Vascular Stent
Zn-based alloys have been recognized as highly promising bioabsorbable materials for cardiovascular stents, due to their biocompatibility and, more importantly, favorable corrosion rates as compared to Mg alloys. However, both low tensile strength and intrinsic mechanical instability arisen from strong strain rate sensitivity and strain softening behavior make development of Zn alloys challenging for vascular scaffolding applications. In our new study published in Acta Biomaterialia and entitled Towards revealing key factors in mechanical instability of bioabsorbable Zn-based alloys for intended vascular stenting, which was carried out in collaboration with the researchers from the University of Sheffield and University of Oxford, we developed, processed and characterized binary Zn-4.0Ag and ternary Zn-4.0Ag-Mn 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 the cold wire drawing, leading to an equiaxed ultrafine grain (UFG) structure with an average size of 670 nm and 240 nm for the Zn-4.0Ag and Zn-4.0Ag-0.6Mn alloys, respectively. Mn showed a powerful grain refining effect as it promoted the dynamic recrystallization and hindered the growth of the UFG grains during the drawing process. In addition, cold working resulted in dynamic precipitation of markedly fine AgZn3 particles, distributing throughout the Zn matrix. Such strain-induced precipitates triggered mechanical degradation through an activation of Zn/AgZn3 boundary sliding, exhibiting maximum elongation of 430% and 203 % at the strain rate of 3.3 × 10−3 s−1 for Zn-4.0Ag and Zn-4.0Ag-0.6Mn, respectively. The observed precipitation softening phenomenon caused strong strain rate sensitivity and distinct strain softening behavior in the cold drawn alloys. Short-time annealing significantly mitigated the mechanical instability by reducing the AgZn3 volume fraction and thus, decreased the contribution of Zn/AgZn3 boundary sliding. The ternary alloy wire showed superior microstructural stability as compared to its Mn-free counterpart due to the pinning effect of nanosized Mn particles on the grain boundaries, restricting the grain coarsening. The corrosion results revealed that the microstructural manipulation strongly influenced the corrosion behavior of the Zn alloys so that the cold drawn wires exhibited intensive pitting corrosion. However, a shift of the corrosion regime from localized to a more uniform was observed after applying the heat treatment, mainly due to the dissolution of AgZn3 precipitates.
