The attraction of these alloys lies in their unusually broad compositional design space. Unlike conventional hydride families, high-entropy alloys and medium-entropy alloys allow phase constitution, hydrogen thermodynamics and microstructure to be adjusted through the choice and proportion of several elements. The source article notes that many of these materials are better understood as complex concentrated alloys, because their behavior is governed not only by configurational entropy but also by enthalpic interactions, lattice distortion and local chemical effects. Refractory elements such as titanium, vanadium, zirconium, niobium and hafnium can provide hydrogen-absorbing structures, while chemical disorder creates a distribution of local hydrogen environments that may affect site occupancy, diffusion and sorption thermodynamics.

Semiempirical descriptors provide the first stage of alloy selection. Atomic-size mismatch estimates lattice distortion; the entropy–enthalpy competition parameter, Ω, weighs configurational entropy against the magnitude of mixing enthalpy; valence electron concentration, or VEC, provides a proxy for phase tendencies; and the radius ratio between larger hydride-forming elements and smaller transition metals offers a geometric guide for Laves-phase packing. Earlier criteria associate VEC values below 6.87 with a greater tendency toward BCC structures, while a value near 6.4 has repeatedly been linked to favorable hydrogen-storage behavior in Laves-related systems. Ratios close to 1.225 are associated with stable C14 or C15 structures, although the review cautions that these are screening tools rather than universal rules. Local ordering, segregation, intermetallic stoichiometry and processing history can overturn predictions based only on average composition.

Those first-pass predictions are increasingly refined with CALPHAD, density functional theory and machine learning. CALPHAD can estimate phase equilibria, hydrogen solubility limits and hydride stability across temperature and pressure, and a model built for single-phase BCC multicomponent alloys reproduced two equilibrium plateaus in the TiVNb system across temperatures from 200 to 235 °C. A separate thermodynamic framework calculated room-temperature pressure–composition–temperature behavior for 440 multicomponent C14 alloys and reproduced the main plateau features and capacities of six experimental alloys. High-throughput workflows can now screen many candidate compositions before expensive atomistic calculations are undertaken, while machine-learning models can learn composition–property relationships from experimental databases. These tools are complementary, however: CALPHAD constrains phase stability, DFT examines local atomic environments and binding energies, and data-driven models depend heavily on the quality and scope of the available datasets.

Density functional theory can estimate formation energies, phase stability, hydrogen solution enthalpies and site-specific hydrogen binding, yet chemical disorder makes accurate modeling expensive. The coherent potential approximation is computationally efficient but averages over local relaxation and short-range order. Special quasi-random structures represent disorder more realistically but require large supercells, while approaches using small sets of ordered structures strike a practical compromise for broad screening. The review stresses that such calculations are most useful for relative comparisons rather than direct forecasts of measured isotherms, activation requirements, kinetic rates or usable capacity. Finite periodic cells commonly represent conditions at 0 K and may omit finite-temperature entropy, surfaces, oxides, defects, segregation and multiphase evolution, so experimental verification remains indispensable.

Performance differences between the major phase families are pronounced. C14 Laves-phase HEAs generally activate more readily, absorb and release hydrogen faster and operate under milder near-room-temperature conditions, but their gravimetric capacity is usually moderate. The equiatomic TiZrCrMnFeNi alloy demonstrated reversible storage of about 1.7 wt% hydrogen at room temperature under moderate pressure, with little or no severe activation requirement. CALPHAD-guided TiZrNbFeNi compositions retained a dominant C14 phase and achieved roughly 1.3–1.8 wt% reversible capacity, depending on phase fraction and homogeneity. By contrast, refractory BCC alloys form hydrides with reported formation enthalpies commonly around −40 to −60 kJ mol⁻¹ H₂, which contributes to low plateau pressures and desorption temperatures often above 300 °C. Their high uptake is therefore not fully accessible under mild operating conditions.

