A hetero-grained L1₂-strengthened high-entropy alloy with remarkable resistance against intermediate-temperature intergranular embrittlement and thermal instability

Alloys with coherent L1₂-type nanoparticles usually possess high strength and ductility, making them promising for wide usage in various technologies and energy-saving strategies. Despite years of effort, most L1₂-strengthened high-entropy alloys (HEAs) still exhibit serious intergranular embrittlement in the intermediate temperature regime (typically around 600–800 °C), limiting their applications in elevated-temperature environments. In this study, we successfully overcome this critical issue by engineering the microstructure with a heterogeneous architecture consisting of columnar grains (CGs) and fine grains (FGs) in the NiCoFeCrAlTiNb-type L1₂-strengthened HEA. Different from the brittle intergranular fracture in the equiaxed counterpart, a ductile fracture at 700 and 800 °C associated with a good ductility of 22.0 and 9.8% is achieved via the heterogeneous-grained architectures, while maintaining a high strength level of 1190 and 820 MPa, respectively. At 700 °C, superlattice intrinsic stacking faults (SISFs) dominate the plastic deformation in the equiaxed grain region, whereas nanotwins are also activated in the heterogeneous grain region. By contrast, dislocation loops, as well as stacking faults, carry the plasticity when tensioned at 800 °C. More significantly, benefitting from the strong pinning of the coherent L1₂-type nanoparticles with low coarsening rates, the heterogeneous structure and associated mechanical properties could be well maintained even after the long-term annealing at 800 °C for 336 h. These findings pave an effective way for the development of high-performance alloys for elevated-temperature applications.