Minimal Al promotes dual-superlattice precipitates in Ni_2.1CoCrFeTa_0.2 high entropy alloy with excellent tensile properties

L1₂ and D0₂₂ superlattices have shown excellent strengthening effects and good thermal stability in high entropy alloys (HEA). Cooperating these two precipitates in one alloy has been proved promising in enhancing the comprehensive properties. However, design of the dual-superlattice precipitates in HEAs is still challenging due to unexplored interactions between the L1₂ and D0₂₂ phases during thermal aging. Here, we showed that addition of minimal L1₂-forming Al (0.5 at. %) led to L1₂-D0₂₂ dual superlattice precipitates in Ni_2.1CoCrFeTa_0.2. Our results showed that as the content of Al increases from 0 to only 1 at. %, the dominant precipitates in Ni_2.1CoCrFeTa_0.2 changed from single D0₂₂ phase to L1₂-D0₂₂ dual phases, and finally single L1₂ phase. Samples were annealed at different temperatures ranging from 650 °C to 750 °C. When the temperature is higher than 700 °C, D0₂₂ phase become unstable, the dual-superlattice structure will be disrupted. Superior tensile strength of 1550 MPa and good fracture elongation of 16 % are achieved by aging the (Ni_2.1CoCrFeTa_0.2)_99.5Al_0.5 HEA at 700 °C for 84 h. These insights provide guidance for the design of precipitation-hardened alloys with better mechanical properties.