Alloying can moderate that trade-off, but usually cannot eliminate it. Controlled chromium additions to Ti–V–Nb alloys raised plateau pressures and partially destabilized the hydride, while excessive aluminum caused capacity loss and secondary phases. In a combined machine-learning, DFT and experimental study, replacing part of chromium with iron in a TiVNbCr system reduced the absorption enthalpy from −51.6 to −41.3 kJ mol⁻¹ H₂ while preserving reversible BCC-to-FCC hydride transformation, but usable hydrogen capacity also declined. Processing is another major variable. Melting and casting provide reference microstructures, although rapid solidification can leave dendritic segregation that broadens pressure–composition plateaus. Homogenization can improve chemical uniformity and phase distribution, but the right temperature matters: in TiZrCrMnFeNi, annealing at 1,000–1,200 °C increased the C14 fraction and enhanced storage, whereas treatment at 900 or 1,300 °C reduced C14 content and sharply lowered capacity.

Mechanical alloying and severe plastic deformation offer another route to faster activation. Milling creates nanocrystalline or partly amorphous structures, high defect densities and chemically homogeneous powders, enabling chemistries such as magnesium-containing alloys that are difficult to produce by melting. High-pressure torsion, commonly applied at about 5–6 GPa, can refine grains into the sub-100-nanometre range and add non-equilibrium interfaces that provide possible pathways for hydrogen transport. These features can lower activation barriers and accelerate sorption, but grain boundaries and dislocations also trap hydrogen. The same defect population that improves entry may retain hydrogen after desorption, increase hysteresis and reduce reversibility, so deformation is not an unambiguously beneficial treatment. Powder oxidation, milling contamination, pyrophoricity and the lack of a scalable severe-deformation route remain additional barriers.

Multiphase architectures offer perhaps the most promising way to move beyond the capacity-versus-reversibility divide. In BCC–C14 systems, the BCC phase generally acts as the main hydrogen reservoir, while Laves-phase domains and their interfaces are associated with improved activation, nucleation and transport. The optimum phase ratio is not simply the one giving maximum total uptake: in one Ti–V–Mn–Fe–Ni series, the alloy with the greatest BCC fraction achieved the highest uptake, about 1.7 wt%, whereas the C14-rich alloy had the best reversible capacity, about 1.2 wt%. Another dual-phase series showed that an intermediate balance best combined accessible diffusion paths with enough storage sites. This phase-synergy concept is promising, but the review notes that quantitative rules for selecting phase balance, elemental partitioning and interface morphology for a particular operating window do not yet exist.

Long-term testing exposes the limits of short-term capacity claims. Repeated hydrogenation can generate dislocations, vacancies and clusters that act as persistent trapping sites, gradually reducing the mobile hydrogen fraction. In BCC materials, lattice expansion and contraction and symmetry-changing transformations also contribute to local cracking, incomplete reversal and decrepitation. Some high-resolution studies suggest that defect accumulation may be more important than wholesale phase decomposition. By comparison, C14-rich alloys often undergo smaller volume changes and can be more mechanically durable. The TiZrFeMnCrV alloy, for example, retained about 97% of its initial 1.80 wt% capacity after 50 cycles, with later capacities remaining between 1.73 and 1.76 wt%. The review concludes that the field now needs common reporting for activation, equilibration, usable reversible capacity, plateau pressure, hysteresis and cycling conditions, alongside inverse-design workflows that combine CALPHAD, DFT and data-driven prediction. Without application-anchored standards, high-entropy hydrides may continue to look promising in isolated measurements without demonstrating a decisive advantage in storage systems.

Subject of Research: Comparative design, processing and hydrogen-storage performance of high- and medium-entropy alloys

Article Title: Recent advances in hydrogen storage properties of high-entropy alloys: from alloy design to performance

Article References: Edalati, P., & Floriano, R. (2026). Recent advances in hydrogen storage properties of high-entropy alloys: from alloy design to performance. Journal of Materials Science. https://doi.org/10.1007/s10853-026-13672-1

Image Credits: AI Generated

DOI: 10.1007/s10853-026-13672-1

Keywords: hydrogen storage, high-entropy alloys, medium-entropy alloys, C14 Laves phase, BCC alloys, CALPHAD, density functional theory, machine learning, hydrogen kinetics, multiphase materials, Recent, advances

The investigators fabricated Ti–Zr–Nb and Ti–Zr–Ta alloy series with different niobium and tantalum contents by vacuum arc melting. Commercially pure titanium and Ti–6Al–4V served as reference materials. X-ray diffraction and electron backscatter diffraction were used to examine phase constitution and microstructure, while electrochemical measurements assessed corrosion behavior and passive-film responses in PBS. Gibbs free energy analysis and early cell-adhesion assessments were also performed, as summarized in the abstract.

