Microstructure, Mechanical Properties and Corrosion Behavior of Extruded Mg-Zn-Ag Alloys with Single-Phase Structure

doi:10.1007/s40195-018-0712-x

Abstract

Abstract:

Mg-Zn-Ag alloys have been extensively studied in recent years for potential biodegradable implants due to their unique mechanical properties, biodegradability and biocompatibility. In the present study, Mg-3Zn-xAg (wt%, x = 0.2, 0.5 and 0.8) alloys with single-phase crystal structure were prepared by backward extrusion at 340 °C. The addition of Ag element into Mg-3Zn slightly influences the ultimate tensile strength and microstructure, but the elongation firstly increases from 12% to 19.8% and then decreases from 19.8% to 9.9% with the increment of Ag concentration. The tensile yield strength, ultimate tensile strength and elongation of Mg-3Zn-0.2Ag alloy reach up to 142, 234 MPa and 19.8%, respectively, which are the best mechanical performance of Mg-Zn-Ag alloys in the present work. The extruded Mg-3Zn-0.2Ag alloy also possesses the best corrosion behavior with the corresponding corrosion rate of 3.2 mm/year in immersion test, which could be explained by the single-phase and uniformly distributed grain structure, and the fewer twinning.

Key words: Mg-Zn-Ag, Single, phase, Backward, extrusion, Mechanical, performance, Corrosion, behavior

Hong Zhao, Li-Qing Wang, Yu-Ping Ren, Bo Yang, Song Li, Gao-Wu Qin. Microstructure, Mechanical Properties and Corrosion Behavior of Extruded Mg-Zn-Ag Alloys with Single-Phase Structure[J]. Acta Metallurgica Sinica (English Letters), 2018, 31(6): 575-583.

Figures/Tables 14

Table 1 Composition of Mg-Zn(-Ag) alloys measured by ICP-AES (balance magnesium)

Alloy	Mg (wt%)	Zn (wt%)	Ag (wt%)
Mg-3Zn	97.20	2.80	0
Mg-3Zn-0.2Ag	96.97	2.83	0.20
Mg-3Zn-0.5Ag	96.61	2.88	0.51
Mg-3Zn-0.8Ag	96.34	2.88	0.78

Table 2 Chemical composition of SBF solution (g/L) used in these experiments

NaCl	NaHCO₃	KCl	K₂HPO₄·3H₂O	MgCl₂·6H₂O	c(HCl) = 1 mol/L (mL)	CaCl₂	NaSO₄	TRIS	c(HCl) = 1 mol/L (mL)
8.035	0.355	0.225	0.231	0.311	39	0.292	0.072	6.118	0-5

Fig. 1 XRD patterns of as-extruded Mg-Zn(-Ag) alloys

Table 3 Lattice parameters of extruded Mg-Zn(-Ag) alloys

Alloys	a (nm)	c (nm)	c/a
Mg-3Zn	3.2034	5.2014	1.6236
Mg-3Zn-0.2Ag	3.2047	5.2008	1.6229
Mg-3Zn-0.5Ag	3.2036	5.2004	1.6232
Mg-3Zn-0.8Ag	3.2031	5.2006	1.6236

Fig. 2 Optical micrographs of solid solute treatment Mg-Zn(-Ag) alloys and SEM-BSE images of as-extruded Mg-Zn(-Ag) alloys a, a1 Mg-3Zn, b, b1 Mg-3Zn-0.2Ag, c, c1 Mg-3Zn-0.5Ag, d, d1 Mg-3Zn-0.8Ag

Fig. 3 SEM-EDS mapping of all the extruded Mg-Zn-Ag alloys: a Mg, b Zn, c Ag

Fig. 4 Grain size distributions of the extruded Mg-Zn(-Ag) alloys: a Mg-3Zn, b Mg-3Zn-0.2Ag, c Mg-3Zn-0.5Ag, d Mg-3Zn-0.8Ag

Fig. 5 EBSD inverse pole figure (IPF) map of as-extruded Mg-Zn(-Ag) alloys: a, a1 Mg-3Zn, b, b1 Mg-3Zn-0.2Ag, c, c1 Mg-3Zn-0.5Ag, d, d1 Mg-3Zn-0.8Ag

Fig. 6 TEM bright field micrographs of the as-extruded Mg-Zn-Ag alloys: a, b Mg-3Zn-0.2Ag, c, d Mg-3Zn-0.8Ag

Fig. 7 Tensile stress-strain curves a and the corresponding yield strength, ultimate strength as well as elongation of extruded Mg-Zn(-Ag) alloys b. The load direction is parallel to extrusion direction

Fig. 8 Electrochemical polarization curves of Mg-Zn(-Ag) alloys in SBF solution

Table 4 Electrochemical parameters of Mg-Zn(-Ag) alloys from the polarization test

Alloy	Corrosion potential, E_corr (V vs. SCE)	Pitting potential, E_b (Vvs.SCE)	Current density, i_corr(μA/cm)	Cathodic slope, β _c (mV/decade) versus SCE	Anodic slope, βa (mV/decade) versus SCE	Polarization resistance, R_p (kΩ cm²)	Corrosion rate, P _i(mm/year)
Mg-3Zn	- 1.704	- 1.549	60.534	135.142	106.784	0.43	1.4
Mg-3Zn-0.2Ag	- 1.679	- 1.548	33.627	104.598	68.322	0.54	0.8
Mg-3Zn-0.5Ag	- 1.667	- 1.546	68.938	181.715	124.388	0.45	1.6
Mg-3Zn-0.8Ag	- 1.645	- 1.541	101.594	202.528	138.116	0.35	2.3

Fig. 9 a Corrosion rate of Mg-Zn(-Ag) alloys after immersion in SBF solution for various times, b X-ray diffraction patterns of the corrosion products after immersion for 14 days

Fig. 10 Macro-morphologies of the corroded surfaces of Mg-Zn(-Ag) alloys after immersion in SBF solution. a Mg-3Zn, b Mg-3Zn-0.2Ag, c Mg-3Zn-0.5Ag, d Mg-3Zn-0.8Ag

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