Effect of B content on microstructure and mechanical properties of (Co30Cr25Fe40Ni5)100-xBx high-entropy alloys

(Co30Cr25Fe40Ni5)(100-x)B-x (x = 0, 1, 2, 4, 6, 8, 10 and 12, x values in molar ratio) high-entropy alloys (HEAs) were prepared by vacuum arc melting, and the effect of B content on crystal structure, microstructure and mechanical properties of (Co30Cr25Fe40Ni5)(100-x)B-x HEAs were systematically studied. In this present work, the results showed that Co30Cr25Fe40Ni5 HEA was mainly composed of FCC and BCC solid solutions. With the addition of B element, the microstructure of the studied alloy first changed to hypoeutectic microstructure (x = 1, 2, 4, 6 and 8), then to fully eutectic microstructure with a mixture of FCC, BCC and boride phases (x = 10), and finally to hypereutectic microstructure (x = 12). With the increased of B content, the hardness of (Co30Cr25Fe40Ni5)(100-x)B-x HEAs show overall upward trend; The strength firstly increases and then decreases. The strengthening and toughening mechanisms of the studied alloys were second phase strengthening, fine grain strengthening and solid solution strengthening.