Microstructure evolution, Cu segregation and tensile properties of CoCrFeNiCu high entropy alloy during directional solidification

CoCrFeNiCu (equiatomic ratio) samples (empty set 8 mm) were directionally solidified at different velocities (10, 30, 60 and 100 mu m/s) to investigate the relationship between solidification velocity and microstructure formation, Cu micro-segregation as well as tensile properties. The results indicate that the morphology of the solid-liquid (S-L) interface evolves from convex to planar and then to concave with the increase of solidification velocity. Meanwhile, the primary and the secondary dendritic arm spacings decrease from 100 mu m to 10 mu m and from 20 mu m to 5 mu m, respectively. They are mainly influenced by the axial heat transfer and grain competition growth. During directional solidification, element Cu is repelled from the FCC phase and accumulates in the liquid owe to its positive mixing enthalpy with other elements. Tensile testing results show that the ultimate tensile strength (UTS) gradually increases from 400 MPa to 450 MPa, and the strain of the specimen prepared at the velocity of 60 mu m/s is higher than those of others. The fracture mode of all specimens is the mixed fracture containing both ductile fracture and brittle fracture, in which ductile fracture plays a fundamental role. In addition, the brittle fracture is induced by Cu segregation. The improvement of UTS is resulted from columnar grain boundary strengthening. (C) 2019 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.