Overcoming the strength and ductility trade-off in Ni-based alloy through tailoring of bimodal grain structures, hierarchical twins and coherent nanoprecipitates

The longstanding strength-ductility trade-off has posed a significant challenge in materials science, limiting the potential applications of numerous structural materials. It is crucial to improve performance by adjusting microstructures to activate a synergistic effect of multiple strengthening/deformation mechanisms. In this study, we introduce a novel strategy to develop a multi-scale heterogeneous structure in a Ni-based alloy, characterized by a bimodal grain distribution with small grains containing high-density hierarchical twins (third-order), oversized grains devoid of twins. The combination of microstructural heterogeneity and deliberate twin distribution enables the alloy to exhibit specific strengthening and deformation mechanisms in different regions, enhancing the matrix and effectively distributing the stress and strain. Simultaneously, nanoscale L1₂ precipitates with an extremely low lattice mismatch (0.193 %) distributed across all grains, reducing the accumulation of elastic strain caused by dislocation movement and thereby preventing crack initiation at interfaces. The unique hindrance and accommodation of dislocations by this structure significantly enhance strength without sacrificing ductility, achieving a yield strength as high as 1498.6 MPa and a uniform elongation of 18 %. During tensile deformation, small grains with twins and oversized grains exhibit different abilities to absorb and constrain dislocations. Hierarchical twins facilitate interactions with dislocations in multiple directions. Various deformation mechanisms, including a high density of tiny stacking faults, Lomer-Cottrell locks, and short twins, are activated, particularly in the oversized grains, which promote increased dislocation multiplication and accumulation, contributing to the high strain hardening ability and excellent ductility. This study offers a novel paradigm and insights for designing ultra-strong and ductile alloys by managing multi-scale microstructural heterogeneities.