Hierarchical nanocomposite formation and strengthening mechanisms in a phase-separated CrFeCoNiCu high-entropy alloy

• CrFeCoNiCu HEA shows unique hierarchical phase separation into LE & HE FCC regions. • Dual FCC phases each embed nanoscale coherent precipitates (NCPs) of the other matrix. • Precipitate shearing mechanism by LE and HE NCPs leads to significant strengthening. • Annealing causes coarsening, triggering Orowan bypass and hardness reduction. • Propose a design strategy for HEA nanocomposites with tunable mechanical properties. This study investigates a CrFeCoNiCu high-entropy alloy (HEA) exhibiting hierarchical phase separation into Cu-rich low-entropy (LE) and CrFeCoNi-rich high-entropy (HE) regions, driven by a miscibility gap and monotectic reaction. A CALPHAD-based pseudo-binary diagram guided the optimization of solidification and annealing conditions, promoting nanoscale coherent precipitate (NCP) formation within each phase. Multiscale analysis (SEM, EPMA, TEM, 3D APT) revealed a dual FCC-phase composite where each region contains NCPs of the other phase. During annealing, secondary NCPs grow more slowly than primary phase-separated regions, enabling effective control through heat treatment. Mechanical testing via nanoindentation and micropillar compression showed that coherent NCPs enhance strength via a shearing mechanism. In particular, LE NCPs in the HE matrix effectively distribute the applied strain and suppress dislocation motion and slip band formation, resulting in stable strain hardening without stress drops. Upon coarsening beyond the size applicable for shearing, transition to the Orowan bypass mechanism resulting in a decrease in nano-hardness. These findings highlight the importance of coherent phase-separated nanostructures in strengthening and deformation control, and present a thermodynamically guided microstructural design strategy for fabricating HEA-based hierarchical nanocomposites with tunable mechanical performance, offering a promising approach for developing next-generation high-strength, ductile structural materials.