Negative Excess‐Energy Interfaces: A New Paradigm for Metal Strengthening Beyond Theoretical Limits

For decades, refining grain or twin structures at the nanoscale has been the cornerstone strategy for enhancing metal strength [1]. However, when structural dimensions shrink to below ∼10 nm, this approach hits an insurmountable bottleneck, that is, material softening triggered by interface instability, which prevents further strength improvement [2-5]. Now, writing in science, Li and colleagues introduce a novel strengthening approach based on nanoscale negative excess-energy interfaces (NEIs) that elevates the mechanical performance of nickel-molybdenum (Ni-Mo) alloys to an unprecedented level [6]. The discovery and engineering of coherent interfaces between face-centered cubic (FCC) and hexagonal close-packed (HCP) lattices in supersaturated Ni(Mo) solid solutions are characterized by a remarkable negative excess energy. Unlike conventional grain boundaries (GBs) or twin boundaries (TBs) that exhibit positive excess energy and inherent instability at high densities, these NEIs possess intrinsic thermodynamic stability—an attribute confirmed through both experimental observations and density functional theory (DFT) calculations (Figure 1A–F). The negative excess energy arises from slightly stronger chemical bonding at the FCC–HCP interfaces compared to the neighboring phases, as revealed by DFT calculations showing interfacial energies ranging from −8.7 to −19.5 mJ m−2 for different configurations. This unique energetic signature enables the formation of extremely dense interface networks with average spacing as small as 0.7 nm, far denser than any stable interfaces reported in bulk metallic materials to date. Microstructures, DFT calculations, and mechanical properties of the Ni-Mo supersaturated solution samples. Typical bright-field TEM images of (A) the as-deposited amorphous alloy and (B) the crystallized sample 26Mo#5 with nanograins containing high-density planar faults. (C) High-resolution high-angle annular dark-field-scanning transmission electron microscope (HAADF-STEM) image of a grain in sample 26Mo#5. (D–F) Intermixed phases of six ABAB layers plus six ABCABC layers (“6 + 6”) separated in different ways (DFT vs. HR-TEM images) and calculated interfacial excess energies. The small and large spheres represent Ni and Mo atoms, respectively. (G) Variations of the yield strength versus microstructure size measured from micropillar compression (solid symbols) and microhardness tests (open symbols, Hv/2.7). (H) Ashby map of yield strength versus Young's modulus for Ni-based samples with different microstructures. Reproduced with permission from Ref. [6]. Copyright 2025, the American Association for the Advancement of Science. The synthesis of these high-density NEIs is achieved by a two-step process: pulse-current electrodeposition of amorphous Ni-Mo alloy foils (with a Mo content of 26.0 at% ± 0.5 at%) followed by controlled crystallization annealing. This thermal processing triggers the formation of supersaturated FCC Ni(Mo) nanograins with a strong <111> texture, interspersed with dense planar faults that evolve into coherent FCC–HCP interfaces. Critically, Mo atoms are homogeneously distributed throughout the Ni lattice without segregation to grain boundaries, a key factor in preventing the formation of intermediate compounds (e.g., Ni3Mo) that would compromise the mechanical performance. Furthermore, the density of NEIs can be precisely tuned by adjusting annealing temperature and duration, reaching a maximum of 39% in samples annealed for 1 h at 848 K followed by 1 h at 923 K (sample 26Mo#5). The mechanical performance of these NEI-reinforced Ni(Mo) alloys is nothing short of transformative. Micropillar compression tests reveal that the alloy with the highest NEI density (26Mo#5) achieves a yield strength of 5.08 ± 0.03 GPa, surpassing not only conventional nanograined and nanotwinned Ni alloys but also approaching the theoretical strength limit of Ni alloys (Figure 1G). Notably, this ultrahigh strength is accompanied by fully elastic deformation up to ∼2% strain, a level seen in amorphous alloys but not in polycrystalline metals [7]. There was no evidence of plastic flow or shear band formation. Repeated loading-unloading cycles confirm the reversibility