Increasing the Nb or Ta content promoted a change from α′/α″/β multiphase structures toward β-dominated matrices, with Nb showing the stronger β-stabilizing effect. Corrosion resistance did not improve monotonically with increasing alloy content. Ta5 had the lowest corrosion current density among the investigated alloys, at 1.13 × 10−6 A cm−2, whereas Nb4 had the lowest passive current density among the designed alloys, at 2.60 × 10−5 A cm−2. Nb10 and Ta10 showed higher interfacial impedance near the open-circuit potential, but they also had higher corrosion and passive current densities and an earlier rise in current during anodic polarization. These measurements indicate that high interfacial impedance near open-circuit potential did not necessarily correspond to stable anodic passivation.

X-ray photoelectron spectroscopy identified passive films based on TiO₂ and ZrO₂ that also contained Nb- or Ta-containing oxide species. The reported Gibbs free energy analysis indicated that formation of the high-valence oxides was thermodynamically favorable. Overall, the abstract reports that relatively low Nb or Ta additions produced a better combined balance of the evaluated corrosion, passivation and early cell-adhesion properties than higher additions. Because the supplied evidence is an abstract rather than the full article, the precise protocols, uncertainty estimates, statistical results and detailed limitations of the cell-adhesion assessment are not available here; the findings should not be interpreted as evidence of implant performance or patient outcomes.

Subject of Research: Experimental evaluation of Nb and Ta alloying effects on corrosion, passive films and early cell adhesion in Ti–Zr medium-entropy alloys

Article Title: Effect of Nb and Ta alloying on corrosion behavior and passive film chemistry of biomedical Ti–Zr-based medium-entropy alloys in PBS solution

Article References: Hu, S., Wei, P., Liu, Y., Chen, Z., & Liu, D. (2026). Effect of Nb and Ta alloying on corrosion behavior and passive film chemistry of biomedical Ti–Zr-based medium-entropy alloys in PBS solution. Journal of Materials Science, 61(43), 34147-34172. https://doi.org/10.1007/s10853-026-13760-2

Image Credits: AI Generated

DOI: 10.1007/s10853-026-13760-2

Keywords: Ti–Zr medium-entropy alloys, niobium alloying, tantalum alloying, corrosion behavior, passive films, electrochemistry, XPS, cell adhesion, Effect, alloying, corrosion, behavior

The alloys in question are a refractory medium-entropy alloy with the composition Nb45Ta25Ti15Hf15 and a chemically simpler binary alloy, Nb70Ti30. Both were arc-melted from high-purity elements, forged, cross-rolled and heat-treated to produce single-phase, largely recrystallized microstructures with grain sizes averaging roughly 259 micrometres in the quaternary alloy and a bimodal distribution of about 51 to 412 micrometres in the binary one. Interstitial impurities, which are notorious embrittlers in bcc metals, were kept low: the quaternary alloy contained only about 60 parts per million by weight of carbon, 58 ppmw of nitrogen and 197 ppmw of oxygen, while the binary alloy held 561 ppmw of oxygen and 111 ppmw of nitrogen.

When the team pulled tensile specimens and loaded compact-tension fracture samples cooled with gaseous helium at the ISIS Neutron and Muon Source in Oxfordshire, the results defied conventional expectations. The yield strength of Nb45Ta25Ti15Hf15 at 20 kelvin was 938.9 plus or minus 22.6 megapascals, statistically indistinguishable from the 936.6 plus or minus 22.4 megapascals measured at 77 kelvin. The binary alloy behaved the same way, with a yield strength of 839.9 plus or minus 18.5 megapascals at 20 kelvin, only 14 megapascals above its 77-kelvin value and within statistical variation. In other words, instead of the steep rise in strength that normally accompanies cooling in bcc metals, these alloys showed a plateau.

