Elsevier

Acta Biomaterialia

Volume 9, Issue 10, November 2013, Pages 8604-8610
Acta Biomaterialia

The processing of ultrafine-grained Mg tubes for biodegradable stents

https://doi.org/10.1016/j.actbio.2013.01.010Get rights and content

Abstract

An investigation was carried out on equal-channel angular pressing (ECAP) and extrusion processing of a ZM21 Mg alloy to obtain an improved candidate material for the manufacturing of biodegradable Mg stents. Ultrafine-grain size billets of the ZM21 alloy were obtained by two-stage ECAP aimed at achieving an initial refining of the structure at 200 °C and then reaching the submicrometer grain size range by lowering the processing temperature down to 150 °C. The investigation revealed a significant improvement in the properties of the ECAP-treated samples compared with the starting coarse-grained ZM21 alloy. The 0.2% yield strength rose from 180 to 340 MPa after 150 °C ECAP processing, while maintaining a fairly high tensile ductility. The ultrafine ZM21 alloy billets were then used for the extrusion of stent precursors having the form of small-size tubes. The grain size after extrusion remained in the submicrometer range while the hardness was revealed to be significantly higher than that of the coarse-grained ZM21 Mg alloy. It was demonstrated that processing of biodegradable Mg stent having an ultrafine-grained microstructure by ECAP and low-temperature extrusion is feasible and that the obtained products feature promising properties.

Introduction

Magnesium and its alloys are promising materials for biodegradable vascular stents owing to their relatively low corrosion resistance in human body fluids and their good biocompatibility [1], [2], [3], [4]. However, studies have also revealed that the rapid corrosion rate of conventional magnesium alloys causes premature loss of stent mechanical properties. The most effective way to enhance both the mechanical properties and the corrosion resistance of engineering magnesium alloys is to add specific alloying elements such as Li and rare-earth (RE) elements [5], [6], [7]. However, the toxicity of these alloying elements in a biomaterial is still a controversial issue among biomedical scientists [2], [3], [8]. Microstructural refinement is an alternative effective way for increasing both the mechanical properties and corrosion resistance of Mg alloys, especially when exploiting severe plastic deformation techniques to produce ultrafine-grained (UFG) materials featuring a submicrometer grain size [9], [10], [11], [12]. Moreover, the achievement of superplastic properties induced by the marked grain refinement could enhance the formability at elevated temperature, allowing easier production of miniaturized devices [13], [14].

Recent studies by Alvarez-Lopez et al. [10] and Argade et al. [15] reported that in a AZ31 Mg alloy the best corrosion behavior in phosphate-buffer solution (PBS) could be achieved after extensive grain refinement by equal-channel angular pressing (ECAP), as revealed by the lower initial corrosion potential and the higher charge transfer resistance values at long immersion periods. The UFG microstructure also showed the highest polarization resistance and the most positive pitting and repassivation potentials as compared to coarse-grained microstructures.

The experimental investigation described in this paper is focused on a ZM21 Mg alloy selected with the aim of exploring an alloy system preferentially formed by non-toxic elements, thus preserving the highest levels of biosafety and biocompatibility. It is reported that Ca, Zn and Mn in fairly low concentrations do not produce harmful effects (these elements are actually essential for the human metabolism) [2], [8], [16], whereas elements such as Al, Zr, Y and other RE elements that are used in other commercial Mg alloys to improve strength and corrosion resistance may give unwanted effects when released into the human body at high rates [2], [3], [4], [17].

The strategy of a significant grain refinement to improve both strength and corrosion behavior was here exploited by adopting ECAP to produce an UFG ZM21 alloy. The UFG billets were then directly used to produce small tubes by a warm extrusion process, followed by laser cutting to produce UFG Mg stent prototypes.

The microstructure obtained after ECAP and extrusion as well as the mechanical properties achieved are here investigated to supply information about possible processing routes for improved Mg biodegradable stents featuring a UFG structure.

Section snippets

Materials and methods

A ZM21 (Mg–1.78Zn–0.89Mn, wt.%) wrought alloy was selected for this investigation. Cylindrical specimens 10 mm in diameter and 150 mm long were machined from commercial extruded bars. The die used for ECAP processing featured two channels intersecting at an angle of 110° with an angle of 20° as the outer arc of curvature. According to the Iwahashi equation [18], this geometry generates an equivalent plastic strain of 0.76 per pass. The die was heated by four electrical resistance heaters

Results

In order to generate a very fine microstructure in Mg alloy billets, the ECAP process has to be performed at as low a temperature as possible. However, processing the starting billets directly at 150 °C led to extensive cracking due to lack of ductility of the original coarse-grained structure, as shown in Fig. 2. Increasing the processing temperature to 200 °C significantly improved the alloy formability and allowed defect-free billets to be obtained. Further tests showed that, once the alloy

Discussion

According to established theories [21], the mechanism of formation of new grains in metallic alloys deformed by ECAP is characterized by the progressive modification of low-angle cell boundaries into more stable equiaxed high-angle grain boundaries. This process is expected to consume a significant fraction of the mobile dislocations in the material structure with increasing number of passes, provided that processing is performed at sufficiently low temperature.

In the low-temperature range,

Conclusions

An investigation was carried out on properties and processing of UFG ZM21 Mg alloy as a possible candidate material for the manufacturing of improved biodegradable Mg stents.

  • 1.

    ECAP was used to achieve a significant grain refining in the submicrometer grain-size range. Processing was performed according to a two-step strategy aimed at achieving a first refining of the structure at 200 °C and then reaching the UFG grain size range by lowering the processing temperature down to 150 °C. A homogeneous

Acknowledgement

The authors would like to thank Fondazione CaRiTRO for partially funding the research under Grant number 2011.0250.

References (27)

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Part of the Biodegradable Metals Conference 2012 Special Issue, edited by Professor Frank Witte and Professor Diego Mantovani.

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