Effect of minor content of Gd on the mechanical and degradable properties of as-cast Mg-2Zn-xGd-0.5Zr alloys

doi:10.1016/j.jmst.2018.10.022

Abstract

Abstract:

RE-containing Mg alloys used as biodegradable medical implants exhibit good promising application due to their good mechanical properties and degradation resistance. In this work, effect of Gd on the microstructure, mechanical properties and biodegradation of as-cast Mg-2Zn-xGd-0.5Zr alloys was investigated. The results showed that there were mainly α-Mg, I-phase, W-phase and MgZn₂ phase in Mg-Zn-Gd-Zr alloys. With increase of the Gd content, the strength of the alloys was enhanced due to the second phase strengthening and grain refinement. The degradation resistance of Mg-2Zn-0.5Zr alloy was increased by adding 0.5-1% Gd due to the uniformly distributed second phases which acted as a barrier to prevent the pitting corrosion. However, increasing Gd content to 2% reduced the degradation resistance of the alloy due to the galvanic corrosion between the matrix and the second phases. The good degradation resistance and mechanical properties of as-cast Mg-2Zn-1Gd-0.5Zr alloy makes it outstanding for biomaterial application.

Key words: Mg-2Zn-0.5Zr-Gd alloy, Microstructure, Mechanical properties, Biodegradation

Junxiu Chen, Lili Tan, Xiaoming Yu, Ke Yang. Effect of minor content of Gd on the mechanical and degradable properties of as-cast Mg-2Zn-xGd-0.5Zr alloys[J]. J. Mater. Sci. Technol., 2019, 35(4): 503-511.

Figures/Tables 20

Table 1 Chemical compositions of as-cast Mg-Zn-Gd-Zr alloys.

Alloys	Nominal composition	Actual composition
		Zn (wt. %)	Gd (wt. %)	Zr (wt. %)	Mg
Alloy 1	Mg-2Zn-0.5Gd-0.5Zr	2.00	0.53	0.48	Bal.
Alloy 2	Mg-2Zn-1Gd-0.5Zr	2.07	0.95	0.48	Bal.
Alloy 3	Mg-2Zn-2Gd-0.5Zr	2.24	1.90	0.47	Bal.
Alloy 4 (control)	Mg-2Zn-0.5Zr	2.22	-	0.40	Bal.

Fig. 1. Optical images of (a) Alloy 1, (b) Alloy 2, (c) Alloy 3 and (d) Alloy 4.

Fig. 2. SEM micrographs of (a) Alloy 1, (b) Alloy 2, (c) Alloy 3 and (d) Alloy 4.

Table 2 EDS analyses of the second phases marked in Fig. 2.

Position	Chemical compositions (at. %)			Phase
Position	Mg	Zn	Gd	Phase
A	97.63	1.86	0.50	I-phase
B	89.38	7.54	3.07	W-phase
C	97.77	1.92	0.30	Mg-Zn phase
D	72.77	18.29	8.94	W-phase
E	80.32	13.12	6.56	W-phase
F	97.03	2.97	-	Mg-Zn phase

Fig. 3. X-ray diffraction patterns of (a) Alloy 1, (b) Alloy 2, (c) Alloy 3 and (d) Alloy 4.

Fig. 4. DSC curves of (a) Alloy 1, (b) Alloy 2, (c) Alloy 3 and (d) Alloy 4.

Fig. 5. The mechanical properties of as-cast Mg-2Zn-xGd-0.5Zr alloys.

Fig. 6. Fracture morphologies of the experimental alloys (a) Alloy 1, (b) Alloy 2, (c) Alloy 3 and (d) Alloy 4.

Fig. 7. Potentiodynamic polarization curves of the alloys in Hank’s solution at 37 °C.

Table 3 Tafel fitting results based on potentiodynamic polarization in Hank’s solution.

Samples	Current density i_corr (μA cm^-2)	Potential E (V vs. SCE)	Corrosion rate (mm y^-1)
Alloy 1	6.45 ± 0.63	-1.61 ± 0.022	0.15 ± 0.014
Alloy 2	4.58 ± 0.76	-1.60 ± 0.033	0.10 ± 0.017
Alloy 3	12.45 ± 1.58	-1.56 ± 0.017	0.28 ± 0.036
Alloy 4	11.25 ± 0.55	-1.56 ± 0.02	0.25 ± 0.012

Fig. 8. EIS curves of the alloys at the OCP in Hank’s solution at 37 °C, (a) Nyquist plots, (b) Bode plots of log |Z| vs. log f, (c) Bode plots of phase angle and (d) equivalent circuit of the alloys in Hank’s solution.

Table 4 Fitting results of the alloys immersed in Hank’s solution.

Samples	R_s (Ω cm²)	Y₀₁ (μΩ cm^-2 s^-1)	n₁	R₁ (Ω cm²)	Y₀₂ (μΩ cm^-2 s^-1)	n₂	R₂ (Ω cm²)	R₃ (Ω cm²)	L (H cm^-2)
Alloy 1	19.87	6.40	0.56	42.73	14.40	0.60	4448	44.01	642.21
Alloy 2	18.96	47.58	0.45	42.83	17.20	0.63	5186	26956	555.86
Alloy 3	20.22	10.86	0.55	26.23	21.21	0.60	3326	10252	235.34
Alloy 4	21.88	21.21	0.50	33.49	19.02	0.62	3531	508.52	578.23

Fig. 9. pH change of Hank’s solution at different immersion time of the alloys.

Fig. 10. SEM micrographs of the alloys after immersion in Hank’s solution for 14 days with degradation products, (a) Alloy 1, (b) Alloy 2, (c) Alloy 3 and (d) Alloy 4.

Table 5 EDS analyses of the degradation products marked in Fig. 10.

Position	Chemical composition (wt. %)
	Mg	Zn	Ca	P	Gd
A	90.52	1.53	2.40	5.23	0.32
B	50.26	1.32	24.84	22.73	0.85
C	96.64	1.53	0.51	-	1.32
D	82.29	-	8.17	9.54	-

Fig. 11. X-ray diffraction patterns attained from the degradation products of (a) Alloy 1, (b) Alloy 2, (c) Alloy 3 and (d) Alloy 4 after immersion in Hank’s solution for 14 days.

Fig. 12. SEM micrographs of the alloys after immersion in Hank’s solution for 7 days, 14 days and 28 days as well as macrographs of the alloys after immersion for 28 days.

Fig. 13. The topography of the alloy observed by laser confocal scanning after immersion in Hank’s solution for 28 days and removal of the degradation products (a) Alloy 1, (b) Alloy 2, (c) Alloy 3 and (d) Alloy 4.

Table 6 The depth difference between the highest place and the lowest place and the surface roughness in Fig. 13.

Alloys	Alloy 1	Alloy 2	Alloy 3	Alloy 4
Depth difference R_max (μm)	163	119	394	266
Surface roughness R_a (μm)	38	21	78	71

Fig. 14. Corrosion rate of the alloys after immersion in Hank’s solution for 28 days.

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