The fracture toughness data were equally striking. Measured with nonlinear elastic J-integral methods and converted to size-independent stress-intensity values, the quaternary alloy showed a crack-initiation toughness of about 96.5 megapascals times the square root of a metre at 20 kelvin, rising to a steady-state crack-growth toughness of roughly 118 megapascals square-root-metres. The binary alloy did even better, with a crack-initiation toughness of 170 megapascals square-root-metres at 20 kelvin, little changed from 188 and 219 at 77 and 298 kelvin respectively. Both alloys also remained ductile in tension, with tensile fracture strains of approximately 14 per cent even at the lowest temperature tested.

To understand why, the researchers turned to an arsenal of electron microscopy techniques, including electron backscatter diffraction, transmission Kikuchi diffraction, high-resolution transmission electron microscopy and four-dimensional scanning transmission electron microscopy, analysed with the py4DSTEM software package. Tensile specimens interrupted shortly after yielding, at a true plastic strain of just 0.4 per cent, revealed the answer. Yielding in these alloys is not initiated by dislocation glide, the usual carrier of plasticity in bcc metals, but by deformation twinning on the {112}⟨111⟩ system, coupled with a local collapse of the crystal lattice into the hexagonal omega phase.

The omega phase appeared as nanometre-thick interfacial layers that expand in the space between nanotwins, and it was also observed as a transitional structure along twin boundaries. The researchers could not determine whether the lattice collapses to omega first and thereby assists twin nucleation, or whether the local shear stress from a growing twin embryo triggers the transformation, but in either case the phase assists twin nucleation and growth. As the twins propagated and slammed into grain boundaries, they generated stress concentrations high enough to nucleate dislocations, which then glided into the grain interiors and, at higher strains, adopted the sluggish screw character typical of bcc plasticity.

This sequence is essentially the reverse of what happens in the celebrated face-centred cubic cryogenic alloys such as CrCoNi, where dislocation glide comes first and twinning and phase transformations follow to spread out damage. In these bcc alloys, twinning comes first, blunts crack tips through a mechanism that is nearly temperature-insensitive, and then serves as an efficient source of dislocations that dissipate stress. Because twinning stresses barely change with temperature, the crack-tip plastic zone does not constrict as the material cools, which is precisely what prevents the ductile-to-brittle transition from appearing.

At a crack tip, however, the picture grows more complicated. The triaxial stress state ahead of a crack, with principal stresses three to five times higher than in uniaxial tension, suppresses shear and promotes partly intergranular fracture, with crystallographic microvoids along grain boundaries. Atom probe tomography showed that titanium and hafnium enrich grain boundaries over a range of about three to four nanometres, raising the group IV concentration locally to about 42 atomic per cent. This chemical heterogeneity promotes dislocation nucleation but also supplies a driving force for stress-induced omega formation, a phase predicted by the Rice parameter to be brittle, making the group IV additions a double-edged sword. Yet even under these harsh conditions, dislocations nucleated at twin and omega boundary intersections in both the parent and neighbouring grains delayed fracture, and some edge dislocations glided far enough to form micrometre-scale kink bands near the fracture surface.

By mapping measured twinning and glide stresses onto the Hutchinson-Rice-Rosengren crack-tip stress fields, the team showed that the local shear stresses at 20 kelvin are high enough to activate twinning in both alloys but not, in the quaternary alloy, high enough to move pre-existing dislocations except very close to the tip. The lower glide barriers of the binary alloy produce a more strain-controlled fracture with higher dislocation densities and more ductile fracture surfaces, while chemical complexity raises glide stresses and high-temperature strength at a modest cost to toughness. Crucially, the researchers conclude that the decisive variable is not chemical complexity but electronic structure: group IV additions lower the unstable stacking-fault energy of group V metals, a proxy for dislocation nucleation barriers, while simultaneously promoting twinning and the omega transformation.

