A high-entropy alloy (HEA) is a multi-component alloy obtained by blending at least five metals with compositions of 5%~35%. Generally, HEAs are produced by arc melting and casting. The cast specimens undergo phase segregation and have heterogeneous microstructures. CoCrFeNi is the most investigated high-entropy alloy system. In this study, we used pure Co, Cr, Fe, Ni, and Zr pure powders as raw materials to prepare CoCrFeNiZrx (x = 0, 0.2, and 0.4) alloy powder by mechanical alloying (MA) and subsequent spark plasma sintering (SPS). X-ray diffraction (XRD) is performed on CoCrFeNi powder, which reveals that it has a single-phase face-centered cubic (FCC) structure. Furthermore, the CoCrFeNiZr0.2 alloy completely dissolves into the CoCrFeNi FCC matrix after 50 h of ball milling, and the structure is FCC as well as body-centered cubic (BCC) when the CoCrFeNiZr0.4 alloy is abundant. According to the XRD results of the sintered samples after SPS (1073 K, 50 MPa, 10 min holding), the sintered CoCrFeNi without Zr addition has the single-phase FCC structure. The FCC and BCC solid solution phases are formed in the CoCrFeNiZr0.2 alloy, indicating some amount of Zr is completely dissolved into the CoCrFeNi FCC matrix. The CoCrFeNiZr0.4 alloy sample has two microstructural domains: a single FCC phase without Zr and Zr-rich microstructure consisting of FCC, BCC, and some ω phases. Moreover, a brittle microstructure appears in the sample with the addition of Zr.
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