Effect of Zr on phase separation, mechanical and corrosion behavior of heterogeneous CoCrFeNiZrx high-entropy alloy

doi:10.1016/j.jmst.2021.08.062

Abstract

Abstract:

CoCrFeNi high entropy alloy (HEA) has attracted extensive attention due to its excellent corrosion resistance, but the low strength limits its engineering application prospects. In order to develop CoCrFeNi based HEAs with high strength, ductility and corrosion resistance, the effects of Zr content on the microstructure, mechanical properties and corrosion resistance of heterogeneous CoCrFeNiZr_x (x = 0, 0.25, 0.5 and 1) HEAs were investigated in this work. The results indicate that the increase of Zr content can significantly affect the phase stability of the alloy, and promote the formation of intermetallic compounds (Ni₇Zr₂ and/or Laves phase) and the transformation of solid solution from face-centered cubic (FCC) structure (x = 0, 0.25 and 0.5) to body-centered cubic (BCC) structure (x = 1). Reasonable control of the Zr content can endow the alloy excellent comprehensive properties. Especially, for CoCrFeNiZr_0.25 alloy, composed of FCC matrix and a small amount of Ni₇Zr₂ phases, the yield strength (∼655 MPa) is increased by nearly four times higher than that of Zr-free alloy, and it also has good ductility (fracture stain > 50%). Meanwhile, the corrosion resistance of CoCrFeNiZr_0.25 alloy is better than that of SS304. The EIS results show that the addition of Zr reduces the stability of the passive film on the alloy, which can be related to the content of the beneficial oxide in the passive film and the thickness of the passive film through XPS analysis. Moreover, the work functions of different phases in CoCrFeNiZr_x alloys were obtained by first-principles calculations, which further confirmed the selective corrosion mechanism of the CoCrFeNiZr_x alloy combining the experimental results.

Key words: High-entropy alloy, Microstructure, Corrosion resistance, First-principles, Work function

Wu Qi, Wenrui Wang, Xiao Yang, Lu Xie, Jiaming Zhang, Dongyue Li, Yong Zhang. Effect of Zr on phase separation, mechanical and corrosion behavior of heterogeneous CoCrFeNiZr_x high-entropy alloy[J]. J. Mater. Sci. Technol., 2022, 109: 76-85.

Figures/Tables 20

Fig. 1. XRD patterns of the CoCrFeNiZrx (x = 0, 0.25, 0.5 and 1) alloys.

Fig. 2. SEM backscatter electron images of CoCrFeNiZrx alloys: (a) Zr0, (b) Zr0.25, (c) Zr0.5, (d) Zr1.

Table 1. Nominal compositions of CoCrFeNiZrx alloys and the EDS results in different regions (at.%).

Alloy	Region	Co	Cr	Fe	Ni	Zr
Zr0	Normal composition	25.00	25.00	25.00	25.00	-
Zr0	P1	25.53	25.06	25.35	24.06	-
Zr0.25	Normal composition	23.53	23.53	23.53	23.53	5.88
	P1	25.18	28.11	28.50	18.15	0.05
	P2	22.75	11.97	15.74	33.35	16.19
Zr0.5	Normal composition	22.22	22.22	22.22	22.22	11.12
	P1	21.57	35.45	28.08	14.78	0.11
	P2	26.48	5.85	12.42	30.85	24.40
Zr1	Normal composition	20.00	20.00	20.00	20.00	20.00
	P1	13.71	52.44	28.36	5.15	0.34
	P2	22.48	6.58	15.50	27.84	27.60

Fig. 3. TEM micrographs and the selected area diffraction patterns (SADP) patterns of CoCrFeNiZrx alloys: (a) Zr0.25, (b) Zr0.5, (c) Zr1.

Fig. 4. Compressive engineering stress-strain curves of CoCrFeNiZrx HEAs.

Table 2. Mechanical properties of CoCrFeNiZrx HEAs.

Alloy	Yield strength, σ_y (MPa)	Fracture strength, σ_max (MPa)	Fracture strain, ε_max (%)
Zr0	180	No fracture	No fracture
Zr0.25	655	> 2765	> 50
Zr0.5	1649	2088	4.8
Zr1	-	1598	1.2

Fig. 5. Compression fracture morphologies of CoCrFeNiZrx alloys: (a,b) Zr0.25, (c) Zr0.5, (d) Zr1.

