Effects of ECAP Extrusion on the Microstructure, Mechanical Properties and Biodegradability of Mg-2Zn-xGd-0.5Zr Alloys

doi:10.1007/s40195-020-01136-7

Abstract

Abstract:

Effects of equal channel angular pressing (ECAP) extrusion on the microstructure, mechanical properties and biodegradability of Mg-2Zn-xGd-0.5Zr (x = 0, 0.5, 1, 2 wt%) alloys were studied in this work. Microstructure analysis, tensile test at ambient temperature, immersion test and electrochemical test in Hank’s solution were carried out. The results showed that Gd could further enhance the grain refinement during the ECAP extrusion. Both Gd addition and ECAP extrusion could improve the mechanical properties of the alloys, and the extrusion played the dominant role. Minor addition of Gd (0.5-1 wt%) could obviously enhance the corrosion resistance of the alloys. To some extent, ECAP extrusion improved the corrosion resistance of the alloys due to the change of second phases distribution and the refinement of grains. Further increase in extrusion pass was detrimental to the improvement of the corrosion resistance as a result of increment of the grain boundaries.

Key words: Mg alloys, Gd addition, ECAP extrusion pass, Mechanical property, Biodegradable behavior

Jun-Xiu Chen, Xiang-Ying Zhu, Li-Li Tan, Ke Yang, Xu-Ping Su. Effects of ECAP Extrusion on the Microstructure, Mechanical Properties and Biodegradability of Mg-2Zn-xGd-0.5Zr Alloys[J]. Acta Metallurgica Sinica (English Letters), 2021, 34(2): 205-216.

Figures/Tables 18

Table 1 Chemical compositions of Mg-Zn-xGd-Zr alloys

Alloys	Actual composition
Alloys	Zn (wt%)	Gd (wt%)	Zr (wt%)	Mg
Mg-2Zn-0.5Zr (0Gd)	2.22	-	0.40	Bal.
Mg-2Zn-0.5Gd-0.5Zr (0.5Gd)	2.00	0.53	0.48	Bal.
Mg-2Zn-1Gd-0.5Zr (1Gd)	2.07	0.95	0.48	Bal.
Mg-2Zn-2Gd-0.5Zr (2Gd)	2.24	1.90	0.47	Bal.

Fig. 1 Schematic illustrations of a ECAP extrusion mould, b dimension of tensile test specimen

Fig. 2 Optical images of Mg-2Zn-xGd-0.5Zr alloys after four passes extrusion, respectively

Fig. 3 Change of the grain size with the extrusion pass

Fig. 4 SEM images of Mg-2Zn-xGd-0.5Zr alloys after four passes extrusion

Fig. 5 EDS analyses of the spots marked in Fig. 4: a spot A, b spot B, c spot C, d spot D

Fig. 6 Textures in the Mg-2Zn-xGd-0.5Zr alloys after four passes extrusion: a 0Gd, b 0.5Gd, c 1Gd, d 2Gd

Fig. 7 Mechanical properties of Mg-2Zn-xGd-0.5Zr alloys: a UTS, YS and EL of the alloys after two passes extrusion, b UTS, YS and EL of the alloys after four passes extrusion, c hardness of alloys after four passes extrusion

Fig. 8 Grain size dependencies of yield strength σ0.2 of the 2Gd alloy (As-cast [11])

Fig. 9 Fracture morphologies of Mg-2Zn-xGd-0.5Zr alloys after four passes extrusion: a 0Gd, b 0.5Gd, c 1Gd, d 2Gd

Fig. 10 pH change of Hank’s solution at different immersion time for Mg-2Zn-xGd-0.5Zr alloys after extrusion: a as-cast, b one pass extrusion, c two passes extrusion, d four passes extrusion

Fig. 11 Corrosion rates of the alloys immersed in Hank’s solution for 14 days after two passes and four passes extrusion

Fig. 12 Microstructures of Mg-2Zn-xGd-0.5Zr alloys after immersion in Hank’s solution for 14 days

Fig. 13 Potentiodynamic polarization curves of the alloys with different extrusion passes in Hank’s solution: a as-cast, b one pass, c two passes, d four passes

Table 2 Tafel fitting results based on potentiodynamic polarizations in Hank’s solution

Alloys	As-cast		One pass		Two passes		Four passes
Alloys	E_corr (V)	i_corr (A cm^-2)	E_corr (V)	i_corr (A cm^-2)	E_corr (V)	i_corr (A cm^-2)	E_corr (V)	i_corr (A cm^-2)
0Gd	- 1.57 ± 0.03	20.22 ± 1.53	- 1.51 ± 0.02	14.62 ± 0.83	- 1.55 ± 0.04	10.52 ± 0.30	- 1.50 ± 0.01	14.84 ± 0.18
0.5Gd	- 1.55 ± 0.01	18.28 ± 3.05	- 1.52 ± 0.02	13.41 ± 2.47	- 1.48 ± 0.01	10.30 ± 2.34	- 1.51 ± 0.04	12.40 ± 2.23
1Gd	- 1.60 ± 0.03	10.38 ± 1.01	- 1.49 ± 0.01	10.26 ± 1.19	- 1.54 ± 0.01	9.50 ± 1.03	- 1.50 ± 0.03	10.31 ± 0.65
2Gd	- 1.61 ± 0.02	25.88 ± 2.12	- 1.47 ± 0.03	18.98 ± 2.39	- 1.52 ± 0.03	10.93 ± 1.71	- 1.51 ± 0.02	14.41 ± 1.02

Fig. 14 Impedance curves of the Mg-2Zn-xGd-Zr alloys after four passes extrusion: a Nyquist plots, b Bode plots of log |Z| versus log f, c Bode plots of phase angle, d equivalent circuit of the alloys in Hank’s solution

Table 3 Fitting results of Mg-2Zn-xGd-Zr alloys immersed in Hank’s solution after four passes extrusion

Specimens	R_s (Ω cm²)	CPE₁		R₁ (Ω cm²)	CPE₂		R₂ (Ω cm²)	R₃ (Ω cm²)	L (H cm^-2)
Specimens	R_s (Ω cm²)	Y₀₁ (S sⁿ cm^-2)	n₁	R₁ (Ω cm²)	Y₀₂ (S sⁿ cm^-2)	n₂	R₂ (Ω cm²)	R₃ (Ω cm²)	L (H cm^-2)
0Gd	19.76	1.0 × 10^-5	0.60	34.02	28.87 × 10^-6	0.80	1.59 × 10³	8.35 × 10³	2.63 × 10³
0.5Gd	13.04	6.76 × 10^-5	0.50	134.70	12.13 × 10^-6	0.90	1.64 × 10³	9.54 × 10³	0.69 × 10³
1Gd	16.91	2.40 × 10^-5	0.70	73.30	12.27 × 10^-6	0.90	1.91 × 10³	7.56 × 10³	1.32 × 10³
2Gd	16.31	4.13 × 10^-5	0.60	52.90	21.32 × 10^-6	0.80	1.70 × 10³	7.56 × 10³	0.88 × 10³

Fig. 15 Schematic illustration of the alloy degradation after extrusion: a large second phases and grain size in the as-cast alloy, b the grain size decreased and the second phases distributed uniformly after two passes extrusion, c the grain size decreased further and many strip-shaped ultra-fine grains formed after four passes extrusion. The corrosion occurred from the ultra-fine grains

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