friction on the dislocation motion and increasing the internal stress via lattice expansion. Thus, an enhanced hardness, a reduced pop-in width/load ratio, and numerous surface steps were detected in the presence of hydrogen. For the API sample, the strengthening γ′′ phases were the stress concentrators, and the dislocations nucleated heterogeneously, demonstrating indistinctive pop-in phenomena. Furthermore, the precipitates in the API sample affected the trapping behavior of hydrogen, thereby resulting in the hardness change, which reflected the competition between solution hardening in the matrix and vacancy softening mechanism in precipitates.

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