Deciphering the compositional complexity and sublattice site occupancy toward strong-yet-ductile L1₂-type multicomponent intermetallic alloys

Breaking through the compositional limitations of conventional intermetallic alloys (IMAs), novel multicomponent L1₂-type chemically complex IMAs (CCIMAs) have emerged with extraordinary structural and functional properties, opening new material frontiers and potentially enabling them to withstand even harsher service conditions, such as those in aero-engines and gas turbines. The design of L1₂-type CCIMAs relies heavily on co-alloying key sublattice-occupying elements such as Al, Ti, Nb, and Ta. However, the resulting compositional complexity often leads to undesired brittle phase formation, which consequently degrades their performance. Here, we successfully address this challenge by revealing the compositional complexity and sublattice site occupancy, controllably developing a new L1₂-type NiCoCrAlTiNbTaB CCIMA that exhibits an outstanding strength-ductility synergy and remarkable thermal stability. The phase formation criterion for L1₂-type intermetallic compounds aligns well with the valence electron concentration (VEC) and mixing enthalpy (ΔH_mix) model. Elevating chemical complexity simultaneously increases the volume fraction, anti-phase boundary (APB) energy, and nanohardness of the multicomponent L1₂ phases in the investigated alloys. The accurate sublattice site occupancy of Al, Ti, Nb, and Ta endows the newly designed CCIMA with a high APB energy of 335.9 ± 13 mJ m⁻², contributing to a superior yield stress (YS) of ∼1004 MPa. The disordered interfacial nanolayer (DINL) near the grain boundary (GB) not only mediates the large tensile ductility (∼23.7%) but also potentially retards grain coarsening of the CCIMA thermodynamically and kinetically. This work provides a valid scientific basis for designing L1₂-type CCIMAs with superior integrated performance by precisely tailoring compositional complexity and sublattice site occupancy.