Fig. 6. Potentiodynamic polarization curves of CoCrFeNiZrx HEAs and SS304 in 3.5 wt% NaCl solution at room temperature.

Table 3. Corrosion dynamics parameters of CoCrFeNiZrx HEAs and SS304.

Alloy	i_corr (A cm^-2)	E_corr (V)	E_pit (V)
Zr0	2.132 × 10^-8	-0.126	0.650
Zr0.25	3.640 × 10^-8	-0.178	0.314
Zr0.5	4.305 × 10^-8	-0.247	0.245
Zr1	3.266 × 10^-7	-0.380	0.109
SS304	6.081 × 10^-8	-0.124	0.257

Fig. 7. SEM image and EDS mappings of the CoCrFeNiZrx alloys after the potentiodynaic polarization tests in 3.5 wt% NaCl solution (a) Zr0, (b) Zr0.25, (c) Zr0.5, (d) Zr1.

Fig. 8. Impedance responses of CoCrFeNiZrx HEAs and SS304: (a) Nyquist plots; (b) Bode plots; (c) equivalent circuit.

Table 4. Impedance fitting parameters of the CoCrFeNiZrx HEAs and SS304 by the equivalent circuit.

Alloy	R_s (Ω cm²)	Q (Ω^-1 cm^-2 s ^- ⁿ)	n	R_p (Ω cm²)
Zr0	21.63	1.915 × 10^-5	0.877	5.978 × 10⁵
Zr0.25	7.587	2.693 × 10^-5	0.899	4.108 × 10⁵
Zr0.5	7.374	2.390 × 10^-5	0.925	3.070 × 10⁵
Zr1	11.27	2.834 × 10^-5	0.914	1.251 × 10⁵
SS304	9.617	1.989 × 10^-5	0.883	3.127 × 10⁵

Fig. 9. Fine-scan XPS spectra of the passive film on the CoCrFeNiZrx HEA (a) Ni 2p3/2, (b) Co 2p3/2, (c) Fe 2p3/2, (d) Cr 2p3/2 and (e) Zr 3d.

Table 5. Ratio of each element compound to the metal state in the passive film of CoCrFeNiZrx alloy.

Element	Zr0	Zr0.25	Zr0.5	Zr1
Co_comp / Co_met	2.67	1.89	1.80	1.58
Cr_comp / Cr_met	10.06	7.42	5.07	4.00
Fe_comp / Fe_met	5.34	3.78	2.99	2.75
Ni_comp / Ni_met	1.72	0.92	0.80	0.49
Zr_comp / Zr_met	-	6.13	5.04	3.66

Table 6. Low index surfaces of the Ni7Zr2 and ZrCo2 phase.

Phase	Surface (hkl)	Energy (eV/A²)
Ni₇Zr₂	100	0.1365
	010	0.1488
	001	0.1342
	110	0.1398
	101	0.1299
	011	0.1460
	111	0.1396
Laves-ZrCo₂	100	0.1534
	110	0.1522
	111	0.1593

Fig. 10. Most preferable surface slab model of each phase for WF calculation.

Fig. 11. Work function histogram of each phase in CoCrFeNiZrx alloy.

Table 7. Calculated parameters of ΔHmix, Δδ, and Ω for CoCrFeNiZrx HEAs.

Alloy	ΔH_mix (kJ mol^-1)	Δδ (%)	Ω	Structure
Zr0	-3.75	1.18	5.72	FCC
Zr0.25	-10.35	6.41	2.30	FCC+Ni₇Zr₂
Zr0.5	-15.51	8.38	1.60	FCC+Laves
Zr1	-22.72	10.40	1.13	BCC+Laves

Table 8. Calculated parameters of VEC for solid solution of CoCrFeNiZrx alloys.

Phase	Zr0-FCC	Zr0.25-FCC	Zr0.5-FCC	Zr1-BCC
VEC	8.23	8.04	7.80	7.18

Fig. 12. Schematic diagram of the corrosion process of Zr0.25, Zr0.5 and Zr1 alloys in 3.5 wt% NaCl solution.

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