Strengthening FCC-CoCrFeMnNi high entropy alloys by Mo addition

doi:10.1016/j.jmst.2018.10.009

Abstract

Abstract:

In order to strengthen the face-centered-cubic (FCC) type CoCrFeMnNi high entropy alloys (HEAs), different contents of Mo (0-16 at.%, similarly hereinafter) were alloyed. Phase evolution, microstructure, mechanical properties and related mechanism of these HEAs were systematically studied. The results show that sigma phase is appeared with addition of Mo, and the volume fraction of it increases gradually from 0 to 66% with increasing Mo content. It is found that Mo is enriched in sigma phase, which indicates that Mo element is beneficial to form sigma phase. Compressive testing shows that the yield strength of the alloys increases gradually from 216 to 765 MPa, while the fracture strain decreases from 50% (no fracture) to 19% with increasing of Mo. The alloy exhibits the best compressive performance when Mo content reaches 11%, the yield strength, fracture strength and fracture strain are 547 MPa, 2672 MPa and 44% respectively. The increased volume fraction of sigma phase plays an important role in improving the compressive strength of (CoCrFeMnNi)_100-_xMo_x HEAs.

Key words: High-entropy alloys, Mo element, Phase transition, Mechanical properties, Valence electron concentration

Gang Qin, Ruirun Chen, Huiting Zheng, Hongze Fang, Liang Wang, Yanqing Su, Jingjie Guo, Hengzhi Fu. Strengthening FCC-CoCrFeMnNi high entropy alloys by Mo addition[J]. J. Mater. Sci. Technol., 2019, 35(4): 578-583.

Figures/Tables 10

Table 1 Effect of alloy elements on the phase structure and mechanical properties of HEAs.

Basis material	Alloy element	Second phase	Yield strength	Fracture strain
CoCrFeNi [34]	Mo	δ	Up	Down
CoCrFeNi [35]	Ti	Laves	Up	Down
CoCrFeNi [37]	Nb	Laves	Up	Down
AlCoCrFeNi [38]	Cu	FCC	Down	Up
CoCrFeMnNi [23]	Al	BCC	Up	Down
CoCrFeNi [24]	Mo	σ	Up	Down
CoCrFeMnNi [25]	V	Tetragonal	Up	Down
CoCrCuFeNi [26]	Ti	Laves	Up	Down
AlCoCrFeNi [47]	Ni	FCC	Down	Up
CoCrCuFeNi [40]	Mo	BCC	Up	Down
AlCoCrFeNi [48]	Co	FCC	Down	Up
CoCrCuFeNi [49]	Nb	Laves	Up	Down
CoCrFeNi [50]	Nb	Laves	Up	Down

Fig. 1. XRD patterns of (CoCrFeMnNi)100-xMox (x = 0, 4, 8, 12, 16) HEAs.

Fig. 2. Compressive engineering stress-strain curves of (CoCrFeMnNi)100-xMox (x = 0-16) HEAs.

Fig. 3. SEM-BSE images of (CoCrFeMnNi)100-xMox HEAs. (a) Mo free. (b) Mo-4. (c) Mo-8. (d) Mo-12. (e) Mo-16. (f) High-magnification of the oxide in Mo-12.

Fig. 4. TEM micrographs of (CoCrFeMnNi)88Mo12 HEA. (a) The matrix and the secondary phase. (b) The interface between FCC and sigma phase. (c) The diffraction spot of the region A.

Fig. 5. EDS maps of (CoCrFeMnNi)88Mo12 HEA. (a). HAADF image. (b), (c), (d), (e), (f), and (g) Element distribution of Co, Cr, Fe, Mn, Ni and Mo, respectively.

Fig. 6. Schematic transformation from the FCC phase to sigma phase with increasing of Mo content.

Fig. 7. Relationships between yield strength and volume fraction of sigma phase with Mo content.

Fig. 8. VEC values and Pauling electronegativity values of (CoCrFeMnNi)100-xMox HEAs. (a) VEC. (b) Pauling electronegativity value.

Fig. 9. Enthalpy of mixing (ΔHmix) of (CoCrFeMnNi)100-xMox HEAs.

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