Co-reduction of multicomponent oxides with hydrogen offers a carbon-neutral approach toward sustainable alloy design. Herein, we use in situ high-energy X-ray diffraction technique to gain insights into multicomponent oxide reduction of two precursor variants: mechanically mixed powders and pre-sintered oxide mixtures, targeting an equiatomic CoFeMnNi alloy. We find distinct reduction pathways and microstructure evolution, depending on initial precursors. Mixed powders are reduced to body-centered-cubic, face-centered-cubic, and MnO phases via halite, spinel, and Mn3O4 intermediates, whereas the pre-sintered complex oxide directly transforms into a mixture of metallic and MnO phases. The post-reduction microstructures were also strongly governed by the precursor state: mixed oxides exhibit loosely packed and coarse morphology, whereas the pre-sintered ceramic material showcases two distinct morphologies, either relatively dense metal-rich regions or regions with metallic nanoparticles supported on nanoporous MnO, highlighting the significant role of initial precursors on the final microstructure. Hence, precursor design strategies may offer a single-step route to nanoporous alloys with potential applications in catalysis and energy technologies.

In metals and alloys, solute segregation at grain boundaries typically undermines cohesion and ductility. Here, we overturn this paradigm by showing that solvent Fe atoms can preferentially enrich low-angle grain boundaries (LAGBs) in a ferrous alloy, dramatically enhancing ductility. Cold rolling and aging generate coherent nanoprecipitates, a high dislocation density, and abundant LAGBs in an austenitic matrix, yielding an ultrahigh tensile yield strength of ∼ 1.74 GPa. Moreover, the solvent Fe enrichment at LAGBs lowers local stacking fault energy and activates austenite-to-martensite transformation under load. This transformation-induced plasticity effect stabilizes plastic flow, enabling a uniform elongation of ∼ 26.2 % despite the alloy’s exceptional strength. Our findings challenge conventional views of segregation and offer a new design strategy for ultra-strong, highly ductile alloys.

Ultrahigh-strength bulk alloys with martensitic structures are essential for heavy-duty applications and infrastructure. However, they often contain small-angle grain boundaries (SAGBs), which enhance ductility but weaken resistance to dislocation motion. This limitation restricts tensile strength to below 2.5 GPa, even when nanoprecipitates or hierarchical architectures are introduced. Here we overcome this limitation by developing a near-single-phase martensitic alloy with a tensile strength exceeding 3 GPa. In the model (Fe₄₉Co₄₀Mo₁₁)_99.6B_0.3C_0.1 (at.%) alloy, cold rolling followed by low-temperature annealing introduces a high density of dislocations and drives Mo, C and B atoms to cosegregate at the SAGBs, forming interface complexes. These complexes stabilize the SAGBs, reinforce barriers to dislocation motion and still permit dislocation transmission across boundaries. As a result, the alloy achieves a tensile yield strength of 3.05 GPa and a fracture elongation of 5.13%, setting a benchmark for ultrahigh-strength, ductile alloys. This simple, scalable process integrates seamlessly with existing manufacturing methods and opens a path to next-generation structural materials.

Body-centered cubic (bcc) alloys can achieve gigapascal-level yield strengths but typically are limited in tensile ductility (<20%), contrasting sharply with elemental metals (the largest elongation of ~50%). Multi-principal-element alloys offer vast compositional space to reach synergistic strength–ductility combinations. However, combinatorial trial-and-error exploration is prohibitively costly, while machine learning (ML) approaches are hindered by data scarcity. Here, we develop an ML-guided framework integrating active learning with physics-informed Bayesian optimization to rapidly converge on optimal compositions. The resulting Ti₃₆V₁₄Nb₂₂Hf₂₂Zr₁Al₅ alloy achieves a yield strength of 953 MPa and a large tensile ductility of 42%. The high strength arises from the substantial lattice distortion, as well as the ~1-nm-sized local chemical fluctuations (LCFs) inherent to the highly concentrated bcc solid solution. The ubiquitous LCFs also substantially promote dislocation multiplication and strain hardening, enabling a large tensile ductility. Our approach demonstrates ML’s efficacy in accelerating the finding of high-performance alloys.

