Microstructures and mechanical properties of Nb-alloyed CoCrCuFeNi high-entropy alloys

doi:10.1016/j.jmst.2017.11.007

Abstract

Abstract:

Nb has a positive effect on improving the mechanical properties of metal materials, and it is expected to strengthen CoCrCuFeNi high-entropy alloys (HEAs) with outstanding ductility and relatively weak strength. In this paper, the alloying effects of Nb on the microstructural evolution and the mechanical properties of the (CoCrCuFeNi)_100-_xNb_x HEA were investigated systematically. The result shows that Nb promotes the phase transition from FCC (face-centered cubic) to Laves phase, and the volume fractions of Laves phase increase from 0% to 58.2% as the Nb content increases. Compressive testing shows that the addition of Nb has a positive effect on improving the strength of CoCrCuFeNi HEA. The compressive yield strength of (CoCrCuFeNi)_100-_xNb_x HEAs increases from 338 MPa to 1322 MPa and the fracture strain gradually reduces from 60.0% (no fracture) to 8.1% as the Nb content increases from 0 to 16 at.%. The volume fraction increase of hard Laves phase is the key factor for the strength increase, and the reduction of the VEC (valence electron concentration) value induced by the addition of Nb is beneficial for the increase of the Laves phase content in these alloys.

Key words: High-entropy alloys, Nb alloying, Phase transition, Mechanical properties, Laves phase

Gang Qin, Shu Wang, Ruirun Chen, Xue Gong, Liang Wang, Yanqing Su, Jingjie Guo, Hengzhi Fu. Microstructures and mechanical properties of Nb-alloyed CoCrCuFeNi high-entropy alloys[J]. J. Mater. Sci. Technol., 2018, 34(2): 365-369.

Figures/Tables 8

Fig. 1. XRD patterns of (CoCrCuFeNi)100-xNbx (x = 0,4,8,12,16) HEAs. Nb content prompts the transformation of the crystal structure from FCC phase structure to Laves phase structure.

Fig. 2. SEM micrographs of as-cast (CoCrCuFeNi)100-xNbx alloys. (a, c, e, g, i. Backscattered images of Nb-0 and Nb-4, Nb-8, Nb-12, Nb-16 alloys, respectively; b, d, f, h, j. Partial magnification backscattered images of Nb-0 and Nb-4, Nb-8, Nb-12, Nb-16 alloys, respectively).

Table 1 Contents of different elements in regions I-III from EDS.

Region	Co (%)	Cr (%)	Cu (%)	Fe (%)	Ni (%)	Nb (%)
I	16.61	15.75	3.51	8.90	11.47	43.76
II	20.76	20.63	12.15	18.41	20.60	7.44
III	20.69	22.50	15.13	20.83	18.91	1.94

Fig. 3. Compressive stress-strain curves of as-cast (CoCrCuFeNi)100-xNbx (x = 0-16) HEAs.

Fig. 4. Relationship between the yield strength of (CoCrCuFeNi)100-xNbx and volume fraction of Laves phase with Nb content increases.

Fig. 5. Atomic diffusion model for (CoCrCuFeNi)100-xNbx: (a) The matrix of Nb-0, FCC; (b, c) The atomic replacement and solid solution of Nb-4 and Nb-8, respectively; (d) The matrix, FCC phase and the Laves phase; (e) Crystal grows.

Table 2 VEC values of some metallic elements.

Element	Co	Cr	Cu	Fe	Ni	Nb
VEC value	9	6	11	8	10	5

Fig. 6. Valence electron concentration (VEC) of (CoCrCuFeNi)100-xNbx (x = 0, 4, 8, 12, 16) HEAs.

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