EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Twinning-induced plasticity (TWIP) steels have been recently proposed for manufacturing biodegradable stents. Thanks to their exceptional mechanical properties, closely resembling those of the commonly used L605 Co-Cr alloy, thinner structures can finally be targeted. However, in vivo studies on Fe-based alloys reported that a compact layer of phosphates formed on the surface during implantation, thus decreasing the degradation rate. In an attempt to reverse this tendence, galvanic coupling was investigated, and the addition of Ag forming noble second phases was studied. Therefore, this work aimed at assessing whether such a strategy could successfully prevent passivation over medium-long degradation time in vitro while preserving excellent mechanical and processing properties. Results showed that the addition of 0.4% wt. Ag did not change the tensile strength nor the yield strength of a Fe-16Mn-0.7C steel, but elongation at failure was reduced by 13%. This was attributed to preferential formation of {111} grains during thermal treatments in the Ag-containing alloy, thus promoting the formation of mechanical twins, limiting dislocation gliding. Furthermore, the presence of Ag promoted galvanic coupling in the first 7 days of immersion. Mn carbonates started depositing at the surface after 7 days, for then forming a nearly compact layer after 28 days, on top of which Fe hydroxide islands were detected. After 60 days, part of this compact layer detached, replaced by Fe phosphates. After 120 days, a new layer of Mn carbonates and Fe hydroxides formed on top of the phosphates attached to the surface. The superposing nature of the degradation layers was further confirmed by impedance spectroscopy. Although the addition of Ag did not prove to effectively prevent phosphate formation, the acquired knowledge on the evolution of the degradation mechanism in TWIP steels can foster the development of new strategies for accelerating their degradation rate when implanted in humans.