Excellent strength-ductility synergy properties of gradient nano-grained structural CrCoNi medium-entropy alloy

doi:10.1016/j.jmst.2021.09.058

Abstract

Abstract:

Tailoring heterogeneities could bring out excellent strength-ductility synergy properties. A gradient nano-grained (GNG) structure, i.e. grain size range from nanometer (∼50 nm) at topest surface layer to micrometer (∼1.3 μm) at center layer, was successfully introduced into CrCoNi medium-entropy alloy (MEA) by means of high energy shot peening in this work. Experimental results demonstrated that this GNG CrCoNi MEA shows excellent strength and ductility combination properties, exhibiting high yield strength and ultimate tensile strength of ∼1215 MPa and ∼1524 MPa, respectively, while remaining a good ductility of ∼23.0%. The extraordinary hetero-deformation induced (HDI) hardening origins from heterogeneous structure, i. e. GNG structure, which contributes to the majority strength enhancement. Dynamical reinforced heterogeneous structure during tension process results in the enhanced HDI hardening effect, which facilitates excellent ductility and strain hardening capacity at high-level strength. Our work provide not only a feasible and effective way to strengthen the CrCoNi MEA, and other low stacking faults energy (SFE) materials, but also an useful insight to understanding HDI hardening in heterogeneous structure.

Key words: Medium-entropy alloy, Gradient nano-grained structure, High energy shot peening, Hetero-deformation induced hardening, Strength-ductility synergy

Wenjie Lu, Xian Luo, Dou Ning, Miao Wang, Chao Yang, Miaoquan Li, Yanqing Yang, Pengtao Li, Bin Huang. Excellent strength-ductility synergy properties of gradient nano-grained structural CrCoNi medium-entropy alloy[J]. J. Mater. Sci. Technol., 2022, 112: 195-201.

Figures/Tables 8

Fig. 1. EBSD investigation for CG CrCoNi MEA, (a) IPF image, (b) grain size distribution and (c) grain boundaries misorientation.

Fig. 2. Microstructure investigation for CG CrCoNi sample processed by HESP treatment, (a)-(c) representative cross-sectional TEM BF image at topmost surface, ～100 μm-deep layer and center layer, respectively, (a) NG structure, inset is the corresponding SADP, (b) twinned UFG structure, the red arrows indicate twin structure, (c) deformation CG structure, the yellow arrows indicate high-density dislocations, (d) and (e) schematic of plate tensile sample processed by HESP, showing two GNG layers sandwiching a deformation CG core, (f) average grain size distribution from center to surface layers.

Fig. 3. Mechanical properties of CG and GNG CrCoNi MEA samples: (a) hardness distributions from surface to center, (b) typical engineering stress-strain curves, and (c) work-hardening rate curves, and the properties of an UFG CrCoNi MEA sample [15] was presented for comparison.

Fig. 4. The comparison results of current GNG CrCoNi MEA with previous literatures about CrCoNi and CrCoNi-based MEAs by different strengthening methods [[32], [33], [34], [35], [36]].

Table 1. Alloys, processing, microstructure and tensile properties of CrCoNi and CrCoNi-based MEAs obtained from previous works and present work.

Alloys	Processing^a	Microstructure	σ_y (MPa)	σ_UTS (MPa)	ε (%)	Refs.
CrCoNi	AC+1473 K/10 h/WQ+CR(80%)+973 K/1 h/WQ	FCC,GS=1.3 μm	750	1096	41.2	Present work
	AC+1473 K/10 h/WQ+ CR(80%)+973 K/1 h/WQ+HESP	FCC,GNG	1215	1524	23
	AC+CR(90%)+1473 K/12 h/WQAC+CR(90%)+1473 K/12 h/WQ+HPT+1173 K/20 min/WQAC+CR(90%)+1473 K/12 h/WQ+HPT+973 K/30 min/WQAC+CR(92%)+1473 K/24 h/WQ+1198 K/1 h/WQAC+1472 K/24 h/WQ+CR(60%)+1073 K/1 hMA+SPSAC+HPTAC+HPT+1073 K/1 h/WQAC+1473 K/12 h/WQ+HF1323 K(90%)+CR(95%)+873 K/1 h/WQ	FCC,GS=111 μmFCC,GS=1.47 μmFCC,GS=0.199 μmFCC,GS=13 μmFCC,GS=∼5-50 μmFCC+BCCFCC,GS=50 nmFCC,GS=3.3 μmFCC, HGS	27038699329843565219015121150	5876801080863875102420958361270	8058.620.8677525.94.73731	[15][32][7][33][34][25]
	SLM	HM	651	907	35.8	[35]
(CoCrNi)₉₂Al₆Ta₂	AC+1498 K/24 h/WQ+CR(70%)+1423 K/3 min/WQ	FCC,GS=8 μm	595	998	52	[36]
(CoCrNi)₉₄Al₃Ti₃	CR(66%)+1433 K/3 min +1073 K/2 h/WQ	FCC+γ′,GS=67 μm	750	1300	44	[19]

Fig. 5. (a) LUR tensile test results of CG and GNG CrCoNi MEA sample, (b) an enlarged view for comparison of hysteresis loops, and (c) calculated HDI hardening of GNG CrCoNi MEA sample, where σr and σu are reloading yield stress and unloading yield stress of an LUR loop [39].

Fig. 6. Representative deformation characters in tensile tested GNG sample by TEM investigation at topest surface, ～100 μm-deep layer and center layer, respectively, (a) a deformed NG in topmost layer, (b) a HRTEM image showing deformation-induced NTs with extended SFs, (c) a deformed twinned-UFG in ～100 μm-deep layer, (d) a HRTEM image showing the enlarge view of different DTs and SFs structures, (e) a deformed CG in center layer, the inset SADP indicate the DTs structure, (f) a HRTEM image showing the thermal twin evolve into incoherent HAGB, the inset is the corresponding FFT.

Fig. 7. Schematics of microstructure evolution in GNG sample with increasing applied stress, which exhibiting a dynamical reinforced heterogeneous grain structure during tensile process, (a) original GNG structure: (b) heterogeneous deformation results in the HDI stress in CG structure, and (c) a more heterogeneous structure forms resulting from DTs and/or SFs separating grains.

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