Fatigue and corrosion fatigue behaviors of biodegradable Zn-Li and Zn-Cu-Li under physiological conditions

doi:10.1016/j.jmst.2022.04.051

Abstract

Abstract:

Zinc (Zn) alloys are emerging as a new class of biodegradable metallic materials due to their good biocompatibility, suitable biodegradability, and nontoxicity. However, the dynamic loading in the human body, along with the corrosive physiological environment, brings great challenges to the application of biodegradable Zn alloys. At present, there are few reports on the fatigue and corrosion fatigue properties of Zn alloys in simulated body fluid (SBF). In the present work, extruded Zn-0.8Li and Zn-2Cu-0.8Li alloys were selected in order to systematically evaluate their fatigue and corrosion fatigue behaviors both in air at ambient temperature and in SBF at 37 °C. Results revealed that the fatigue limits of the extruded Zn-0.8Li and Zn-2Cu-0.8Li alloys were about 135 and 180 MPa, respectively, in air at ambient temperature. However, the fatigue limits of the two alloys decreased to 65 and 80 MPa, respectively, in SBF at 37 °C and showed a linear relationship between the corrosion fatigue life and the stress amplitude. The sources of fatigue cracks in air were internal microstructural defects or weak mechanical properties of the material, while the initiation of corrosion pits on the surface was the main reason for the source of the formation of fatigue cracks in SBF. The fracture mode of the extruded Zn-0.8Li and Zn-2Cu-0.8Li alloys was quasi-cleavage fracturing. Compared to static immersion testing, cyclic loading significantly increased the corrosion rates of the two experimental alloys in a corrosion fatigue environment.

Key words: Biodegradable metals, Biomedical applications, Corrosion fatigue behaviors, Fatigue behaviors, Zinc alloys

Huafang Li, Yan Huang, Xiaojing Ji, Cuie Wen, Lu-Ning Wang. Fatigue and corrosion fatigue behaviors of biodegradable Zn-Li and Zn-Cu-Li under physiological conditions[J]. J. Mater. Sci. Technol., 2022, 131: 48-59.

Figures/Tables 20

Table 1. Chemical compositions of Zn-0.8Li and Zn-2Cu-0.8Li alloys (wt.%).

Alloy	Cu	Li	Zn
Zn-0.8Li	-	0.78	Bal.
Zn-2Cu-0.8Li	2.00	0.76	Bal.

Fig. 1. (a) Shape and dimensions of tensile test specimens; (b) shape and dimensions of fatigue test specimens.

Fig. 2. Schematic diagram of the device for corrosion fatigue tests.

Fig. 3. XRD patterns of Zn-0.8Li and Zn-2Cu-0.8Li alloys.

Fig. 4. SEM micrographs of (a) extruded Zn-0.8Li and (b) extruded Zn-2Cu-0.8Li.

Fig. 5. Tensile stress-strain curves of extruded (a) Zn-0.8Li and (b) Zn-2Cu-0.8Li alloys.

Table 2. Tensile mechanical properties of extruded Zn-0.8Li and Zn-2Cu-0.8Li alloys at ambient temperature.

Alloy	TYS (MPa)	UTS (MPa)	Elongation (%)
Zn-0.8Li	420.67±4.04	570.33±0.58	25.50±2.60
Zn-2Cu-0.8Li	369.67±4.16	503.33±2.31	42.83±0.58

Fig. 6. Stress-life (S-N) curves for (a) Zn-0.8Li alloy and (b) Zn-2Cu-0.8Li alloy tested in air and in SBF.

Table 3. Fatigue and corrosion fatigue experimental results for Zn-0.8Li alloy.

Stress (MPa)	Air (cycles)	SBF (cycles)
360	4.86/5.52 × 10³	-
300	1.46/2.02 × 10⁴	-
250	3.34/4.75/4.92 × 10⁴	2.07/2.59 × 10⁴
200	1.88/1.96 × 10⁵	3.02/3.33 × 10⁴
180	2.32/3.04/3.46 × 10⁵	4.68/5.14 × 10⁴
160	3.15/3.28/3.80 × 10⁵	3.49/5.35/5.42 × 10⁴
140	4.16/4.86 × 10⁵	-
130	6.5/6.5 × 10⁵	8.95/1.09/1.24/2.46 × 10⁵
90	-	3.01/3.14 × 10⁵
80	-	2.23/4.76//5.46 × 10⁵
70	-	6.37 × 10⁵
65	-	6.5 × 10⁵

Table 4. Fatigue and corrosion fatigue experimental results for Zn-2Cu-0.8Li alloy.

