Significantly improved corrosion resistance of Mg-15Gd-2Zn-0.39Zr alloys: Effect of heat-treatment

doi:10.1016/j.jmst.2019.03.027

Abstract

Abstract:

The effects of heat-treatment on corrosion behavior of Mg-15Gd-2Zn-0.39 Zr alloys were investigated through microstructure characterization, corrosion tests, and scanning Kelvin probe force microscope (SKPFM) analysis. In long-term corrosion experiments, the corrosion rates of Mg-Gd-Zn-Zr alloys were mainly determined by the effects of micro-galvanic corrosion. During heat-treatment, the β-(Mg,Zn)₃Gd eutectic phase in as-cast alloys transformed into a long-period stacking ordered (LPSO) phase, coupled with the precipitation of small precipitates. As heat-treatment proceeded, the local potential and the volume fraction of the LPSO phases reduced gradually compared with the eutectic phase, which resulted in a remarkable decrease of the micro-galvanic effect between the second phase and Mg matrix. As a result, the corrosion resistance of heat-treated alloys improved significantly.

Key words: Mg-Gd-Zn-Zr alloy, Corrosion behavior, Heat-treatment, LPSO phase, SKPFM

Jing Liu, Lixin Yang, Chunyan Zhang, Bo Zhang, Tao Zhang, Yang Li, Kaiming Wu, Fuhui Wang. Significantly improved corrosion resistance of Mg-15Gd-2Zn-0.39Zr alloys: Effect of heat-treatment[J]. J. Mater. Sci. Technol., 2019, 35(8): 1644-1654.

Figures/Tables 18

Table 1 Mechanical property of Mg-RE alloys under different heat-treatment conditions.

Alloy compositions (at.%)	Heat-treatment conditions	UTS (MPa)	YS (MPa)	Elongation	Ref.
Mg-1.7Gd-0.4Zn	As-cast	196	159	3.0	[17]
	T4 (773 K,10 h)	299	181	5.3
	T4+T6(498 K, peak-aged)	416	235	7.2
	Extruded	335	191	9.0
	Extruded + T6	384	203	5.6
Mg-1Zn-2Gd	As-cast	162	150	63	[10]
	T4 (793 K,2 h)	190	150	41
	T4+T6 (473 K,60 h)	240	190	67
	T4+T6 (573 K,10 h)	235	190	45
Mg-9Gd-3Y-1Zn- 0.8Mn	T4 (793 K,10 h)	382	302	12.3	[16]
Mg-9Gd-3Y-1Zn- 0.8Mn	T4+T6 (473 K,1 h)	451	342	6.1	[16]
Mg-9Gd-3Y-1Zn- 0.8Mn-1.4Ag	T4 (793 K,10 h)	433	350	10.6	[16]
Mg-9Gd-3Y-1Zn- 0.8Mn-1.4Ag	T4+T6 (473 K,1 h)	533	399	9.0	[16]
Mg-5Gd-1Zn-0.6Zr	T4 (793 K,12 h)	220	110	15	[15]
Mg-5Gd-1Zn-0.6Zr	T4+ Extruded	265	214	31	[15]
Mg-2.5Gd-1Zn- 0.18 Zr	As-cast	220	139	4.2	Our research results
	T4 (773 K, 35 h)	236	129	10.6
	T4+T6 (473 K, 128 h)	290	163	10.4

Fig. 1. XRD patterns of Mg-Gd-Zn-Zr alloys under different heat-treatment conditions.

Fig. 2. SEM images of Mg-Gd-Zn-Zr alloys under different heat-treatment conditions: (a) as-cast, (b) T4, (c) T6-U, (d) T6-P, (e) T6-O.

Table 2 EDS results of different regions in Fig. 2.