The implications reach well beyond the laboratory. Refractory multi-principal-element alloys are candidates for hypersonic vehicles, energy technologies and structural applications where bcc materials have always been limited by their brittle transition. This work demonstrates an unconventional pathway, independent of alloying complexity, for suppressing that transition: let twinning blunt the crack and nucleate the dislocations that glide cannot. The authors caution that the balance is delicate, since interstitial oxygen above roughly one atomic per cent would likely push the transition temperature back above 20 kelvin, and grain size and recrystallization state both modulate the mechanism. But for now, two niobium-titanium alloys have shown that at the edge of deep cryogenics, a well-orchestrated competition between twinning, transformation and dislocation nucleation can keep a metal tough where metals are supposed to fail.

Subject of Research: Cryogenic deformation mechanisms and fracture toughness of body-centred cubic niobium-titanium alloys

Article Title: Competing mechanisms toughen bcc binary and medium-entropy alloys at 20 K

Article References: Cook, D. H., Chen, X., O’Leary, M., Rao, S., Wang, W., Belcher, C. H., Payne, M., Borges, P., Lee, T. L., Devaraj, A., Asta, M., Lavernia, E. J., Apelian, D., Minor, A. M., Liu, D., Kumar, P., & Ritchie, R. O. (2026). Competing mechanisms toughen bcc binary and medium-entropy alloys at 20 K. Nature. https://doi.org/10.1038/s41586-026-11065-7

Image Credits: AI Generated

DOI: 10.1038/s41586-026-11065-7

Keywords: body-centred cubic alloys, ductile-to-brittle transition, medium-entropy alloys, deformation twinning, omega phase, fracture toughness, cryogenic temperature, dislocation nucleation, niobium-titanium alloys, refractory alloys, strain hardening, electron microscopy

The research, published in Nature Materials, examined crystal formation in nanoparticles made from high- and medium-entropy alloys. These materials contain several metallic elements in roughly comparable proportions, unlike conventional alloys such as steel, which are dominated by one principal element. Their chemically complex and highly disordered atomic environments make them challenging systems for studying how crystals emerge. They also offer an unusually revealing setting because the atoms do not begin in a simple, uniform arrangement. By observing thousands of nuclei at different stages of development, the researchers were able to test whether the classical model’s assumption of a uniform crystal core and abrupt interface accurately reflects what happens in real materials.

The team developed a strategy for effectively freezing the nucleation process in place. The nanoparticles were heated to temperatures above 3,000 degrees Fahrenheit and then supercooled, dropping from extreme heat to room temperature within a few hundredths of a second. During this rapid thermal journey, crystal nuclei began to form, but the sudden cooling arrested them at different stages of growth. This created a collection of atomic snapshots representing early, intermediate and more advanced nuclei. Because the actual nucleation event occurs far faster than conventional data acquisition, directly watching a single nucleus form is extraordinarily difficult. By stopping many particles at different moments, however, the researchers transformed a fleeting process into a large statistical dataset.

They then used atomic electron tomography, an advanced three-dimensional imaging technique capable of mapping the positions of individual atoms within a nanoparticle. The method allowed the researchers to reconstruct each particle atom by atom and calculate how strongly ordered different regions were. In total, the study analyzed more than 8,000 nuclei, ranging from clusters containing fewer than 10 atoms to structures containing more than 1,000. Rather than finding a consistent crystalline region with a clean outer surface, the researchers observed a striking gradient. The innermost portion of each nucleus displayed the highest degree of crystallinity, while atomic order gradually declined with distance from the center. The transition into the surrounding disordered material was diffuse rather than sharply defined.

“The crystals were not uniform with a sharp boundary from the disordered atoms around them, as predicted by classical nucleation theory,” said corresponding author Jianwei “John” Miao, a professor of physics and astronomy at UCLA and a member of the California NanoSystems Institute. “Instead, we saw a gradient. Every nucleus had a core of highest crystallinity and then became more disordered as you go from that core to the boundary.” The observation challenges one of the most familiar visual images in materials science: a small, perfectly formed crystal embryo surrounded by atoms that have not yet joined it. In the new picture, the nucleus is more like a continuously evolving landscape, in which the distinction between crystal and non-crystal becomes progressively less certain as one moves outward.

