Thermal stability and deformation mechanisms in Ni-Co-Fe-Cr-Al-Ti-Nb-type nanoparticle-strengthened high-entropy alloys

The precipitate morphologies, coarsening kinetics, elemental partitioning behaviors, grain structures, and tensile properties were explored in detail for L1₂-strengthened Ni_39.9Co₂₀Fe₁₅Cr₁₅Al₆Ti_4–xNb_xB_0.1 (x = 0 at.%, 2 at.%, and 4 at.%) high-entropy alloys (HEAs). By substituting Ti with Nb, the spheroidal-to-cuboidal precipitate morphological transition, increase in the coarsening kinetics, and phase decomposition upon aging at 800 °C occurred. The excessive addition of Nb brings about the grain boundary precipitation of an Nb-rich phase along with the phase decomposition from the L1₂ to lamellar-structured D0₁₉ phase upon the long-term aging duration. By partially substituting Ti with Nb, the chemically complex and thermally stable L1₂ phase with a composition of (Ni_58.8Co_9.8Fe_2.7)(Al_12.7Ti_5.8Nb_7.5Cr_2.3) ensures the stable phase structure and clean grain boundaries, which guarantees the superb high-temperature mechanical properties (791 ± 7 MPa for yielding and 1013 ± 11 MPa for failure) at 700 °C. Stacking faults (SFs) were observed to prevail during the plastic deformation, offering a high work-hardening capability at 700 °C. An anomalous rise in the yield strength at 800 °C was found, which could be ascribed to the multi-layered super-partial dislocations with a cross-slip configuration within the L1₂ particles.