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

doi:10.1016/j.jmst.2019.08.019

Abstract

Abstract:

CoCrFeNiCu (equiatomic ratio) samples (ø 8 mm) were directionally solidified at different velocities (10, 30, 60 and 100 μm/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 μm to 10 μm and from 20 μm to 5 μm, 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 μm/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.

Key words: High entropy alloy, Directional solidification, Segregation, Tensile properties

Huiting Zheng, Ruirun Chen, Gang Qin, Xinzhong Li, Yanqing Su, Hongsheng Ding, Jingjie Guo, Hengzhi Fu. Microstructure evolution, Cu segregation and tensile properties of CoCrFeNiCu high entropy alloy during directional solidification[J]. J. Mater. Sci. Technol., 2020, 38: 19-27.

Figures/Tables 15

Fig. 1. Directionally solidified CoCrFeNiCu HEA sample.

Fig. 2. S-L interface of CoCrFeNiCu HEA directionally solidified at 10 μm/s: (a) S-L interface; (b) mushy zone; (c) dendrite and dendritic tip; (d) inter-dendritic region.

Fig. 3. Solidification front of CoCrFeNiCu HEA at different solidification velocities: (a) 10 μm/s; (b) 30 μm/s; (c) 60 μm/s; (d) 100 μm/s.

Fig. 4. Dendritic arm spacing of the samples prepared at different velocities: (a) the primary dendritic arm spacing; (b) the secondary dendritic arm spacing.

Fig. 5. Evolution of S-L interface during directional solidification.

Fig. 6. Testing positions by EDS in CoCrFeNiCu HEA directionally solidified at velocity 10 μm/s: (a) solidification front; (b) mushy zone. A-inter-dendritic region; B-dendrite; C-grains in the mushy zone; D-grain boundary in the mushy zone.

Table 1 Chemical composition (at.%) of directionally solidified CoCrFeNiCu HEA.

Position	Cu	Cr	Fe	Co	Ni
A	76.68	4.64	4.04	3.68	10.95
B	16.41	21.22	23.26	19.54	19.56
C	9.63	24.26	22.76	23.52	19.84
D	70.9	6.91	6.48	5.62	10.09

Fig. 7. EPMA selected area of CoCrFeNiCu HEA directionally solidified at 10 μm/s.

Fig. 8. EPMA-BSE image of the element distribution map in the dendrite tip of CoCrFeNiCu HEA directionally solidified at 10 μm/s.

Fig. 9. EPMA-BSE image of the elements distribution map of the mushy zone of CoCrFeNiCu HEA directionally solidified at 10 μm/s.

Fig. 10. Schematic diagram of CoCrFeNiCu HEA solidification process.

Fig. 11. Tensile properties of CoCrFeNiCu HEA samples.

Fig. 12. Fracture morphologies of directionally solidified CoCrFeNiCu HEA specimens: (a) 10 μm/s; (b) 30 μm/s; (c) 60 μm/s; (d) 100 μm/s.

Fig. 13. EDS result of the fracture surface of directionally solidified CoCrFeNiCu HEA at dimple region.

Fig. 14. EDS result of the fracture surface of the directionally solidified CoCrFeNiCu HEA at area of the dimple and tearing ridge.

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