Sustainable hydrogen-based direct reduction (HyDR) of iron oxide is an effective approach to reduce carbon emissions in steel production. As the reduction behaviour is closely related to the microstructure evolution, it is important to understand the microscopic reduction mechanisms. Industrial hematite pellets are microstructurally intricate systems with inherent porosity, defects, and impurities. Therefore, in the present study we investigated the HyDR of single crystal hematite (at 700 °C) to elucidate the reduction behaviour and microstructure evolution in a model system. The reduction kinetics of the single crystal (SC) were compared to those of industrial polycrystalline porous pellets using thermogravimetric analysis. Additional SC samples were prepared such that their faces are parallel to the (0001), (101¯0) and (12¯10) crystallographic planes of hematite, and then partially reduced to 16 and 80 % reduction degree. Their microstructure was thoroughly examined by scanning electron microscopy and electron backscatter diffraction (EBSD). Reaction fronts were thus shown to advance into the hematite by a shrinking core model while creating a percolating pore network in the magnetite layer; this was closely followed by wüstite and iron formation, as well as pore coarsening, with the retained oxides proceeding to reduce homogenously throughout the sample abiding by the pore/grain models. Notably, a “cell-like” morphology develops in the magnetite near the hematite/magnetite interface, with finely porous “cell interiors” surrounded by coarsely porous “cell walls”. Furthermore, the hierarchal pore formation, phase transformations, texture, and orientation relationships are considered.

Coherent precipitation-hardened alloys often struggle to achieve both ultrahigh strength and exceptional ductility due to their limited resistance to dislocation motion and vulnerability to glide plane softening. Here, we tackle these challenges by introducing multicomponent precipitates with much increased antiphase boundary (APB) energy. In a model Ni₃Al-type (L1₂) precipitation-hardened face-centered cubic (FCC) NiCo-based alloy, we incorporate multiple elements at the Al sublattice sites within the precipitates, reducing antisite defects and enhancing ordering degree. This process yields multicomponent precipitates with an ultrahigh APB energy (~308 ± 14 millijoules per square meter), which notably strengthens the alloy. Moreover, the exceptionally high APB energy transforms the deformation mechanism from dislocation shearing to stacking fault shearing, thereby avoiding glide plane softening. These result in a tensile yield strength of 1616 ± 9 megapascals, an ultimate tensile strength of 2155 ± 22 megapascals, and a uniform elongation of 10.1 ± 0.3% for the alloy.

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.

Hydrogen-based direct reduction offers a sustainable pathway to decarbonize the metal production industry. However, stable metal oxides, like Cr2O3, are notoriously difficult to reduce, requiring extremely high temperatures (above 1300 °C). Herein, we explain how reducing mixed oxides can be leveraged to lower hydrogen-based reduction temperatures of stable oxides and produce alloys in a single process. Using a newly developed thermodynamic framework, we predict the precise conditions (oxygen partial pressure, temperature, and oxide composition) needed for co-reduction. We showcase this approach by reducing Cr2O3 mixed with Fe2O3 at 1100 °C, significantly lowering reduction temperatures (by ∼200 °C). Our model and post-reduction structural and chemical analyses elucidate that the temperature-lowering effect is driven by the lower chemical activity of Cr in the Fe-Cr solid solution phase. This strategy achieves low-temperature co-reduction of mixed oxides, dramatically reducing energy consumption and CO2 emissions, while unlocking transformative pathways toward sustainable alloy design.

