Towards ultrastrong and ductile medium-entropy alloy through dual-phase ultrafine-grained architecture

doi:10.1016/j.jmst.2022.02.052

Abstract

Abstract:

Advanced materials with superior comprehensive mechanical properties are strongly desired, but it has long been a challenge to achieve high ductility in high-strength materials. Here, we proposed a new V_0.5Cr_0.5CoNi medium-entropy alloy (MEA) with a face-centered cubic/hexagonal close-packed (FCC/HCP) dual-phase ultrafine-grained (UFG) architecture containing stacking faults (SFs) and local chemical order (LCO) in HCP solid solution, to obtain an ultrahigh yield strength of 1476 MPa and uniform elongation of 13.2% at ambient temperature. The ultrahigh yield strength originates mainly from fine grain strengthening of the UFG FCC matrix and HCP second-phase strengthening assisted by the SFs and LCO inside, whereas the large ductility correlates to the superior ability of the UFG FCC matrix to storage dislocations and the function of deformation-induced SFs in the vicinity of the FCC/HCP boundary to eliminate the stress concentration. This work provides new guidance by engineering novel composition and stable UFG structure for upgrading the mechanical properties of metallic materials.

Key words: Medium-entropy alloy, FCC/HCP dual-phase, Strength and ductility, Ultrafine-grained (UFG), Multiple hardening mechanisms

Zhen Chen, Hongbo Xie, Haile Yan, Xueyong Pang, Yuhui Wang, Guilin Wu, Lijun Zhang, HuTang, Bo Gao, Bo Yang, Yanzhong Tian, Huiyang Gou, Gaowu Qin. Towards ultrastrong and ductile medium-entropy alloy through dual-phase ultrafine-grained architecture[J]. J. Mater. Sci. Technol., 2022, 126: 228-236.

Figures/Tables 11

Fig. 1. XRD patterns of the alloys with different treatment states. (a) XRD patterns of CR, CR725, and CR900, respectively. (b) Lebail refinement of the XRD pattern of CR725.

Fig. 2. Microstructure of CR. (a) SEM-BSE image showing typical deformation microstructure. (b) TEM-BF image exhibiting high-density dislocations and the corresponding SAED from one grain. (c) HRTEM image of deformed grains showing a large number of SF bundles inside FCC grains. The inset is the fast Fourier transform (FFT) pattern corresponding to the yellow box. (d) HAADF-STEM image showing a representative SF.

Fig. 3. Fully-recrystallized dual-phase UFG architecture in CR725. (a) TKD phase map showing FCC/HCP dual-phase UFG structure. The black and yellow lines are related to high angle boundaries and twin boundaries, respectively. (b) TKD inverse pole figure (IPF) map. (c) TEM-BF image of dual-phase UFG structure showing low-density dislocations. (d) HRTEM image of FCC/HCP dual-phase region. The inset is the FFT pattern of the region marked by the cyan box in HCP lamellae. (e) Corresponding SAED patterns of the FCC solid matrix and the HCP solution lamellae. (f) HAADF-STEM image of the atomic arrangements of FCC/HCP dual-phase region.

Fig. 4. Microstructure of CR900. (a) EBSD inverse pole figure (EBSD-IPF) map. (b) EBSD phase map showing FCC single-phase FG microstructure. The black and yellow lines are related to high angle boundaries and twin boundaries, respectively.

Table 1. Phase constituents, volume fraction, and grain size (lamellar thickness) of CR, CR725, and CR900.

Sample	FCC		HCP
Sample	Fraction (%)	Size (μm)	Fraction (%)	Thickness (μm)
CR	100	/	0	/
CR725	72.6	0.361 ± 0.208	27.4	0.126 ± 0.61
CR900	100	2.02 ± 1.04	0	/

Fig. 5. Corresponding chemical compositions of FCC matrix and HCP lamellae in dual-phase UFG structure. (a) STEM image showing FCC/HCP dual-phase UFG structure. (b) STEM-EDS mapping of the region identical to (a), exhibiting the distributions of V, Cr, Co, and Ni, respectively, in the two phases. (c) Enlarged view of the region marked by the red box in (a) and the corresponding EDS composition profiles of the selected region marked by the yellow line.

Fig. 6. Tensile properties of (a) CR, CR725, and CR900 at room temperature. The inset presents the corresponding strain hardening rate (dσ/dε) of CR725. (b) Maps of εUE vs σYS of typical HEAs and MEAs at room temperature.

Table 2. Tensile properties of CR, CR725, and CR900, respectively.

Sample	YS (MPa)	UTS (MPa)	UE (%)	TE (%)
CR	1693	1822	1.7	3.5
CR725	1476	1578	13.2	16.8
CR900	682	1123	35.9	40.1

Fig. 7. Theoretically generalized stacking fault energies (SFEs) at 0 K for the FCC structured (a, c) VxCr1-xCoNi (x = 0, 0.5, and 1) alloys and (b, d) V14.35Cr21.12Co31.84Ni32.69 (measured composition of the FCC phase, termed as V14.35), V0.5Cr0.5CoNi (nominal composition, termed as V16.67), and V19.45Cr12.48Co33.31Ni34.76 (measured composition of the HCP phase, termed as V19.45). γisf, γusf, and γutf represent the intrinsic SFE, the unstable SFE, and the unstable twinning fault energy, respectively. (c) and (d) are the enlarged views of the local regions marked by the purple boxes in (a) and (b), respectively.

Fig. 8. Relative total energy between the paramagnetic FCC (FCC PM) and the paramagnetic HCP (HCP PM) structured phases plotted as a function of Wigner-Seitz radius for the V0.5Cr0.5CoNi alloy at 0 K.

Fig. 9. Plastic deformation mechanisms of CR725. (a) TEM-BF image after a tensile fracture. (b) SAED patterns corresponding to No. 1 FCC grain and No. 2 HCP grain in (a). The inset in the SAED of the HCP phase shows the enlarged view corresponding to the blue box. (c) View of another electron beam incident direction corresponding to the region in (a). The inset shows the corresponding SAED of No. 1 FCC grain. (d) Enlarged view of the identical region outlined by the yellow box in (c). (e) HRTEM image of SFs. The inset shows an FFT image corresponding to the cyan box. (f) Corresponding inverse fast Fourier transform (IFFT) image showing a representative SF.

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