Open Access

The quasi-nearest atom (QNA) parameter has shown promise in characterizing packing disorder in metallic glasses, yet its application has so far been limited to multicomponent alloys, where compositional complexity obscures the purely geometric nature of the defects it identifies. Here, we use QNA to identify structural defects in monoatomic metallic glasses produced by molecular dynamics simulations and assess their impact on mechanical behavior. We show that loosely packed atoms share similar structural signatures, including disrupted medium-range order, unusual ∼90° bond angles, reduced five-fold symmetry, and a preference for energetically unfavorable 2- and 4-atom connection modes. When subjected to uniaxial tension, glasses with a lower density of loosely packed atoms exhibit superior mechanical properties. Regions with few QNA per atom constitute load-bearing backbones supporting high von Mises stress with limited shear strain, while high-QNA regions accommodate greater plastic deformation at lower stress and serve as preferential sites for shear transformation zones. The normalized QNA distributions collapse onto a single curve across the four FCC and BCC monoatomic glasses studied, with consistent spatial correlations and connection-mode fractions (within ±4%) at each NQ level, suggesting that the packing topology is dominated primarily by geometry rather than by element-specific bonding. These results confirm that QNA is a promising structural descriptor for capturing important trends linking local packing geometry to mechanical behavior in monoatomic MGs.

Solid electrolytes (SEs) enable the use of alkali-metal negative electrodes (NEs) in solid- and semi-solid-state sodium batteries, increasing both energy density and safety for stationary and portable applications. NASICON (NA Super Ionic CONductor) ceramics provide 3-dimensional Na+ migration pathways and exhibit room-temperature (RT) ionic conductivities of ~1 mS/cm, a wide electrochemical stability window, and high thermal, mechanical and thermodynamic stability. As a result, they are among the most promising candidates for high-energy, RT sodium-sulfur (Na-S) batteries. A major challenge in the development of these systems is the poor interfacial contact between the SE and metallic NE, which leads to locally high current densities and frequent cell failure. Current research at BAM is focused on the development of novel NASICON electrolytes for room-temperature Na-S cells employing a liquid sodium-potassium (Na-K) alloy at the SE/NE interface. The Na-K alloy can significantly improve interfacial contact, but it also introduces new challenges related to interfacial stability and materials compatibility. To study and improve the stability of this interface, NASICON electrolytes are synthesized and tested at BAM. Different stoichiometries and synthesis routes are explored. The high-temperature reaction behavior is studied by thermogravimetric analysis (TGA), while key material properties - including densification, phase purity, grain size, and grain-boundary composition - are characterized by XRD, SEM/EDX, TGA, and ICP-OES. The conductivity of electrolytes is measured at different temperatures in a blocking electrode setup. Half cells employing Na metal and Na-K alloy interfaces are tested to assess interfacial stability and cyclability of the different electrolytes. The analysis establishes a link between synthesis routes, material properties, and electrochemical performance. This systematic study of the SE/NE interface provides fundamental insight into interfacial stability in the presence of Na–K alloys. It forms the basis for future work aimed at the rational design of tailored NASICON electrolytes with improved stability and performance for room-temperature Na–S batteries.

By evaluating the long-wavelength limit of Khachaturyan’s microelasticity theory, we show that it recovers the Kamachali–Wang continuum formulation for multi-component solid solutions [Scripta Mater 206 (2022) 114226]. A corresponding expression for the elastic energy of multi-component solid solutions under cubic anisotropy is also obtained and discussed in comparison with the isotropic case. The framework is demonstrated on the Fe–Mn–Ni–Co–Cu alloy system to perform phase stability analysis at 873 K, including spinodal and binodal part of the phase diagram. The resulting cubic-anisotropic (incoherent) phase diagrams show that the spinodal promotion is reduced compared to the isotropic case, consistent with the derivations. The limit transition is examined using a Gaussian window function, which allows the definition of a critical coarse-graining length above which the continuum description becomes valid. This study provides a rigorous microscopic foundation for continuum multi-component elastic energy formulations, with broad computational implications in thermodynamic and phase-field modeling of chemically complex alloys and microstructures.

