The pursuit of strong yet ductile metallic materials is a long-standing challenge in materials science and engineering. Grain refinement to the nanoscale via the introduction of numerous grain boundaries (GBs) significantly increases strength. However, this refinement hinders dislocation accumulation, leading to poor work-hardening and limited ductility (uniform elongation, εu, below 5%). Furthermore, the high density of GBs accelerates grain coarsening, resulting in poor thermal stability. This strength-ductility-stability trade-off has hampered the practical application of nanocrystalline (NC) metals. Existing strategies to enhance ductility, such as creating heterogeneous nanostructures or utilizing chemical heterogeneity, often compromise strength or only modestly improve ductility. Achieving a combination of high strength (>2 GPa), sufficient ductility (>10%), and adequate thermal stability (>0.5Tm) remains a significant challenge. Ordered superlattice structures, while offering enhanced dislocation interlocking and accumulation due to suppressed dislocation cross-slip by anti-phase boundaries (APBs), are often brittle due to weak GBs. This research addresses these challenges by developing a novel core-shell nanostructure.

Numerous studies have explored methods to enhance the ductility of nanocrystalline materials. Key strategies focus on boosting dislocation emission and encouraging dislocation accumulation within grains. Creating heterogeneous nanostructures to generate dislocations or utilizing chemical heterogeneity within nanograins are well-known approaches. However, these methods often compromise strength or yield only modest improvements in ductility (δ < 10%). Ordered superlattice structures, with their high APB energy, show potential for improving work-hardening; however, their inherent brittleness due to weak GBs has limited their applications. Previous research on high-strength alloys, including those strengthened by multiple phases, has achieved good results but not the combination of high strength, ductility, and thermal stability sought in this work.

This study employs a rapid solidification method, specifically melt-spinning, to prepare a nanocrystalline Ni42.4Co22.4Fe9.8Al11.0Ti12.6B1.8 (at.%) alloy. The high cooling rates ( >1 × 106 K s−1) result in diffusion-limited solidification and significant microstructure refinement. The resulting microstructure is characterized using transmission Kikuchi diffraction (TKD), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), atomic resolution energy-dispersive X-ray spectroscopy (EDS) mapping, and three-dimensional atom probe tomography (3D-APT). The cooling rate is tuned to produce samples with varying grain sizes. Mechanical properties are evaluated via in-situ tensile testing, including micro-tensile tests on samples with varying dimensions to assess sample size effects. Thermal stability is determined by annealing experiments, with hardness and grain size measured after annealing at various temperatures. Detailed TEM characterization of deformed samples is performed to elucidate the underlying deformation mechanisms. The analysis includes quantification of phase composition, elemental distribution, dislocation density, and grain boundary characteristics.

The core-shell nanostructure in the NiCoFeAlTiB alloy consists of an L12-type ordered superlattice core and a ≈3 nm thick disordered face-centered-cubic (fcc) nanolayer (DINL) at the grain boundaries. 3D-APT reveals enrichment of Fe, Co, and B, and depletion of Ni, Al, and Ti in the DINL. The NC NiCoFeAlTiB samples exhibit exceptional mechanical properties: yield strength (σy) up to 2.20 GPa, ultimate tensile strength (σu) up to 2.65 GPa, and uniform elongation (εu) up to 17%. These values surpass those of other NC and ultrafine-grained (UFG) alloys. The high yield strength is attributed to ordering strengthening (Δσos ≈ 900 MPa) and Hall-Petch strengthening (ΔσHP ≈ 1.8 GPa for d = 79 nm). The exceptional work-hardening capability stems from the high dislocation accumulation capacity facilitated by the ordered core and the ductile DINL. In-situ tensile testing shows multiple slip bands, indicating extensive dislocation activity and strain delocalization, preventing shear band formation and intergranular cracking. Fracture surfaces exhibit a dimpled structure, confirming ductile fracture. The high thermal stability, with an onset grain coarsening temperature (Ton) of 1173 K, is attributed to the thermodynamic stabilization by reduced GB energy and kinetic stabilization due to the DINL and nanoprecipitates acting as pinning sites. This Ton value surpasses that of most NC alloys, including NC HEAs and NT Ni alloys.

The findings demonstrate the effectiveness of the order-disorder core-shell strategy in achieving a remarkable strength-ductility-stability combination in NC alloys. The high APB energy in the ordered core, combined with the ductile DINL acting as dislocation sources, leads to significant dislocation accumulation and enhanced work-hardening. The DINL also plays a crucial role in accommodating plastic deformation and preventing intergranular cracking, contributing to the high ductility. The reduced GB energy and pinning effect of the DINL and nanoprecipitates are responsible for the exceptional thermal stability. The results surpass the performance of previously reported NC and UFG alloys, suggesting that this core-shell design philosophy is highly promising for developing high-performance nanostructured materials across various alloy systems.

This research successfully demonstrates a novel core-shell nanostructure strategy for achieving superior strength, ductility, and thermal stability in NC alloys. The combination of ordered superlattice cores and disordered interfacial nanolayers provides a powerful approach to overcome the inherent limitations of NC materials. Future research could explore the applicability of this strategy to other alloy systems and investigate further optimization of the core-shell structure for enhanced performance.

The study focuses on a specific alloy system (NiCoFeAlTiB). While the results are highly promising, further investigation is needed to determine the generalizability of the core-shell strategy to other alloy compositions and processing methods. The sample size in the micro-tensile tests, although minimizing size effects, may still influence the results. Further studies with larger sample sizes would increase confidence in the results.