Towards revealing key factors in mechanical instability of bioabsorbable Zn-based alloys for intended vascular stenting

Abstract

Zn-based alloys are recognized as promising bioabsorbable materials for cardiovascular stents, due to their biocompatibility and favorable degradability as compared to Mg. However, both low strength and intrinsic mechanical instability arising from a strong strain rate sensitivity and strain softening behavior make development of Zn alloys challenging for stent applications. In this study, we developed binary Zn-4.0Ag and ternary Zn-4.0Ag-xMn (where x = 0.2-0.6wt%) alloys. An experimental methodology was designed by cold working followed by a thermal treatment on extruded alloys, through which the effects of the grain size and precipitates could be thoroughly investigated. Microstructural observations revealed a significant grain refinement during wire drawing, leading to an ultrafine-grained (UFG) structure with a size of 700 nm and 200 nm for the Zn-4.0Ag and Zn-4.0Ag-0.6Mn, respectively. Mn showed a powerful grain refining effect, as it promoted the dynamic recrystallization. Furthermore, cold working resulted in dynamic precipitation of AgZn₃ particles, distributing throughout the Zn matrix. Such precipitates triggered mechanical degradation through an activation of Zn/AgZn₃ boundary sliding, reducing the tensile strength by 74% and 57% for Zn-4.0Ag and Zn-4.0Ag-0.6Mn, respectively. The observed precipitation softening caused a strong strain rate sensitivity in cold drawn alloys. Short-time annealing significantly mitigated the mechanical instability by reducing the AgZn₃ fraction. The ternary alloy wire showed superior microstructural stability relative to its Mn-free counterpart due to the pinning effect of Mn-rich particles on the grain boundaries. Eventually, a shift of the corrosion regime from localized to more uniform was observed after the heat treatment, mainly due to the dissolution of AgZn₃ precipitates. STATEMENT OF SIGNIFICANCE: Owing to its promising biodegradability, zinc has been recognized as a potential biodegradable material for stenting applications. However, Zn's poor strength alongside intrinsic mechanical instability have propelled researchers to search for Zn alloys with improved mechanical properties. Although extensive researches have been conducted to satisfy the mentioned concerns, no Zn-based alloys with stabilized mechanical properties have yet been reported. In this work, the mechanical properties and stability of the Zn-Ag-based alloys were systematically evaluated as a function of microstructural features. We found that the microstructure design in Zn alloys can be used to find an effective strategy to not only improve the strength and suppress the mechanical instability but also to minimize any damage by augmenting the corrosion uniformity.

Keywords: Biodegradable; Corrosion; Precipitation softening; Room temperature superplasticity; Zinc alloys.

Fig. 1.

Optical microstructure of the as-cast alloys: (a) A4, (b) AM42, c) AM44, (d) AM46 and their corresponding solution heat-treated (a’–d’).

Fig. 2.

XRD patterns of the investigated alloys in the as-cast condition.

Fig. 3.

Microstructure of the extruded alloys: (a) A4, (b) AM42, (c) AM44 and (d) AM46.

Fig. 4.

IPF map of the extruded (a) A4, (b) AM44 and (c) AM46 alloys.

Fig. 5.

Mechanical properties of the investigated Zn-Ag-based alloys compared to Zn alloys explored so far [13, 14, 26, 35].

Fig. 6.

Bright field STEM images and their corresponding EDS elemental maps obtain from the CD (a) A4 and (b) AM46 alloys. (c) SAD pattern obtained from the AM46 alloy showing reflections for the Zn, AgZn₃ and Mn phases.

Fig. 7.

(a) Bright field TEM image obtained from the AM46 alloy. (b) Dark field TEM image from the area shown in (a) which is formed by screening those diffracted beam marked by a yellow circle (i.e. 1/d ≈ 5.3 1/nm and 1/d ≈ 5.5 1/nm) in the inset. (c) and (d) display high resolution TEM images obtained from the Mn particle shown in (a) at different magnifications.

Fig. 8.

Engineering stress-strain tensile curves for (a) A4 and (b) AM46 alloys in EX and CD conditions.

Fig. 9.

(a) tensile curves for the CD A4 alloy at different HT times and (b) their corresponding Hall-Petch relationship.

Fig. 10.

Microstructure of the CD A4 alloy after (a) 2s (b) 10s, (c) 20s, (d) 1 min and (e) 2 min of HT.

Fig. 11.

The effect of strain rate on the engineering stress-strain curves for the (a) CD and (b) 20 s-annealed A4 alloy. (c) the variation of flow stress and fracture elongation versus strain rates obtained from (a) and (b).

Fig. 12.

(a) tensile curves of the CD AM46 alloy at different HT times and (b) their corresponding Hall-Petch relationship.

Fig. 13.

Bright field STEM images obtained from the AM46 alloy after (a) 1 min and (b) 20 min of HT and their corresponding EDS elemental maps.

Fig. 14.

The effect of strain rate on the engineering stress-strain curves for the (a) CD and (b) 20 min-HT AM46 alloy. (c) the variation of flow stress versus strain rates obtained from (a) and (b).

Fig. 15.

Cross-sectional morphologies of the corroded A4 (a-d) and AM46 wires (e-h) after 10 (a, c, e, g) and 40 days (b, d, f, h) of static immersion tests, (k, l) corresponding EDS analyses, and (m) corrosion rates on the wires after 40 days of immersion tests.