Microstructure and mechanical behavior in a spherical L1 2 nanoprecipitates strengthened Ni 35 Co 27.5 Cr 27.5 Al 5 Ti 5 high-entropy alloy fabricated by direct casting and heat treatment

Intricate thermomechanical processing (TMP) including casting, homogenization, cold rolling, annealing and aging, has been widely used to achieve L1 2-strengthened CoCrNi-based medium- and high-entropy alloys (MEAs/ HEAs). However, a large number of parts are usually manufactured by direct casting. We fabricated a nonequiatomic Ni 35 Co 27.5 Cr 27.5 Al 5 Ti 5 HEA consisting of spherical L1 2 nanoprecipitates and FCC matrix using casting, homogenization and aging. Notably, both the homogenized and aged Ni 35 Co 27.5 Cr 27.5 Al 5 Ti 5 HEA samples exhibit equiaxed grains, with the latter showcasing uniformly distributed spherical L1 2 nanoprecipitates in the FCC matrix. The average size and volume fraction of these nanoprecipitates in the aged HEA are-10.9 nm and-41.9 vol%, respectively. Mechanical testing reveals remarkable improvements in the aged HEA, with a yield strength of-916 MPa, an ultimate tensile strength of-1220 MPa, and a total elongation of-21.5 %. The observed enhancement is primarily attributed to precipitation strengthening mechanism. Digital image correlation (DIC) analysis of tensile deformation demonstrates uniform strain distribution in both the homogenized and aged HEA samples. In addition, electron channeling contrast imaging (ECCI) and transmission electron microscopy (TEM) examinations unveil a complex interplay of deformation mechanisms, including multiple planar slip, dislocation shearing coherent L1 2 precipitates, superpartials, stacking fault (SF) networks, and Lomer-Cottrell (L-C) locks. These mechanisms collectively impede dislocation motion, thereby contributing to the excellent strain-hardening ability and outstanding strength-ductility synergy of the HEA.