Effect of trace HA on microstructure, mechanical properties and corrosion behavior of Mg-2Zn-0.5Sr alloy

doi:10.1016/j.jmst.2017.06.013

Abstract

Abstract:

Effect of the addition of trace HA particles into Mg-2Zn-0.5Sr on microstructure, mechanical properties, and bio-corrosion behavior was investigated in comparison with pure Mg. Microstructures of the Mg-2Zn-0.5Sr-xHA composites (x = 0, 0.1 and 0.3 wt%) were characterized by optical microscopy (OM), scanning electron microscopy (SEM) equipped with energy dispersion spectroscopy (EDS) and X-ray diffraction (XRD). Results of tensile tests at room temperature show that yield strength (YS) of Mg-2Zn-0.5Sr/HA composites increases significantly, but the ultimate tensile strength (UTS) and elongation decrease with the addition of HA particles from 0 up to 0.3 wt%. Bio-corrosion behavior was investigated by immersion tests and electrochemical tests. Electrochemical tests show that corrosion potential (E_corr) of Mg-2Zn-0.5Sr/HA composites significantly shifts toward nobler direction from -1724 to -1660 mV_SCE and the corrosion current density decreases from 479.8 to 280.8 μA cm^-2 with the addition of HA particles. Immersion tests show that average corrosion rate of Mg-2Zn-0.5Sr/HA composites decreases from 11.7 to 9.1 mm/year with the addition of HA particles from 0 wt% up to 0.3 wt%. Both microstructure and mechanical properties can be attributed to grain refinement and mechanical bonding of HA particles with second phases and α-Mg matrix. Bio-corrosion behavior can be attributed to grain refinement and the formation of a stable and dense CaHPO₄ protective film due to the adsorption of Ca²⁺ on HA particles. Our analysis shows that the Mg-2Zn-0.5Sr/0.3HA with good strength and corrosion resistance can be a good material candidate for biomedical applications.

Key words: Magnesium alloys, Bio-composites, Hydroxyapatite, Microstructure, Mechanical property, Bio-corrosion behavior

Jian-Xing Li, Yuan Zhang, Jing-Yuan Li, Jian-Xin Xie. Effect of trace HA on microstructure, mechanical properties and corrosion behavior of Mg-2Zn-0.5Sr alloy[J]. J. Mater. Sci. Technol., 2018, 34(2): 299-310.

Figures/Tables 16

Table 1 Actual chemical composition of the alloys (wt%).

Alloy	Mg	Zn	Sr	Al	Cu	Fe	Ni	HA
Pure	Bal.	/	/	0.056	<0.001	0.022	<0.001	/
Mg-2Zn-0.5Sr	Bal.	2.03	0.54	0.074	<0.001	0.0033	<0.001	/
Mg-2Zn-0.5Sr/0.1HA	Bal.	2.10	0.54	0.063	<0.001	0.0026	<0.001	0.0251
Mg-2Zn-0.5Sr/0.3HA	Bal.	1.96	0.60	0.074	<0.001	0.0033	<0.001	0.2359

Table 2 Chemical composition of the Kokubo simulated body fluid (SBF) compared to that of human blood plasma.

Solution	Ion concentration (mmol/L)
	Na⁺	K⁺	Ca²⁺	Mg²⁺	HCO₃^-	Cl^-	HPO₄^2-	SO₄^2-
Plasma	142.0	5.0	2.5	1.5	27.0	103.0	1.0	0.5
Kokubo(c-SBF)	142.0	5.0	2.5	1.5	4.2	147.8	1.0	0.5

Fig. 1. Optical microscopic image of the samples of (a) pure Mg, (b) Mg-2Zn-0.5Sr, (c) Mg-2Zn-0.5Sr/0.1 HA and (d) Mg-2Zn-0.5Sr/0.3HA.

Fig. 2. SEM micrographs of the cross-sections of (a, b) Mg-2Zn-0.5Sr, (c, d) Mg-2Zn-0.5Sr/0.1HA and (e, f) Mg-2Zn-0.5Sr/0.3HA.

Table 3 Composition of different regions in the microstructure of the specimens in wt% (marked as A, B and C in Fig. 2) obtained by energy-dispersive X-ray spectroscopy.

Alloy	Region A				Region B				Region C
	Mg	Zn	Sr	Ca	Mg	Zn	Sr	Ca	Mg	Zn	Sr	Ca
Mg-2Zn-0.5Sr	98.25	1.75	-	-	73.33	6.78	19.89	-	-	-	-
Mg-2Zn-0.5Sr/0.1HA	98.06	1.94	-	-	75.92	5.61	13.18	5.29	89.41	1.93	5.92	2.74
Mg-2Zn-0.5Sr/0.3HA	98.05	1.95	-	-	60.54	4.01	23.66	11.79	70.72	2.91	18.53	7.84
Mg-2Zn-0.5Sr/0.3HA	98.05	1.95	-	-	60.54	4.01	23.66	11.79	70.72	2.91	18.53	7.84

Fig. 3. X-ray diffraction patterns of specimens, wherein α-Mg, Mg17Sr2, MgZn and HA are detected.

Fig. 4. Schematic diagram of the formation of the second phases and the HA particle enrichment.

Fig. 5. Tensile stress-strain curves of the experimental materials generated at room temperature.

Fig. 6. The pH measured in the SBF from different samples during the immersion test as a function of immersion time.

Fig. 7. Potentiodynamic polarization curves of the experimental materials in the SBF.

Table 4 Electrochemical parameters of the samples in SBF obtained from the polarization test.

Alloy	E_corr (mV/SCE)	I_corr (μA cm^-2)	E_bd (mV)	P_i (mm/year)	β_C (mV/decade)	βa (mV/decade)	R_P (kΩ cm²)
Pure Mg	-1719	288.4	-1549	6.6	-369.5	431.3	0.30
Mg-2Zn-0.5Sr	-1724	479.8	-1549	11.0	-433.8	280.1	0.15
Mg-2Zn-0.5Sr/0.1HA	-1696	290.8	-1551	6.6	-388.5	274.3	0.24
Mg-2Zn-0.5Sr/0.3HA	-1660	280.8	-1548	6.2	-367.3	253.9	0.24

Fig. 8. Corrosion rates of experimental materials calculated by the weight loss after 10 d of immersion and the Tafel curve in the SBF at (37 ± 1) °C.

Fig. 9. SEM and EDS of the surface of experimental materials after 10 d of immersion in the SBF at (37 ± 1) °C, for (a) pure Mg, (b) Mg-2Zn-0.5Sr, (c) Mg-2Zn-0.5Sr/0.1HA and (d) Mg-2Zn-0.5Sr/0.3HA.

Fig. 10. X-ray diffraction patterns of the corrosion products on the sample surfaces, wherein α-Mg, Mg(OH)2 and CaHPO42H2O are detected.

Fig. 11. Three-dimensional corrosion morphology of the samples after corrosion product removal for (a) pure Mg, (b) Mg-2Zn-0.5Sr, (c) Mg-2Zn-0.5Sr/0.1HA and (d) Mg-2Zn-0.5Sr/0.3HA.

Fig. 12. Schematic corrosion process of the experimental materials (biodegradable magnesium alloys) immersed in SBF.

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