Effect of molybdenum content on the microstructure and corrosion behavior of FeCoCrNiMox high-entropy alloys

doi:10.1016/j.jmst.2019.10.020

Abstract

Abstract:

The effect of Mo content on the microstructure revolution and corrosion behavior of cast FeCoCrNiMo_xhigh-entropy alloys in chloride environments were investigated. Results indicate that FeCoCrNi and FeCoCrNiMo_0.1alloys are in single FCC solid solution. The precipitates form in FeCoCrNiMo_0.3 and increase in FeCoCrNiMo_0.6 alloys. Pitting occurs on FeCoCrNi and FeCoCrNiMo_0.1alloys while FeCoCrNiMo_0.3 and FeCoCrNiMo_0.6alloys suffer from preferential localized corrosion at the regions depleted in Cr and Mo. The higher Cr₂O₃/Cr(OH)₃ ratio and the incorporation of Mo oxides make the passive film more protective and the corrosion resistance of the FeCoCrNiMo_0.1alloy is thus enhanced. The correlation between microstructure and corrosion behavior and the corresponding corrosion mechanism were clarified.

Key words: High-entropy alloys, Corrosion behavior, Precipitate

Chunduo Dai, Tianliang Zhao, Cuiwei Du, Zhiyong Liu, Dawei Zhang. Effect of molybdenum content on the microstructure and corrosion behavior of FeCoCrNiMo_x high-entropy alloys[J]. J. Mater. Sci. Technol., 2020, 46: 64-73.

Figures/Tables 15

Table 1 Chemical compositions of as-cast FeCoCrNiMox alloys (wt%).

Alloy	Fe	Co	Ni	Cr	Mo
Mo₀	24.77	26.14	26.03	23.06	-
Mo_0.1	23.76	25.07	24.97	22.12	4.08
Mo_0.3	21.97	23.18	23.08	20.45	11.32
Mo_0.6	19.74	20.82	20.74	18.37	20.33

Fig. 1. XRD patterns of FeCoCrNiMox HEAs with different Mo contents.

Fig. 2. Microstructures of the FeCoCrNiMox alloys in different statuses: (a-d) SEM microstructure of the alloys after metallographic etching; (e-h) SEM microstructure of the alloys after electrochemical polishing. (a, e) Mo0, (b, f) Mo0.1, (c, g) Mo0.3 and (d, h) Mo0.6.

Table 2 Chemical composition analysis of the regions marked in Fig. 2 (wt%).

Alloy	Structure and phase	Fe	Co	Ni	Cr	Mo
Mo_0.1	Dendrite, 1	24.27	25.52	24.94	21.23	4.04
Mo_0.1	Inter-dendrite, 2	22.89	23.90	24.32	23.07	5.82
Mo_0.3	Dendrite, 1	22.51	24.44	24.07	19.95	9.03
	Inter-dendrite, 2	21.54	21.96	22.30	21.23	12.97
	Second phase, 3	17.51	19.01	17.48	22.05	23.95
Mo_0.6	Dendrite, 1	22.03	22.69	22.78	18.61	13.89
Mo_0.6	Second phase, 2	18.43	19.78	17.21	19.91	24.67

Fig. 3. TEM image and SEAD patterns of the as-cast Mo0.6 alloy.

Fig. 4. Potentiodynamic polarization curves of the alloys with various Mo contents: (a) 0.25 M NaCl solution, (b) 1 M NaCl solution.

Fig. 5. Corrosion morphology of Mo0 and Mo0.1?Mo0.6 alloys after immersion in 6 wt% FeCl3 + 0.05 M HCl solution at 60 °C for 36 h and 120 h, respectively: (a) Mo0 alloy after immersion for 36 h, (b) Mo0.1 alloy after immersion for 120 h, (c, d) Mo0.3 alloy after immersion for 120 h, (e, f) Mo0.6 alloy after immersion for 120 h.

Fig. 6. SEM and 3D images of the anodic dissolution morphologies of the FeCoCrNiMox alloy after polarization tests in 1 M NaCl at ambient temperature, (a1?c1) Mo0 alloy (a2?c2) Mo0.1 alloy, (a3?c3) Mo0.3 alloy, (a4?c4) Mo0.6 alloy. (a1?a4) and (b1?b4) SEM images and 3D morphologies after polarization for 600 s; (c1?c4) 3D morphologies after polarization for 1800s.

Fig. 7. EPMA mapping of the elements in the Mo0, Mo0.1 and Mo0.3 alloys after polishing away the anodic dissolution layer: (a1) Mo0 alloy, (a2) Mo0.1 alloy, (a3) Mo0.3 alloy.

Fig. 8. Current density varies during the formation of passive films at 0 VSCE in deaerated 1 M NaCl solution.

Table 3 Atomic compositions of Fe, Co, Cr, Ni and Mo elements in the passive films (at.%).

Alloys	Fe	Co	Cr	Ni	Mo
Mo₀	23.95	15.73	42.35	17.97
Mo_0.1	29.32	14.86	37.68	14.21	3.93

Fig. 9. XPS spectra of the passive film formed on the Mo0 and Mo0.1 alloys: (a) Cr 2p3/2, (b) Fe 2p3/2, (c) Ni 2p3/2, (d) Co 2p3/2, (e) O 1s and (f) Mo 3d peaks in Mo0.1 alloy.

Table 4 Binding energies and contents of the main components extracted from XPS analysis.

Elements	Compounds	Binding Energy (eV) Mo₀ alloy/Mo_0.1 alloy	Mo₀ alloy (%)	Mo_0.1 alloy (%)
Fe 2p_3/2	Fe₂O₃	710.4 / 710.4	5.17	9.42
	FeOOH	711.8 / 711.8	19.85	23.19
Cr 2p_3/2	Cr₂O₃	575.9 / 576.2	18.67	20.25
	Cr(OH)₃	577.1 / 577.1	35.97	28.33
Ni 2p_3/2	NiO	853.5 / 853.9	7.32	1.50
	Ni(OH)₂	855.8 / 855.5	3.61	5.30
Co 2p_3/2	Co₃O₄	780.1 / 779.4	7.78	2.36
	Co(OH)₂	782.1 / 781.0	1.63	5.80
Mo 3d	Mo⁴⁺ 3d3/2	231.6	—	3.85
	Mo⁴⁺ 3d5/2	228.5	—
	Mo⁶⁺ 3d3/2	235.6	—
	Mo⁶⁺ 3d5/2	232.4	—

Fig. 10. AES sputtering depth profiles of the alloy elements in the passive film formed on Mo0 and Mo0.1 alloys.

Fig. 11. Topographical and Volta potential mappings of the Mo0.3 alloy and line-scan section analysis results obtained from the SKPFM analysis. (a) Topographical mappings (b) Volta potential mappings (c) line-scan section analysis obtained from Fig. 11(b).

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