Effect of silver in thermal treatments of Fe-Mn-C degradable metals: Implications for stent processing

Abstract

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.

Keywords: Biomaterials; Recrystallization; Silver; Steels; Twinning.

Graphical abstract

Fig. 1

Evolution of Vickers microhardness as a function of the thermal treatment parameters for the 0Ag and 0.4Ag alloys: a) 0Ag and 0.4Ag after 10 min as a function of temperature; b) 0Ag at 700 °C, 800 °C and 900 °C as a function of time; c) 0.4Ag at 700 °C, 800 °C and 900 °C as a function of time.

Fig. 2

SEM micrographs detailing the microstructural evolution of the studied alloys as a function of the recrystallization temperature after a 10-min treatment.

Fig. 3

EDS elemental mapping of 0.4Ag alloy in the unrecrystallized state, when a-e) cold rolled (CR25) and f-j) treated at 500 °C (500).

Fig. 4

Second phase particle size from ImageJ analyses: a) average carbide size and area fraction; b) average size and area fraction of the Ag-rich particles.

Fig. 5

XRD spectra of a) 0Ag and b) 0.4Ag alloys after different recrystallization treatments for 10 min.

Fig. 6

Thermodynamic simulation of the phases formation as a function of temperature for Fe16Mn0·7C alloy (0Ag sample). a) Equilibrium calculation; b) Scheil-Gulliver non-equilibrium calculation.

Fig. 7

EBSD maps of both alloys in both cold rolled (CR25) and treated (800 °C;10 min) (800) states. From left to right: orientation image maps (OIM), inverse pole figures (IPF) and orientation distribution functions (ODF) at φ₂ = 45°. The map of orientations in the IPFs is described in the top center figure. The relative intensity of each orientation is represented by the color scale, detailed in the histograms each IPF. The map of the ideal texture components for ODFs is detailed in the top right figure.

Fig. 8

Band slope image quality maps. Red lines inside the grains indicate twin boundaries; the other lines are deformation bands.

Fig. 9

Grain size distribution for the studied materials before (CR25) and after annealing at 800 °C for 10 min (800): a) 0Ag CR25; b) 0Ag 800; c) 0.4Ag CR25; d) 0.4Ag 800.

Fig. 10

Schmid factor data of both alloys in the cold rolled (CR25) and annealed (800 °C; 10 min) (800) states along the normal direction. Schmid factor map (0Ag CR25:a); 800:b); 0.4Ag CR25:d); 800:e)); comparison of Schmid factors intensities in CR25 and 800 states (0Ag:e), 0.4Ag:f)).

Fig. 11

Results from biological assessment of both 0Ag and 0.4Ag alloys compared to 316L stainless steel. a) relative viability towards endothelial cells (ECs) at 1% elution; b) relative viability towards smooth muscle cells (SMCs) at 1% elution; c) relative hemocompatibility from clotting time tests at 0, 15, 30, 45, 60 min; d) relative hemolysis. *p < 0.001 vs CTRL+ 15’; **p < 0.001 vs SS316L 30’; $ p < 0.01 vs SS316L 30’; #p < 0.001 vs CTRL+ and SS316L 45’; & p < 0.001 vs CTRL+ 60’; % p < 0.05 vs CTRL+ 60’.