Thermal stability of (CoCrFeNi)₉₄Ti₂Al₄ alloy containing coherent nanoprecipitates at intermediate temperatures

The thermal stability of (CoCrFeNi)₉₄Ti₂Al₄ alloys containing coherent nanoprecipitates is investigated at intermediate temperatures of 723–923 K. The microstructure evolution is characterized both inside grains and near grain boundaries upon aging for 12–168 h. The primary L1₂ precipitates within grains maintain good stability, but unlike the cases at temperatures above 1000 K, the coarsening rate in this temperature regime is not significantly lower than the conventional Ni-based alloys. High density of fine-scaled secondary L1₂ precipitates are formed outside a narrow precipitate-free zone surrounding the primary precipitates during aging at 923 K, and the A sublattice in such A₃B-type precipitates are occupied by higher fraction of Ni in the secondary precipitates than in the primary ones. Furthermore, Heusler phases and discontinuous-precipitate area are observed near grain boundaries, between which and grain interior shows a linearly-arranged precipitate row. Despite such complex microstructure, overall good thermal stability is identified at grain boundaries in this intermediate-temperature regime.