Stress (MPa)	Air (cycles)	SBF (cycles)
360	3.89/5.81 × 10³	-
300	1.20/1.49 × 10⁴	-
250	2.52/8.46/8.55 × 10⁴	1.96/2.42 × 10⁴
220	1.27/1.91/2.66 × 10⁵	-
200	2.97/3.09 × 10⁵	2.13/3.08 × 10⁴
190	4.84 × 10⁵	-
180	6.18/6.5/6.5 × 10⁵	3.96/4.07/5.41 × 10⁴
160	-	5.35/7.26/7.99/9.20 × 10⁴
130	-	1.02/1.18/1.37 × 10⁵
90	-	2.04/2.11/3.44 × 10⁵
80	-	6.5 × 10⁵
70	-	6.5 × 10⁵

Fig. 7. High-cycle fatigue fracture overall-view fractographic images in air at ambient temperature of (a, c, e) extruded Zn-0.8Li alloy samples: (a) failure at 300 MPa stress with 2.02 × 104 cycles, (c) failure at 250 MPa stress with 4.75 × 104 cycles and (e) failure at 160 MPa stress with 3.28 × 105 cycles; and (b, d, f) extruded Zn-2Cu-0.8Li alloy samples: (b) failure at 300 MPa stress with 1.49 × 104 cycles, (d) failure at 250 MPa stress with 8.55 × 104 cycles and (f) failure at 190 MPa stress with 4.84 × 105 cycles. Three distinct regions, i.e. fatigue crack initiation zone, crack propagation zone, and final overload zone, are labeled A, B, and C, respectively, in images.

Fig. 8. High-cycle fatigue fracture fractographic images of (a) extruded Zn-0.8Li alloy (failure at 250 MPa stress with 4.75 × 104 cycles) and (c) extruded Zn-2Cu-0.8Li alloy (failure at 250 MPa stress with 8.55 × 104 cycles) in air at ambient temperature; (b, d) magnified images taken from the fatigue crack initiation zones and crack propagation zones of the Zn-0.8Li alloy (b) and Zn-2Cu-0.8Li alloy (d).

Fig. 9. High-cycle corrosion fatigue fracture overall-view fractographic images in SBF at 37 °C after removing corrosion products of extruded Zn-0.8Li alloy samples: (a) failure at 250 MPa stress with 2.07 × 104 cycles, (c) failure at 180 MPa stress with 4.68 × 104 cycles and (e) failure at 130 MPa stress with 1.09 × 105 cycles; and extruded Zn-2Cu-0.8Li alloy samples: (b) failure at 250 MPa stress with 2.42 × 104 cycles, (d) failure at 200 MPa stress with 3.08 × 104 cycles and (f) failure at 160 MPa stress with 9.20 × 104 cycles. Three distinct regions, i.e. fatigue crack initiation zone, crack propagation zone, and final overload zone, are labeled A, B, and C, respectively, in images.

Fig. 10. Corrosion fatigue sources of extruded Zn-0.8Li and Zn-2Cu-0.8Li alloy samples at different stresses in SBF at 37 °C: (a-d) corrosion fatigue sources of the extruded Zn-0.8Li failure at 180 MPa with 5.14 × 104 cycles (a, b) and 130 MPa stress with 2.46 × 105 cycles (c, d), respectively; (e-h) corrosion fatigue sources of the extruded Zn-2Cu-0.8Li failure at 200 MPa with 2.13 × 104 cycles (e, f) and 160 MPa stress with 7.26 × 104 cycles (g, h), respectively (without removing corrosion products (a), (c), (e), (g); removing corrosion products (b), (d), (f), (h)).

Fig. 11. Corrosion rate comparison between immersion in static SBF (static immersion) and under cyclic loading in circulating SBF (corrosion fatigue environment in this study) of (a) Zn-0.8Li and (b) Zn-2Cu-0.8Li. *p < 0.05, **p < 0.01.

Fig. 12. Corrosion rates of experimental materials at different time points immersed in static SBF.

