Influence of alloying and processing on the degradation behaviour of Mg-Zn based alloys
Designing a biodegradable Mg alloy with good corrosion resistance, mechanical integrity, biocompatibility and functionality is desired. Zn, an essential trace element in the body, is widely used as an alloying element in Mg. To further improve the corrosion performance and biofunctionality of Mg-Zn binary system, additions of extra alloying elements and processings are usually performed. It is of vital importance to understand how the microstructures and the corrosion performances respond with alloying and processing. In this work, elements capable of promoting bone regenerations (Ca and Sr) and antibacterial effects (Ag and Cu) were selected. Moreover, In (Indium) was also added as it is widely used in dental materials. The addition amount of Zn was selected as 0.5 wt.% and that of X was 0.2 wt.% to reduce the formation of intermetallics and to downscale galvanic corrosion. The corrosion performances were evaluated mainly by electrochemical methods, H2 evolution test and corrosion morphology observations. For the as-cast samples, the grain refinement efficiency and intermetallics forming ability differed greatly among the alloying elements. Si impurity also played a vital role in the precipitates formation in all Mg-Zn(-X) systems. Generally Mg-Zn-Ca possessed the highest corrosion resistance due to the refined grain size, limited intermetallics and uniform corrosion, whereas Mg-Zn-Cu showed the highest corrosion rate due to the coarse grains, intermetallics activity and severe pitting corrosion. Except Mg-Zn-Cu, the corrosion performances of Mg-0.5Zn(-0.2X) were comparable with those of the higher concentrated systems in literature, indicating the feasibility of these micro-alloying systems for biomedical applications. As Mg-Zn-Ca had the best corrosion performance, further studies were carried out to investigate how the corrosion initiated and developed. Mg2Ca phase was found to be anodic and was preferentially corroded within the first hour of immersion. Whereas, Ca2Mg6Zn3 and MgCaSi phases continuously act as cathodes until 24 h immersion. After solution annealing, the more homogeneous microstructure and reduced galvanic couples resulted in a higher corrosion resistance in Mg-Zn-Ca. In contrast, the intrinsic impurities (Fe and Si) started to dominate the corrosion behaviour of Mg-0.5Zn after solution annealing. The corrosion resistance of Mg-0.5Zn deteriorated with the extension of solution treatment duration as the Fe concentrations in the Fe-Si particles were promoted. To investigate the impacts of further hot extrusion, the as-extruded Mg-0.5Zn-(0.2X) alloys were also characterised. The as-extruded alloys had comparable grain sizes (~30 μm) so that the effect of grain size on corrosion performance could be neglected. In contrast to the widely-recognised observation from literature that extrusion enhances the corrosion performance, the corrosion rates of as-extruded Mg-Zn, Mg-Zn-Ag and Mg-Zn-In were significantly accelerated compared to those of the as-cast counterparts. Whereas the corrosion rate of Mg-Zn-Ca was further decelerated after annealing and extrusion, due to the finer grain size, weakened fibre texture and limited intermetallics. Moreover, the corrosion resistance of as-extruded Mg-0.5Zn was as poor as the solution annealed material. Instead, superior corrosion performance of as-extruded Mg-0.5Zn could be determined if no heat treatment was conducted prior to extrusion. It is suggested that shorter treatment duration or cold working should be considered in the case of micro-alloying Mg systems.
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