Decarbonization solutions enabling the use of low-grade iron ores are essential for a sustainable steel industry, reducing dependence on scarce high-grade ores and environmental impact. Current processes mainly require high-grade ores, highlighting the need for efficient methods to process lower-quality feedstocks. This study explores a hydro-pyrometallurgical approach for sustainable production. Dihydrate ferrous oxalate, obtained via oxalic acid extraction of iron oxide, was analyzed by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), with postmortem samples characterized by X-ray powder diffraction. Non-isothermal experiments were conducted from 25 to 800 °C at 2, 5, and 10 °C/min under inert (argon) and reducing (carbon monoxide and hydrogen) atmospheres. The curves show three main steps: dehydration, decomposition, and, under reducing conditions, reduction to metallic iron. In carbon monoxide, iron carbide formation and graphitic carbon deposition were also observed. DSC revealed endothermic peaks for dehydration and decomposition and in carbon monoxide, a strong exothermic peak, due to the reverse Boudouard reaction.Activation energies were calculated using the Kissinger method. Dehydration showed an activation energy of 62 kJ/mol in argon and carbon monoxide, and slightly lower in hydrogen (58 kJ/mol), likely due to faster diffusion. Decomposition appeared gas independent, with an activation energy of 90 kJ/mol. A mathematical model was developed to relate reaction conversion to time at a fixed heating rate. The model accurately fits the experimental data and remains valid even at higher heating rates, comparable to industrial conditions. This kinetic model supports simulation and scale-up of the iron oxalate reduction process.

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.

The co-reduction of metal oxide mixtures using hydrogen as a reductant in conjunction with compaction and sintering of the evolving metallic blends offers a promising alternative toward sustainable alloy production through a single, integrated, and synergistic process. Herein, we provide fundamental insights into hydrogen-based direct reduction (HyDR) of distinct oxide precursors that differ by phase composition and morphology. Specifically, we investigate the co-reduction of multicomponent metal oxides targeting a 25Co-25Fe-25Mn-25Ni (at.%) alloy, by using either a compacted powder (mechanically mixed oxides) comprising Co3O4-Fe2O3-Mn2O3-NiO or a pre-sintered compound (chemically mixed oxides) comprising Co,Ni-rich halite and Fe,Mn-rich spinel phases. Thermogravimetric analysis (TGA) at a heating rate of 10 °C/min reveals that the reduction onset temperature for the compacted powder was ∼175 °C, whereas it was significantly delayed to ∼525 °C for the pre-sintered sample. Nevertheless, both sample types attained a similar reduction degree (∼80 %) after isothermal holding for 1 h at 700 °C. Phase analysis and microstructural characterization of reduced samples confirmed the presence of metallic Co, Fe, and Ni alongside MnO. A minor fraction of Fe remains unreduced, stabilized in the (Fe,Mn)O halite phase, in accordance with thermodynamic calculations. Furthermore, ∼1 wt.% of BCC phase was found only in the reduced pre-sintered sample, owing to the different reduction pathways. The kinetics and thermodynamics effects were decoupled by performing HyDR experiments on pulverized pre-sintered samples. These findings demonstrate that initial precursor states influence both the reduction behavior and the microstructural evolution, providing critical insights for the sustainable production of multicomponent alloys.

About 1.9 gigatons of steel is produced every year, emitting 8% (3.6 gigatons) of global CO₂ in the process. More than 50% of the CO₂ emissions come from a single step of steel production, known as ironmaking. Hydrogen-based direct reduction (HyDR) of iron oxide to iron has emerged as an emission-free ironmaking alternative. However, multiple physical and chemical phenomena ranging from nanometers to meters inside HyDR reactors alter the microstructure and pore networks in iron oxide pellets, in ways that resist gaseous transport of H₂/H₂O, slow reaction rates, and disrupt continuous reactor operation. Using synchrotron nano X-ray computed tomography and percolation theory, we quantify the evolution of pores in iron oxide pellets and demonstrate how nanoscale pore connectivity influences micro- and macroscale flow properties such as permeability, diffusivity, and tortuosity. Our modeling framework connects disparate scales and offers opportunities to accelerate HyDR.

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.

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.

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.

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.

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.

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.

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.

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.