Twin-induced strengthening biodegradable ZnMg alloys for the orthopedic implants

Biodegradable zinc (Zn) has limitations in the orthopedic conditions due to its low mechanical strength. Alloying with magnesium (Mg) and thermomechanical process are effective ways to overcome the limitations. This work demonstrates that the average grain size decreases from 10.4 μm in the as-extruded Zn-0.2 Mg alloys to the lowest value of 3.2 μm after multi-directional compression (MDC) for 18 passes. A notable increase in twin boundary density was observed in the MDC-treated samples, reaching a peak of 53.52%. During processing, dislocation density initially rose and subsequently declined. The refinement of grains and the proliferation of twin boundaries were key factors enhancing the alloy's strength. The as-extruded sample exhibited a tensile strength of 309 MPa and an elongation of 27.8%. Following MDC, strength increased to 408 MPa for both 12p and 18p samples, while peak elongation reached 30.6%. Regarding corrosion behavior, corrosion rates slightly decreased from 0.174 mm/year in the as-extruded condition to 0.139 mm/year after 12p-MDC, a result associated with the formation of stable corrosion films that suppressed localized attack. Furthermore, alloy extracts promoted high cell viability over extended culture durations or under one-fold dilution. Compared to bioinert titanium, the Zn alloy extracts also demonstrated enhanced in vitro osteogenic potential. The in-vivo experiments also demonstrate the formation of more new bone around biodegradable implants. Collectively, these findings suggest a promising route to boost mechanical strength in biodegradable Zn alloys, supporting their potential use in orthopedic implant applications.