Designing ultrahigh-strength lightweight compositionally complex alloys through heterostructural composite engineering

Achieving a simultaneous enhancement in strength, ductility, and crack resistance remains a formidable challenge in structural alloys, particularly those employing dual-phase heterostructures. Although hetero-interfaces in such systems can promote heterogeneous deformation and strain hardening, they often act as stress concentrators, triggering interfacial strain localization and premature failure. Here, we report a novel heterostructural composite engineering strategy that concurrently minimizes local strain mismatch and enhances interfacial crack tolerance. By introducing Al and C into a lightweight CoNiV-based compositionally complex alloy (CCA), we engineer a unique face-centered cubic (FCC)/L2₁ composite architecture via a short-time annealing (900 °C, 1 min). This microstructure comprises L2₁-decorated recrystallized and non-recrystallized FCC regions, as well as FCC-embedded L2₁ islands, forming a gradient in both hardness and strain. Such a configuration significantly reduces the interphase hardness contrast (∼1.5 GPa), thereby activating pronounced heterogeneous deformation-induced (HDI) hardening while suppressing strain localization through cooperative plasticity between FCC and L2₁ phases. The resulting CCA exhibits an ultrahigh specific yield strength of 226 MPa·cm³/g and a uniform ductility of 14 %, outperforming previously reported precipitation-strengthened dual-phase CCAs. Deformation is governed by multistage strain hardening mechanisms, involving HDI hardening, dislocation slip, nanotwinning, and formation of 9R phases. Additionally, nanotwins and ductile FCC domains within L2₁ islands act as effective crack arresters, delaying crack initiation and propagation. This work establishes a new paradigm for designing CCAs with exceptional mechanical performance through hierarchical composite engineering.