Effect and Mechanism of B Microalloying on the Microstructure and Mechanical Properties of CoNiV Medium-Entropy Alloy
NAN Yong1, GUAN Xu1, YAN Haile1(), TANG Shuai2, JIA Nan1(), ZHAO Xiang1, ZUO Liang1
1 Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), School of Materials Science and Engineering, Northeastern University, Shenyang 110819, China 2 State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang 110819, China
Cite this article:
NAN Yong, GUAN Xu, YAN Haile, TANG Shuai, JIA Nan, ZHAO Xiang, ZUO Liang. Effect and Mechanism of B Microalloying on the Microstructure and Mechanical Properties of CoNiV Medium-Entropy Alloy. Acta Metall Sin, 2024, 60(12): 1647-1655.
CoNiV is a novel medium-entropy alloy with excellent mechanical properties. Currently, alloying CoNiV with Al has been extensively employed to improve its mechanical strength. Unfortunately, the microstructure of CoNiV changes from a single phase to a dual phase due to the addition of Al, which considerably reduces its corrosion resistance. Therefore, developing new strategies to improve its mechanical properties is imperative. In this study, the microstructure and static tensile mechanical properties of (CoNiV)100 - x B x alloys (x = 0, 0.1, and 0.2, atomic fraction, %) are systematically investigated. The results revealed that the strength and ductility of CoNiV can be significantly improved by doping a small amount of B. With the introduction of 0.2%B, the yield strength, ultimate tensile strength, and elongation of CoNiV are improved, increasing by 12%, 10%, and 30%, respectively. The crystal structure, grain size, crystallographic orientation, and plastic deformation mechanism of CoNiV are not affected due to microalloying with B. At room temperature, (CoNiV)99.8B0.2 exhibits fcc structure. The plastic deformation mechanism during static tensile deformation is manifested as dislocation slip, while martensitic transformation and twin effects induced by stress are not observed. The results of the nanohardness tests indicated that doping with trace amounts of B could remarkably enhance the grain/twin boundary hardness, confirming the grain/twin boundary strengthening effect of B on CoNiV. The strengthening of grain/twin boundaries leads to increased resistance of dislocations and provides the ability to hinder crack expansion, resulting in the simultaneous enhancement of the strength and ductility of (CoNiV)99.8B0.2. Moreover, the B element dissolved into the matrix would serve as a pinning site for dislocation, thus contributing to the increased strength of CoNiV.
Fig.1 Static tensile mechanical properties of CoNiV, (CoNiV)99.9B0.1, and (CoNiV)99.8B0.2 alloys at room temperature (a) engineering stress-strain curves (b) true stress-true strain curves (c) strain-hardening rate curves
Alloy
Yield strength / MPa
Ultimate tensile strength / MPa
Fracture elongation / %
CoNiV
968.7
1291.7
33.6
(CoNiV)99.9B0.1
1030.1
1331.6
34.7
(CoNiV)99.8B0.2
1087.4
1419.3
44.1
Table 1 Yield strengths, ultimate tensile strengths, and fracture elongations of CoNiV, (CoNiV)99.9B0.1, and (CoNiV)99.8B0.2 alloys
Fig.2 XRD spectra (a1-a3), phase distribution maps (fcc phase colored in red) overlapped with Σ3 (white), Σ5 (green), Σ7 (blue), and Σ9 (purple) coincidence site lattice (CSL) boundaries (b1-b3), and orientation maps represented by inverse pole figures (IPFs) (c1-c3) for CoNiV (a1-c1), (CoNiV)99.9B0.1 (a2-c2), and (CoNiV)99.8B0.2 (a3-c3) alloys (RD—rolling direction, ND—normal direction, LD—loading direction)
Fig.3 Quantitative characteristics of the CoNiV, (CoNiV)99.9B0.1, and (CoNiV)99.8B0.2 alloys (a) relation between mean grain size (dm) and the content of doped B (b) grain size (d) distributions (c) length fractions of Σ3, Σ5, Σ7, and Σ9 CSL boundaries against the total length of all large angle grain boundaries
Fig.4 Crystal structures and microstructure characterizations of the deformed CoNiV (a1-e1) and (CoNiV)99.8B0.2 (a2-e2) alloys (a1, a2) XRD spectra (b1, b2) phase distribution maps (fcc phase colored in red) (c1, c2) orientation maps represented by IPFs (d1, d2) kernel average misorientation (KAM) maps (e1, e2) TEM images of the dislocation plugging at the interface (Insets show the selected area electron diffraction patterns. The subscripts γ and T denote γ matrix and twin, respectively)
Fig.5 Hardnesses of grain interior (HGI), grain boundaries (HGB), and twin boundaries (HTB) (Inset shows the schematic of nanoindentation test)
Alloy
HGB / HGI
HTB / HGI
CoNiV
1.19
1.21
(CoNiV)99.8B0.2
1.28
1.46
Table 2 Ratios of hardness measured at different regions of CoNiV and (CoNiV)99.8B0.2 alloys
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