Abstract: Chemically complex intermetallic alloys (CCIMAs) have gained particular interest because of their attractive physical and mechanical properties. However, the CCIMAs based on the strict alloying stoichiometry often show serious brittleness with very low ductility at ambient temperature, which seriously hinders their practical use in engineering applications. Here we demonstrate an off-stoichiometry strategy to achieve large tensile plasticity (-30%) together with an ultrahigh tensile strength (-1.4 GPa) in a novel Co_39.3Ni_39.6Al_13.8Ti_3.1Ta_2.8Nb_1.4 (at.%) CCIMA system. Such an ultrahigh strength is primarily ascribed to the high anti-phase boundary energy via multiple alloying additions (i.e., Ti, Ta, and Nb). Simultaneously, the nanoscale disordered phases at grain boundaries (GBs) can efficiently enhance dislocation mobilities and plastic deformation compatibility, thus resulting in a large ductility. The off-stoichiometry strategy provides an effective avenue for the innovation of ultra-strong yet ductile multicomponent intermetallic-based alloys.

Key words: Chemically complex intermetallic alloys, Microstructure, Mechanical behavior, Off-stoichiometry, Anti-phase boundary energy, Nanoscale disordered phase