Accelerating Corrosion and Improving Mechanical Properties by Heterogeneous Interstitial Carbon in Biodegradable Fe-Mn-C Alloys

Carbon addition is effective to promote biodegradation of iron-based implanted materials. However, how to accelerate degradation without compromising the mechanical properties lacks a detailed composition designing strategy. In this study, impurity contents, microstructure evolution, mechanical properties and in vitro corrosion of Fe-35Mn-C alloys are investigated. It is found that (i) the oxygen impurity reduced rapidly and carbides gradually precipitated (C ≥ 0.53 wt.%) with increasing carbon content; (ii) the optimal mechanical properties (σ_s = 723 MPa, 𝜀 = 30%) were obtained in carbides-free region (C = 0.40 wt.%); (iii) corrosion rate still increased to 6.7 mm/a (3 days) gradually although there were no carbides. Through the calculation of stacking fault energy and EBSD analysis, it is found that the twin-induced plastic deformation (TWIP) mechanism caused by the appearance of {111} twins is the main reason for the substantial increase in strength and elongation. At the same time, accelerating degradation mechanism without carbide precipitates was hypothesized for the first time. It can be seen from the calculation of density functional theory that as the interstitial carbon content increases, the matrix tends to be unstable. Therefore, even if there is no carbide formation, the degradation rate of the matrix can also be increased accordingly. This provides a theoretical basis for carbon content control and design of iron-based degradable vascular stents