of the elastic response, highlighting the exceptional structural stability of the NEI network. Microhardness measurements further validate the strengthening effect, with values increasing from 12.18 GPa (low NEI density) to 13.54 GPa (high NEI density), more than those of 1.6 times higher than the as-deposited amorphous alloy (7.04 GPa) and the intermetallic Ni3Mo compound (8.39 GPa). Equally impressive is the significant enhancement in Young's modulus, which scales linearly with NEI density. As shown in Figure 1H, the alloy with the densest NEIs exhibits a Young's modulus of 254.5 ± 2.8 GPa, well above pure Ni (187.6 ± 14.3 GPa), the as-deposited Ni-Mo glass (122.0 ± 3.1 GPa), and even the Ni3Mo intermetallic compound (195.2 ± 11.1 GPa). This stiffening effect is attributed to interface-induced atomic and electronic structure modifications. Specifically, planar faults create local lattice distortions and altered electronic states with higher charge density near interfaces, strengthening atomic bonding [8, 9]. Notably, the NEI-induced stiffening is observed in Ni(P) and Ni(MoW) supersaturated solutions, demonstrating the versatility of the strategy across different Ni-based systems. When plotted on an Ashby map of yield strength versus Young's modulus, the NEI-reinforced Ni-based alloys form a distinct cluster that bridges the gap between conventional metallic alloys and ceramics. This unique property space opens new avenues for applications in extreme environments. Thermal stability is another critical advantage of the NEI-reinforced alloys. Annealing experiments show that the transformation temperature to the ordered Ni3Mo phase increases by ∼100 K with increased NEI density, indicating that the dense interface network inhibits atomic diffusion and phase separation. This thermal stability exceeds that of nanograined metals stabilized by solute segregation, addressing a long-standing challenge in nanomaterials engineering. DFT calculations further predict that NEIs can form in other Ni binary systems (e.g., Ni-W, Ni-Ta, Ni-Nb) owing to their inherent negative excess energy, suggesting broad applicability beyond Ni(Mo) alloys. Beyond its immediate technological implications, this study redefines the paradigm of metallic strengthening. By tailoring interfaces at the atomic scale to achieve negative excess energy, the inherent stability limitations of traditional microstructural features have been overcome, creating more effective obstacles to dislocation motion. This approach moves beyond the Hall–Petch relation that governs grain refinement hardening and introduces a new dimension of materials design: interfacial energy engineering. The ability to control interfacial thermodynamics at the nanoscale thus provides a powerful tool for uplifting material properties, not only in crystalline metals but potentially in metallic glass, semiconductors, ceramics, and composite materials [10]. In summary, this landmark research demonstrates that nanoscale negative excess-energy interfaces are a game-changing strengthening mechanism for Ni alloys, enabling ultrahigh strength approaching the theoretical limit. The findings challenge long-held assumptions about interface behavior in nanomaterials and provide a scalable, generalizable strategy for developing the next-generation high-performance metallic materials. As the global demand for stronger, lighter, and more durable materials continues to grow, this work paves the way for a new era of materials engineering. Song Tang: funding acquisition, writing – original draft. Si Lan: funding acquisition, writing – review and editing. Xun-Li Wang: funding acquisition, writing – review and editing. This work was financially supported by the National Key R&D Program of China (Grant No. 2021YFA1200203), the National Natural Science Foundation of China (Grant No. 12261160364), and the National Natural Science Foundation of China/Research Grants Council Joint Research Scheme (Grant No. N_CityU173/22). Si Lan is an editorial board member for Rare Metals and was not involved in the editorial review or the decision to publish this article. All authors declare that there are no competing interests. These data were derived from the following resources available in the public domain: 2025, 390(6773): 617–621. https://doi.org/10.1126/science.aea4299.