To describe this behavior, the researchers introduced what they call the gradient nucleation pathways model. The model extends classical nucleation theory by allowing the degree of atomic order to vary throughout a nucleus. Classical theory remains within the new framework as a special limiting case: if the gradient is replaced by a perfectly uniform crystalline interior and a sharp interface, the classical results are recovered. Yet the experiments indicate that this idealized condition may rarely occur in the systems examined. The revised model therefore treats nucleation not as the sudden appearance of a finished miniature crystal, but as a multistep pathway through intermediate structures whose atomic order changes gradually.

This distinction also alters the way scientists think about the energy barrier involved in nucleation. In classical theory, forming a stable nucleus requires a system to overcome a sharply defined energetic obstacle. Small clusters are energetically unfavorable because creating an interface between ordered and disordered matter costs energy; only clusters that surpass a critical size can continue growing. The UCLA-led analysis suggests that the transition may instead involve a series of intermediate states. Miao compared the conventional energy barrier to a wall that must be climbed, while the gradient pathway resembles a ladder that allows matter to move upward through multiple smaller steps. If the system can pass through partially ordered configurations, nuclei may overcome the overall barrier more efficiently than the classical picture predicts.

The study also uncovered an unexpected pattern in how nuclei grow and eventually combine. As nuclei increased in size, their central regions became more highly ordered, indicating that crystallinity strengthens from the inside outward rather than appearing all at once. When separate nuclei approached one another, many were already oriented in nearly the same direction, even before they merged. Crystal orientation describes the alignment of atomic planes and axes, and matching orientations can make the final coalescence energetically easier. The observation suggests that neighboring nuclei may interact over short distances or follow correlated low-energy pathways during formation. Instead of growing independently and colliding randomly, they may become partially coordinated before joining into a larger crystal.

The implications extend well beyond the particular alloys used in the experiments. Nucleation is a universal step in processes ranging from cloud formation and ice growth to pharmaceutical crystallization, food production, semiconductor fabrication and the manufacture of structural metals. A more realistic description of how nuclei form could help scientists control when and where materials crystallize, as well as the size, orientation and properties of the resulting crystals. High- and medium-entropy alloys are especially attractive for engineering because their unusual compositions can produce combinations of strength, flexibility, thermal stability and catalytic activity. Understanding their earliest structural transformations could guide the design of more durable components, more efficient catalysts and advanced materials for demanding environments.

The researchers emphasize that their model is intended to apply broadly to nucleation systems, not only complex metallic nanoparticles. If gradients in order are common, they could influence models of freezing, condensation, precipitation and phase separation across physics, chemistry, Earth science and engineering. The ability to map atomic structures in three dimensions also opens a new experimental route for testing theories that previously relied heavily on indirect measurements. Classical nucleation theory is not discarded by the findings; rather, it is placed within a more general framework that recognizes the complexity of real atomic arrangements. “We believe this work will change how researchers think about nucleation,” Miao said, expressing hope that textbooks will eventually be revised. By revealing that crystals may begin not as sharply bounded objects but as smoothly evolving patterns of order, the study offers a fresh view of one of nature’s most fundamental transformations.

Subject of Research: Crystal nucleation and growth in high- and medium-entropy alloy nanoparticles; atomic-scale mechanisms of phase transitions.

Article Title: Crystal nucleation and growth in high-entropy alloys revealed by atomic electron tomography

News Publication Date: 25-Aug-2026

Web References: https://www.nature.com/articles/s41563-026-02727-y

References: Nature Materials; UCLA; California NanoSystems Institute; U.S. Department of Energy, Office of Science, Basic Energy Sciences program.

Image Credits: Miao Lab/UCLA

Keywords

Crystal nucleation, phase transitions, atomic electron tomography, high-entropy alloys, medium-entropy alloys, crystallography, materials science, nanotechnology, solid-state physics, UCLA, crystal growth, nucleation theory