Microstructure refinement in biodegradable Zn-Cu-Ca alloy for enhanced mechanical properties, degradation homogeneity, and strength retention in simulated physiological condition

doi:10.1016/j.jmst.2022.03.006

Abstract

Abstract:

Zn-1.0Cu-0.5Ca (TA15) alloy has shown promising characteristics of enhanced mechanical properties and biodegradability for absorbable cardiovascular stents, endovascular devices, and wound closure devices applications. In this study, the TA15 alloy for bioabsorbable biomedical applications is investigated. In the conventionally cast TA15 (TA15-C) alloy, CaZn₁₃ phase are present as a large dendritic network with an average size of 73.25 ± 112.84 μm. Hot rolling of the TA15-C alloy has broken the long and dendritic network of the CaZn13 phases, however, the refined phases are observed as segregations and the distribution is non-uniform. These segregated CaZn₁₃ suffered heavy localised corrosion which lead to poor mechanical properties in the as-fabricated condition and after biodegradation. Ultrasonic treatment (UST) during casting is identified as an effective technique for the refinement and redistribution of CaZn₁₃ particles in TA15 alloy, which successfully reduce the size of the CaZn₁₃ phase to 10.91 ± 4.65 μm in the as-solidified condition. After hot rolling, the UST processed TA15 (TA15-UST) shows improved mechanical properties due to grain refinement and the reduction in microstructural defects, i.e. the broken CaZn₁₃ phase. Results of 8-week immersion corrosion tests showed that both alloys possess very similar corrosion rate. However, TA15-UST has markedly improved corrosion homogeneity compared to TA15-N which favours the retention of mechanical properties even after prolonged exposure to physiological fluids.

Key words: Biodegradable Zn alloy, Peritectic phase refinement, In vitro corrosion, Mechanical property retention, Cytocompatibility

Nan Yang, Nagasivamuni Balasubramani, Jeffrey Venezuela, Helle Bielefeldt-Ohmann, Rachel Allavena, Sharifah Almathami, Matthew Dargusch. Microstructure refinement in biodegradable Zn-Cu-Ca alloy for enhanced mechanical properties, degradation homogeneity, and strength retention in simulated physiological condition[J]. J. Mater. Sci. Technol., 2022, 125: 1-14.

Figures/Tables 12

Fig. 1. Microstructure of (a) as cast TA15-N-C, (b) as cast TA15-UST-C under OM, (c) Magnified SEM image of TA15-UST-C and associated element mapping of (c1) Ca and (c2) Ti, (c3) spot analysis on the elemental composition of the maked region. (d) CaZn13 phase size distribution in both as cast alloys (n=650).

Fig. 2. Microstructure of (a) hot rolled TA15-N under bright field OM, (a1) under SEM and (a2) under polarised OM. (b) hot rolled TA15-UST under bright field OM, (b1) under SEM and (b2) under polarised OM. (c) XRD patterns of the hot rolled pure Zn, TA15-N and TA15-UST. (d) Coefficient of variation of the second phase volume fraction of TA15-N and TA15-UST measured at different field size. (e) CaZn13 phase size distribution in both hot rolled alloys (n=650).

Table 1. Area fraction of CaZn13 phase in TA15-N and TA15-UST measured at different field sizes (n=50).

Field size[μm²]	Average A(CaZn₁₃) [%]		COV [%]		Min A(CaZn₁₃) [%]		Max A(CaZn₁₃) [%)]
Field size[μm²]	N	UST	N	UST	N	UST	N	UST
2500	14.08	14.23	98.80	37.27	0	6.82	52.40	25.69
10000	13.4	13.51	54.82	26.13	2.21	7.26	27.42	20.53
40000	13.33	13.17	41.51	20.01	4.65	8.13	23.06	17.89
160000	12.97	12.84	21.01	13.42	6.61	10.05	18.2	14.78

Fig. 3. (a-f) Corrosion morphology and (g) corrosion rate of Zn, TA15-N and TA15-UST after 4 and 8 weeks of immersion in HBSS. (h1)-(h5), (i1)-(i5) cross-sectional view of the corrosion morphology of TA15-N and TA15-UST plates after 8 weeks of immersion test.

Fig. 4. Corrosion product composition and distribution on pure Zn, TA15-N, and TA15-UST after 8 weeks of immersion test in HBSS solution. SEM images on the cross section of (a) Zn, (b) TA15-N, (c) TA15-UST, and the associated elemental mapping images of C, O, Ca, Zn, P, Cl. (d) Normalised weight percentages of C, O, Ca, Zn, P, Cl in the marked region in Figure (b) and (c), obtained from EDS point analysis. (e) FTIR analysis on the corrosion product composition.