Current recycling methods for polymer matrix composites produce short, unoriented carbon fibres, limiting their use to intermediate-strength applications. A method is therefore needed to recycle fibres while preserving their original length and orientation, enabling a circular economy for carbon fibre composites. This study proposes a novel hierarchical composite, designed to retain fibre length and orientation during recycling. Virgin carbon fibres are encased in an insoluble epoxy matrix to form tapes that act as the primary units of the structure. These tapes are protected by the primary matrix from chemical degradation and maintain the fibre orientation. The tapes are then embedded in a secondary recyclable matrix, Elium®, a thermoplastic polymer soluble in acetone with mechanical properties comparable to epoxy. The composite is recycled by dissolving the secondary Elium matrix in acetone and recovering the primary tape units with intact fibre length and orientation. The primary units can then be used to assemble new composites. In this study, hierarchical composite laminates underwent recycling up to three times. Their mechanical properties were assessed after each cycle. Laminates with an Elium secondary matrix retained 60–90% of the mechanical performance of epoxy-based laminates. Minimal degradation was observed between cycles, and fibre length, orientation, and volume fraction were fully preserved. Pushout tests confirmed that fibres in the primary tapes were sufficiently shielded during the recycling process. These results validate the feasibility of a hierarchical recyclable composite that combines recyclability with high mechanical performance, serving as a proof of concept and providing opportunities for future development.

Inconel 718 (IN718) is the most commonly used nickel-based superalloy for high-temperature structural applications due to its remarkable strength, as well as its resistance to creep, fatigue, and corrosion up to 650 °C. This study investigated the room- and high-temperature (650 °C) tensile and low-cycle-fatigue (LCF) behavior of IN718 produced by laser powder bed fusion (PBF-LB/M). A bidirectional scanning strategy with 90° rotation after each layer and a four-step heat treatment was applied, and the results were compared to the conventional wrought material. The hierarchical microstructure after heat treatment was characterized on different length scales using microscopic methods. Distinct microstructural characteristics generated during the PBF-LB/M process, such as grain size and morphology, and the periodically graded arrangement of stacked columnar grains interspersed with regions of elongated grains aligned with the build direction, are largely preserved following the applied heat treatment. Additionally, the heat treatment reduces microsegregation and dislocation density associated with the cellular structure, promoting a more uniform precipitation of γ’/γ’’ strengthening phases. At both room and elevated temperatures, the elastic and yield properties of the PBF-LB/M material are comparable to those of the wrought variant. However, the additively manufactured material shows slightly reduced tensile strength, ductility, and strain hardening capability. As a result, it exhibits slightly lower inelastic strain under LCF conditions at both temperatures. While the fatigue life of the PBF-LB/M material is slightly lower than that of the wrought alloy at room temperature, it is vice versa at 650 °C. Both materials demonstrate cyclic softening behavior, which becomes more pronounced at the higher test temperature. Crack propagation is primarily influenced by grain orientation, morphology, and the presence of δ phase at grain boundaries. Under LCF loading at both room and elevated temperatures, multiple crack initiation sites are observed on the surfaces of PBF-LB/M specimens. In microstructurally heterogeneous regions, pronounced crack branching and deflection occur, suggesting that crack paths are shaped by sharp micromechanical gradients and localized clusters of grains with ⟨001⟩ orientation, which are favored for crack growth. The tendency of the PBF-LB/M material to exhibit systematic crack branching and deflection results in irregular crack fronts and mixed-mode propagation behavior. This crack path complexity contributes to additional energy dissipation during fatigue loading. Consequently, despite the relatively large average grain size and a pronounced formation of slip bands, the fatigue life at room temperature approaches that of the wrought material.