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Twinning-induced plasticity (TWIP) Fe–Mn–C steels are biodegradable metals with far superior mechanical properties to any biodegradable metal, including Mg alloys, used in commercially available devices. For this reason, the use of Fe–Mn–C alloys to produce thinner and thinner implants can be exploited for overcoming the device size limitations that biodegradable stents still present. However, Fe–Mn steels are known to form a phosphate layer on their surface over long implantation times in animals, preventing device degradation in the required timeframe. The introduction of second phases in such alloys to promote galvanic coupling showed a short-term promise, and particularly the use of Ag looked especially effective. Nonetheless, the evolution of the corrosion mechanism of quaternary Fe–Mn–C–Ag alloys over time is still unknown. This study aims at understanding how corrosion changes over time for a TWIP steel alloyed with Ag using a simple static immersion setup. The presence of Ag promoted some galvanic coupling just in the first week of immersion; this effect was then suppressed by the formation of a mixed carbonate/hydroxide layer. This layer partly detached after 2 months and was replaced by a stable phosphate layer, over which a new carbonate/hydroxide formed after 4 months, effectively hindering the sample degradation. Attachment of phosphates to the surface matches 1-year outcomes from animal tests reported by other authors, but this phenomenon cannot be predicted using immersion up to 28 days. These results demonstrate that immersion tests of Fe-based degradable alloys can be related to animal tests only when they are carried out for a sufficiently long time and that galvanic coupling with Ag is not a viable strategy in the long term. Future works should focus more on surface modifications to control the interfacial behavior rather than alloying in the bulk.

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Twinning-induced plasticity (TWIP) steels are considered excellent materials for manufacturing products requiring extremely high mechanical properties for various applications including thin medical devices, such as biodegradable intravascular stents. It is also proven that the addition of Ag can guarantee an appropriate degradation while implanted in human body without affecting its bioactive properties. In order to develop an optimized manufacturing process for thin stents, the effect of Ag on the recrystallization behavior of TWIP steels needs to be elucidated. This is of major importance since manufacturing stents involves several intermediate recrystallization annealing treatments. In this work, the recrystallization mechanism of two Fe-Mn-C steels with and without Ag was thoroughly investigated by microstructural and mechanical analyses. It was observed that Ag promoted a finer microstructure with a different texture evolution, while the recrystallization kinetics resulted unaffected. The presence of Ag also reduced the effectiveness of the recrystallization treatment. This behavior was attributed to the presence of Ag-rich second phase particles, precipitation of carbides and to the preferential development of grains possessing a {111} orientation upon thermal treatment. The prominence of {111} grains can also give rise to premature twinning, explaining the role of Ag in reducing the ductility of TWIP steels already observed in other works. Furthermore, in vitro biological performances were unaffected by Ag. These findings could allow the design of efficient treatments for supporting the transformation of Fe-Mn-C steels alloyed with Ag into commercial products.

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