Formation of cuboidal B2 nanoprecipitates and microstructural evolution in the body-centered-cubic Al0.7NiCoFe1.5Cr1.5 high-entropy alloy

The present work primarily investigated the formation of coherent cuboidal B2 nanoprecipitates in body-centered-cubic (BCC) Al_0.7NiCoFe_1.5Cr_1.5 high-entropy alloy (HEA) and its microstructural evolution with temperature. Alloy samples were prepared by suction-cast processing and then heat-treated at different temperatures of 673–1273 K for 2 h, respectively. It was found that the coherent microstructure with cuboidal B2 nanoprecipitates in BCC matrix can be formed in the as-cast state and be stabilized up to 823 K with a particle size of 60–120 nm. Even after being heat-treated at 773 K for 1080 h, the cuboidal B2 nanoprecipitates were still stabilized without any coarsening. It was due to a moderate lattice misfit (ε = 0.4–0.5%) that favored the formation of coherent cuboidal B2 nanoprecipitates, resulting in a good compressive mechanical property. Meanwhile, the Fe/Cr-rich σ phase appeared and dominated after the treatment at 873 K, causing heavy brittleness. The face-centered-cubic (FCC) phase appeared at 973 K and became dominant above 1173 K, which softened the alloy. In addition, the thermodynamic calculations were carried out for further understanding the microstructural evolution with temperature. The influence of microstructures constituted by different phases on mechanical properties was studied, in which high strength caused by cuboidal B2 nanoprecipitates was discussed in light of the precipitation-strengthening mechanism.