Table 5. Summary of reported fatigue behaviors of biodegradable Zn-based, Mg-based, and Fe-based alloys.

Material	Mechanical property			Frequency (Hz)	Corrosion fatigue		Fatigue strength in air (MPa)	Fatigue ratio ^b	Refs.
Material	TYS (MPa)	UTS (MPa)	Elongation (%)	Frequency (Hz)	Medium	Corrosion fatigue strength (MPa)	Fatigue strength in air (MPa)	Fatigue ratio ^b	Refs.
AM porous iron	28	-	-	15	r-SBF	18.2 (N = 3 × 10⁶)	19.6 (N = 3 × 10⁶)	-	[33]
As-extruded Mg-Zn-Y-Nd	159	229	26.7	5	SBF	50 (N = 3.5 × 10⁶)	65 (N = 10⁷)	0.28	[34]
As-extruded HP-Mg	121	208	10.9	10	SBF	52 (N = 4 × 10⁶)	89 (N = 4 × 10⁶)	0.43	[23]
As-extruded Mg-1Ca	148	276	13.8	10	SBF	70 (N = 4 × 10⁶)	90 (N = 4 × 10⁶)	0.33	[23]
As-extruded Mg-2Zn-0.2Ca	118	211	24.4	10	SBF	68 (N = 4 × 10⁶)	87 (N = 4 × 10⁶)	0.41	[23]
Sand-cast AZ91D ^a	105	180	-	5	m-SBF	17 (N = 5 × 10⁵)	57 (N = 10⁷)	0.32	[25]
Die-cast AZ91D	71	171	4.3	10	SBF	20 (N = 10⁶)	50 (N = 10⁷)	0.29	[13]
As-extruded WE43	217	298	21.7	10	SBF	40 (N = 10⁷)	110 (N = 10⁷)	0.37	[13]
AM porous zinc (SO4)	11	-	-	15	r-SBF	8.8 (N = 3 × 10⁶)	7.7 (N = 3 × 10⁶)	-	[32]
AM porous zinc (SO402)	6	-	-	15	r-SBF	4.8 (N = 3 × 10⁶)	4.2 (N = 3 × 10⁶)	-	[32]
As-extruded Zn-0.8Li	421	570	25.5	2	SBF	65 (N = 6.5 × 10⁵)	135 (N = 6.5 × 10⁵)	0.24	Present work
As-extruded Zn-2Cu-0.8Li	370	503	42.8	2	SBF	80 (N = 6.5 × 10⁵)	180 (N = 6.5 × 10⁵)	0.36	Present work

Fig. 13. Fatigue strength of as-extruded Zn-0.8Li and Zn-2Cu-0.8Li alloys compared to other previously reported biodegradable alloys tested in air and SBF. Data collected from Refs [32], [33], [34], [40]. r-SBF: revised simulated body fluid; m-SBF: modified simulated body fluid.

Fig. 14. Comparison of fatigue strengths of extruded Zn-0.8Li, Zn-2Cu-0.8Li, and Mg alloys for biomedical application at 5 × 105 cycles tested in air and SBF. Note that uniaxial tension-compression (R = -1) fatigue testing was conducted for extruded Mg-Zn-Y-Nd alloy in Ref. [34] with a frequency of 5 Hz, for extruded HP-Mg, Mg-1Ca, and Mg-2Zn-0.2Ca alloys in Ref. [23] with a frequency of 10 Hz, and for die-cast AZ91D and extruded WE43 alloys in Ref. [13] with a frequency of 10 Hz, and uniaxial tension-compression (R = -1) fatigue testing was conducted for sand-cast AZ91D in Ref. [25] with a frequency of 5 Hz.

Table 6. RRFS values of biomedical Zn-based alloys and Mg-based alloys in SBF at the same number of cycles to failure (5 × 105).

Material	RRFS	Refs.
As-extruded Mg-Zn-Y-Nd	13%	[34]
As-extruded HP-Mg	19%	[23]
As-extruded Mg-1Ca	22%	[23]
As-extruded Mg-2Zn-0.2Ca	11%	[23]
Sand-cast AZ91D	70%	[25]
Die-cast AZ91D	69%	[13]
As-extruded WE43	33%	[13]
As-extruded Zn-0.8Li	44%	Present work
As-extruded Zn-2Cu-0.8Li	51%	Present work

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