Microstructural evolution and mechanical properties of novel nanoparticles strengthened CoCrFeNi based high entropy alloy

A novel nanoparticles-strengthened CoCrFeNi(CuTi)x alloys (x=0.2∼1.0 at.%) were designed and synthesized to study the influence of CuTi concentrations on the microstructural evolution, phase composition and mechanical behavior of high-entropy alloys (HEAs) fabricated by Mechanical Alloying (MA) and Spark Plasma Sintering (SPS). The microstructure of the (CuTi)0.2 alloy is composed of CoCrFeNi-rich grey matrix, CrFe rich flap-like structure and Cu rich phases. With increasing CuTi content, the crystal structure changes from FCC to FCC+laves+η phases because of the formation of novel nanoparticles. A large amount of CoTi rich spherical particles and NiTi rich needle-like phases are observed in the (CuTi)0.6 alloy. The alloy with a density of 99.19% after SPS exhibits excellent comprehensive properties. Compared to single-phase FCC CoCrFeNi alloy, the compressive fracture strength increases from 871 MPa to 2083 MPa (∼139%). The solid solution strengthening of the FCC matrix and the formation of the secondary phases (Laves and η phases) are the main factors involved in improving the strengthening of the alloy. Most importantly, the Vickers hardness of the (CuTi)1.0 alloy is probably 1.7 times higher than that of the (CuTi)0.2 alloy.