Hierarchical microstructure and two-stage corrosion behavior of a high-performance near-eutectic Zn-Li alloy

Abstract In order to improve mechanical and corrosion properties of biodegradable pure Zn, a knowledge-based microstructure design is performed on Zn-Li alloy system composed of hard β-LiZn4 and soft Zn phases. Precipitation and multi-modal grain structure are designed to toughen β-LiZn4 while strengthen Zn, resulting in high strength and high ductility for both the phases. Needle-like secondary Zn precipitates form in β-LiZn4, while fine-scale networks of string-like β-LiZn4 precipitates form in Zn with a tri-modal grain structure. As a result, near-eutectic Zn-0.48Li alloy with an outstanding combination of high strength and high ductility has been fabricated through hot-warm rolling, a novel fabrication process to realize the microstructure design. The as-rolled alloy has yield strength (YS) of 246 MPa, the ultimate tensile strength (UTS) of 395 MPa and elongation to failure (EL) of 47 %. Immersion test in simulated body fluid (SBF) for 30 days reveals that Li-rich products form preferentially at initial stage, followed by Zn-rich products with prolonged time. Aqueous insoluble Li2CO3 forms a protective passivation film on the alloy surface, which suppresses the average corrosion rate from 81.2 μm/year at day one down dramatically to 18.2 μm/year at day five. Afterwards, the average corrosion rate increases slightly with decrease of Li2CO3 content, which undulates around the clinical requirements on corrosion resistance (i.e., 20 μm/year) claimed for biodegradable metal stents.