Fig. 5. Electrochemical behaviours of pure Zn, TA15-N, and TA15-UST: (a) PDT curve, (b) Nyquist plot and (c) Bode plot.

Table 2. Corrosion potential and corrosion rate measured from electrochemical polarisation test for Zn, TA15-N and TA15-UST alloys in Hanks solution.

Empty Cell	Zn	TA15-N	TA15-UST
Corrosion potential, E_corr [V vs. SCE]	-1.05 ± 0.04	-1.12 ± 0.02	-1.11 ± 0.04
Corrosion current density, i_corr [μA cm^-2]	1.22 ± 0.23	2.25 ± 0.36	2.65 ± 0.32
Corrosion rate, CR [um y^-1]	18.29 ± 3.44	33.87 ± 5.41	35.13 ± 4.24

Table 3. Circuit parameters obtained from mathematical fitting of EIS data of the studies alloys.

Material	R_s[Ω cm²]	R_ct[kΩ cm²]	CP_Edl-T[μF sⁿ^-1 cm^-2]	n₁	R_f[kΩ cm²]	CP_Ef-T[μF sⁿ^-1 cm^-2]	n₂	Chi square, χ²
Zn	3.25 ± 0.11	9.41 ± 0.44	37.50 ± 4.60	0.53 ± 0.02	0.32 ± 0.01	8.81 ± 0.37	0.96 ± 0.01	0.0022
TA15-N	7.44 ± 0.09	4.23 ± 0.15	248.64 ± 7.53	0.52 ± 0.02	1.22 ± 0.05	9.23 ± 0.29	0.87 ± 0.03	0.0004
TA15-UST	2.46 ± 0.17	3.04 ± 0.22	341.63 ± 32.48	0.67 ± 0.05	1.40 ± 0.21	9.90 ± 0.69	0.92 ± 0.01	0.0056

Fig. 6. (a) typical stress-strain curve, and (b) bar chart showing the changes in mechanical properties of TA15-N and TA15-UST after 8 weeks of storage in HBSS/air. Facture surfaces of (c) pure Zn, (d) TA15-N, (e) TA15-UST without experiencing biodegradation; facture surfaces of (d) TA15-N, (e) TA15-UST after 8 weeks of immersion in HBSS.

Fig. 7. Cell viability of (a) L929 and (b) HUVEC incubated in 12.5%, 25%, 50%, 100% extracts for 1 and 3 days.

Table 4. CaZn13 phase size and mechanical properties in the current Zn-Ca based alloys.

Alloy system	CaZn₁₃ size [µm]	Mechanical properties			Refs.
Alloy system	CaZn₁₃ size [µm]	YS [MPa]	UTS [MPa]	e_f [%]	Refs.
Zn-1.0Cu-0.5Ca ^C	73.58 ± 112.84	-	-	-	This work
Zn-1.0Cu-0.5Ca ^UST-C	10.91 ± 4.65	-	-	-	This work
Zn-1.0Cu-0.5Ca ^C-HR	13.76 ± 7.08(cracks)	179.31 ± 1.80	250.89 ± 3.86	27.52 ± 2.3	This work
Zn-1.0Cu-0.5Ca ^UST-HR	10.81 ± 4.69	190.34 ± 4.47	282.01 ± 5.88	29.38 ± 3.1	This work
Zn-1.5Mg-0.1Ca ^C	Approx. 150¹(cracks)	173.81 ± 15.09	241.38 ± 0.39	1.72 ± 0.01	[34]
Zn-0.2Mg-0.15Ca ^HE	15-50	202.8 ± 2.0	259.3 ± 2.3	6	[40]
Zn-0.6Mg-0.1Ca ^C	30.9 ± 10.6	-	<200	<2.5	[33]
Zn-0.6Mg-0.1Ca ^{ECAP (12P)}	∼30(cracks)	-	∼320	∼7	[33]
Zn-0.8Mn-0.4Ca ^C	26	112.2 ± 3.4	120.3 ± 6.3	0.3 ± 0.1	[35,36]
Zn-0.8Mn-0.4Ca ^HE	17	253.4 ± 1.3	343.2 ± 1.6	8.0 ± 1.4
Zn-0.8Mn-0.4Ca ^HE+WCR	12.9(cracks)	244.9 ± 5.7	322.6 ± 11.2	11.8 ± 0.9
Zn-0.1Li-0.4Ca ^HE	23.67 ± 12.58²	∼366	∼415	∼4	[29]
Zn-0.8Li-0.4Ca ^HE	39.55 ± 19.64³	∼245	∼444	∼10	[29]
C: casting, HR: hot rolling, HE-hot extrusion, ECAP: equal channel angular pressing, WCR: warm calibre rolling

Fig. 8. Schematic diagram showing the degradation mechanisms of TA15-N and TA15-UST alloys in current study.

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