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of the grain interior was identified using brightfield transmission electron microscopy (TEM) (Fig. 1A).We conducted aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) analyses to probe the detailed atomic structures and chemistries (Fig. 1, B to D). The inner grain exhibits a highly Ll2-type chemical ordering with distinct atomic complexities of the constituent sublattices. We used atomicresolution energy-dispersive x-ray spectroscopy (EDX) mapping (Fig. 1B) to show that Ti and Al atoms mainly occupy the vertices (B sublattice) of the Ll2 unit cell, whereas the faced centers (A sublattice) are occupied mainly by Ni and Co atome. Iron atoms occupy both the sublattice sites of the Ll2 unit cell and allow the alloy to be stoichiometric. This sorting of the complex atomic site-occupation behavior is remarkably different from that of conventional binary ordered alloys, in which each sublattice is essentially occupied by a single type of element (fig. S1). Partial replacement of Ni with Fe and Co decreases the electron density (the average number of electrons per atom outside the inert gas shell) of the ordered structure. This structure helps suppress the formation of brittle hexagonal or tetragonal ordered phases (15). We also can clearly identify a distinctive interfacial disordering. This creates an ultrathin disordered layer ( 5nm in thickness) along the grain boundary with a face-centered cubjc (fcc) solid-solution structure (Fig. 1C). The volume fraction of interfacial disordered nanolayer can be estinated to be 0.13% (14). We found that this disordered nanolayer is highly coherent with that of the host ordered grain with a small lattice mismatch of 0.2% (14). Moreover, we found that the Fe and Co atoms show a strong tendency to segregate at the boundary regions, whereas the Ni,Al, and atoms are largely depleted (Fig. 1D). This distinctive interfacially disordered superlattice structure (Fig. 1E) is substantially different from that of conventional ordered alloys reported previously. We used three-dimensional atom probe tomography (3D-APT) to provide a quantitative compositional analysis at the atomic scale. This technique is especially important to quantify the light element boron (Fig 2A). The LT type ordered superlattice of the grain interior is compositionally homogeneous without elemental clustering. We identified it as the (Ni,Co,Fe)(Al,TlFe)-type compositionally complex ordered superlattice with a small amount of boron occupying the interstitial positions (16). By contrast, we found that the Fe and Co are strongly enriched inside the DINL accompsnied with codepletion of Ni,Al, and TI elements, which is consistent with our TEM analyses. We were also able to clearly identify local boron enrichment within the DINL with 3D-APT, as EDX is not capable of accurately determining Superlattice architecture with nanoscale disordered interfaces Nano-disordered FCC I: Ordered superlattice grain (OSG); II: Disordered interfacial nanolayer (DINL) Fig. 1. Unusual nanoscale interfacially disordered structure of the superlattice the sublatice cocoupations. (C) Hgi-resolition HAADF-STEM image revealing the utramaterials. (A) Bright-field TEM image showing the polycrystaline morphology. (inset) thin disordered layer at the grain boundares with a nanoscale thickness. The images A corresponding selected-area electron dffraction pattern collected from the grain on the rigit. show the corresponding fast Fourier transform (FFT) patterrs. (D) EDK maps ferior, which shows the Lla-type ordered structure (B) Atomicresolution HAADF- showing the compostional distribution of the DINL (E) Scheratic ilustration highEM mage and corresponding EDX maps taken from the inner Lla-type OSG, revealing lighting the nanoscale interfacialy disordered structure. FOC, face-centered cubic. A Fig. 2. Three-dimensional compositional distributions and nanoscale interfacial cosegregation of the NDI-SMs. (A) Atom maps reconstructed using 3D-APT that show the distribution of each element. Fe,Co, and B are enriched at the DINL, whereas Ni, Al, and TI are depleted correspondingly. (B) Twodimensional compositional contour maps revealing the multielement cosegregation behaviors of Fe,Co, and B elements within the DINL. (C) One-dimensional compositional profile that quantitatively reveals the elemental distributions B C across the OSG and DINL. within this temperature range. Notably, we also discovered an extremely sluggish grain growth over long-duration annealing (up to 120 hours) at a high temperature of 1050C (Fig. 3D). Most traditional structural materials tend to coarsen rapidly at these temperatures (fig. S4) (29-32). By contrast, the NDI-SM retains a uniform grain size distribution with almost negligible coarsening ( 13.19.4m on average). Moreover, an appreciable strength can still be maintained when tested at a high temperature of 1000C (fig. S5). We believe that these initial high-temperature observations suggest a high thermal stability of our NDI-SM, which may render this type of alloy spedifieally suitable for high tempernture strue tural applications. We attribute the ultrahigh yield strength of our NDI-SM mainly to the high antiphase boundary (APB) energy of the highly ordered superlattice grain, which produces a strong barrier against both the nucleation and motion of dislocations. Previous studies have suggested that in the ordered NisAl alloy, the Fig. 3. Mechanical properties and thermal stability of the NDI-SMs. (A) Tensile (C) Variations of Vickers hardness (HV) of the NDI-SM at elevated temperatures stress-stran curve of the NDI-SM tested at 20C in air. The stress-strain curve of compared with those of conventional ordered alloys (27, 28). (D) Grain size high-strength NizAl-type (NizAl-2.5 at \% B) alloy (9) is also plotted for a direct variations as a function of heating durations at a high temperature of 1050C. (Inset) comparison. (Inset) Tensile fractography showing the ductile dimpled structures. A typical EBSD inverse pole figure (PF) map showing the grain size of the sample compared with various corventional buk ardered alloys (8-10,12,13,17, 18,22-26). resistance against the themally drwen softening and grain coarsening. ductilization. (A and B) TEM images of the plasticaly deformed specimens under showing the remarkable ductiliation response dominated by the nanoscale various tensile strains at room temperature, showing that pronounced dislocation interfacial disordering. The DiNL serves as a ductle buffer zone between adjacent activities can be steadily acoommodated at the vicinity of the DINL without ordered grains, which enables excelent plastic-deformation compatibity by intergranular cracks. (C) EBSD IPF map. (D) Corresponding kernel average enhancing dislocation mobilities at grain boundaries, and thus results in the large misorientation map taken from the fractured specimen, which shows the obvious tensile ductilty at an ultrahigh-strength level