Critical roles of sublattice chemistry in governing mechanical behavior and deformation mechanism of L12-type chemically complex intermetallic alloys

L1 2 -type chemically complex intermetallic alloys (CCIMAs) with the ordered A 3 B structure have emerged as novel high-temperature materials due to their exceptional strength-ductility synergy and thermal stability. However, quantitative insight into the effects of sublattice chemistry on deformation and fracture mechanisms remains elusive. Here, we systematically elucidate the critical roles of the Co/Ni ratio in the A-site sublattice on the microstructure and mechanical behavior of a model Co 76.5-x Ni x Al 13.5 Ti 3 Ta 3 Nb 3.5 B 0.5 (at. %) CCIMA. All alloys exhibit fully recrystallized L1 2 -dominated grains with a small fraction of borides, indicating that their phase constitution is insensitive to the A-site chemistry. However, the Co/Ni ratio significantly influences the elemental segregation behavior at grain boundaries (GBs) and associated dislocation activities, giving rise to markedly distinct mechanical properties and fracture morphologies. An increased Co/Ni ratio reduces the anti-phase boundary (APB) energy, leading to a reduction in yield strength from ∼839 MPa (26Co-50.5Ni alloy) to ∼680 MPa (46.5Co-30Ni alloy). Concurrently, the deformation substructure evolves from APB-coupled superdislocations to superlattice intrinsic stacking faults (SISFs) and dense SISF networks, which sustains work hardening to larger strains. Moreover, in the CCIMAs with Co/Ni ≥ 1, Co enrichment accompanied by depletion of Ni and L1 2 -forming elements at GBs is associated with the formation of disordered interfacial nanolayers (DINLs) and suppressed intergranular fracture, enabling elongations exceeding ∼35% while maintaining a tensile strength of ∼1.5–1.6 GPa. This study establishes a correlation between A-site sublattice chemistry, planar fault energetics, GB chemistry/structure, and mechanical properties, providing a new paradigm for controllably designing advanced intermetallic alloys for extreme conditions.