Grain boundary (GB) engineering is an effective tool for improving the mechanical properties of metallic materials. In this study, we added 30 appm boron (B) to an equiatomic TiNbZrHfTa refractory high-entropy alloy (HEA), and found that the GB chemistry has been altered. B preferentially segregates to GBs with the co-segregation of Zr and depletion of Nb and Ta, as revealed by atom probe tomography probing. This change in GB chemistry affects the macroscopic load-bearing performance of the HEA, where a reduction in the yield strength by ∼6.2 ± 0.7% is observed, i.e. , 869.8 ± 10.4 MPa of the B-doped HEA vs. 920.7 ± 3.7 MPa of the B-free HEA. Additionally, a correlation between GB chemistry and the plastic deformation at GBs is found, indicated by changes in the crystallographic slip traces and the Luster-Morris compatibility factor among adjacent crystals. More specifically, we find that (i) B-decorated GBs accommodate higher adjacent plastic strain levels, which is manifested as an out-of-plane offset (via GB shear localization) between two neighboring grains; and (ii) in the B-doped case, slip transfer across GBs requires a lower misalignment between slip planes or slip directions as compared with the B-free sample. This study thus enhances our understanding of the role of GB chemistry in the mechanical properties of polycrystalline body-centered cubic HEAs, leveraging opportunities for GB segregation engineering in this field.

Direct reduction of iron oxide using hydrogen offers a sustainable route to lower carbon emissions in steelmaking. Although iron oxide feedstocks consist of polycrystalline pellets, the influence of initial hematite grain size on direct reduction remains unexplored. Herein, the effect of grain size on reduction kinetics and microstructure evolution were uncovered using model polycrystalline hematite samples with large (~ 30 µm) and ultrafine (~ 1 µm) grains. Thermogravimetric analysis showed grain-size-dependent reduction behavior, while microstructural examination of partially reduced samples revealed that large-grained hematite forms finer directional pore channels due to fewer grain boundaries and orientation changes. Consequently, large-grained samples reduce faster initially as the pore network develops, while ultrafine-grained samples achieve more efficient reduction in later stages facilitated by a more homogenous pore network. These results demonstrate how grain size dictates porosity and texture evolution, providing fundamental insights relevant not only to hydrogen-based iron production but also to the design of porous materials by solid-state reduction processes. (Figure presented.)

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.

The engineering stress-strain curve is often observed to be flat across a wide range of plastic strains after yielding in a uniaxial tensile test, for alloys with gigapascal yield strength, especially those recently developed body-centered-cubic (bcc) multi-principal-element alloys (MPEAs). The near-zero slope of these curves is often perceived as an apparent lack of strain hardening capability, which would set off necking immediately after yielding due to a violation of the Considère criterion. If so, how could the alloy manage to sustain the large uniform elongation? Here, we resolve this puzzle by re-analyzing the data in terms of true stress versus true strain to demonstrate that the perception above is a misjudgment. Thanks to the MPEA's gigapascal yield strength, the wide plateau does not mean that the strain hardening rate is negligible, but rather is at GPa level, adequate to guarantee no onset of the necking instability. Inside a tensile-strained TiZrNb MPEA, the dislocation density was observed to increase by nearly two orders of magnitude, even after a tensile strain of only 8%, indicating obvious dislocation accumulation as the mechanism for strain hardening to delay necking. All these corroborate that an alloy yielding at GPa stress with a “perfect-plastic” curve is ultra-strong yet highly ductile, despite its very high (i.e., almost ∼1) yield ratio.

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.

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.

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.

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.

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.

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.

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.

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.

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.