Alloys	Region	Mg	Gd	Zn	Zr
As-cast alloy	I	98.78	0.80	0.18	0.24
As-cast alloy	II	87.60	7.70	4.70	0.00
T4 alloy	I	97.87	1.60	0.39	0.14
T4 alloy	II	91.27	5.11	3.62	0.00
T6-U alloy	I	97.96	1.46	0.34	0.24
T6-U alloy	II	89.38	5.94	4.46	0.22
T6-P alloy	I	98.14	1.53	0.27	0.06
T6-P alloy	II	90.19	5.72	4.00	0.09
T6-O alloy	I	97.97	1.56	0.32	0.15
T6-O alloy	II	90.49	5.54	3.92	0.05

Fig. 3. (a) Bright-field TEM image of LPSO phase in T4 alloys. (b) SAED pattern of LPSO phase. Electron beam parallel to the 1$\bar{2}$10 direction. (c) HRSTEM image of LPSO phase, indicating a 14H-LPSO structure.

Fig. 4. HAADF-STEM images of precipitates in Mg-Gd-Zn-Zr alloys under different heat-treatment conditions: (a) T4, (b) T6-U, (c) T6-P, (d) T6-O. Especially, (a2) is the EDS mappings of a Zn-Zr particle by HRSTEM.

Fig. 5. Three-dimensional distribution images of (a) as-cast, (b) T4, (c) T6-U, (d) T6-P, and (e) T6-O alloys by HRTXT.

Fig. 6. Hydrogen evolution rate (HER) of Mg-Gd-Zn-Zr alloys at different immersion time.

Fig. 7. EIS of Mg-Gd-Zn-Zr alloys after immersion for (a) 0 h, (b) 24 h, (c) 72 h, and (d) 192 h. (a1-d1) are Nyquist plots and (a2-d2) are Bode plots.

Fig. 8. Equivalent circuits for (a) as-cast alloys at the beginning of the immersion, and (b) as-cast alloys immersed for a period and heat-treated alloys.

Fig. 9. 1/Rp of Mg-Gd-Zn-Zr alloys during various immersion time obtained from EIS.

Fig. 10. Potentiodynamic polarization curves of Mg-Gd-Zn-Zr alloys under different heat-treatment conditions.

Table 3 Electrochemical parameters obtained from potentiodynamic polarization curves in Fig. 10.

	As-cast	T4	T6-U	T6-P	T6-O
E_corr (V)	-1.54	-1.41	-1.41	-1.41	-1.41
i_corr (A·cm^-2)	0.43 × 10^-4	1.16 × 10^-4	0.95 × 10^-4	1.23 × 10^-4	1.39 × 10^-4

Fig. 11. SEM images of the initial corrosion morphologies of (a1) as-cast, (b) T4, (c) T6-U, (d) T6-P and (e) T6-O alloys after immersed in corrosion solution for 30 min. (a2) is the corrosion morphology of as-cast alloys immersed for 1 h.

Fig. 12. SEM images of the cross-section corrosion morphologies of (a) as-cast, (b) T4, (c) T6-U, (d) T6-P, and (e) T6-O alloys after immersed in corrosion solution for 192 h.

Fig. 13. SKPFM results of the local potential distribution between second phases and α-Mg matrix in (a) as-cast, (b) T4, (c) T6-U, (d) T6-P and (e) T6-O alloys.

Table 4 Parameters related to the microstructure characteristic and corrosion rate of Mg-Gd-Zn-Zr alloys.

Alloys	Second phases	Potential relative to matrix (mV)	Volume fraction (%)	HER of 192 h (mL·cm^-2·h^-1)
As-cast	β-(Mg,Zn)₃Gd eutectic phase	290	42.8	1.02
T4 alloy	LPSO	243	27.0	0.12
T4 alloy	ZnZr	426	2.13	0.12
T6-U alloy	LPSO	236	20.0	0.14
T6-U alloy	ZnZr γ"	426 /	2.04 /	0.14
T6-P alloy	LPSO	207	22.0	0.14
T6-P alloy	ZnZr γ"+γ'	426 /	2.26 /	0.14
T6-O alloy	LPSO	170	20.0	0.08
T6-O alloy	ZnZr γ"+γ'+β'+β	426 /	2.30 /	0.08

Fig. 14. Schematic diagram of the relationship between the microstructure characteristic and corrosion rate of Mg-Gd-Zn-Zr alloys.

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