Analysis of the microstructure and the mechanical properties of an alloy Fe-Mn-Ni-Al-C for biodegradable bio-medical applications

Details

Supervisors

Jacques, Pascal

Faculty

Ecole polytechnique de Louvain

Degree label

Master [120] : ingénieur civil en chimie et science des matériaux

Abstract

The artery diseases that originate from a substance called plaque which builds up inside the arteries are very common nowadays. Because of the presence of this plaque, the artery becomes narrower leading to a reduction of the blood flow which can cause, in the worst case, a complete obstruction of the artery. Several treatments exist including medication treatments or surgeries such as an artery bypass or a stent implantation. In this work, a special attention will be devoted to the stents, which are small mesh tubes that are placed with a catheter in the weak or narrow arteries to restore a correct blood flow and to prevent the arteries from bursting. The current issues with this treatment are the potential late thrombosis, restenosis and the recurrence of a narrowing of the artery. Hence, this research focuses on a new generation of stents: the biodegradable stents. The properties of a Fe-Mn-Ni-Al-C TRIP-maraging steel that could lead to medical biodegradable applications such as the coronary stents have been studied. Specifically, the impact of heat treatments on its microstructure and its mechanical properties have been investigated. In addition, the corrosion properties and the degradability of the TRIP-maraging alloy as a function of different heat treatments have been analysed thanks to several immersion tests. Furthermore, the austenite phase fraction has been approximated with a X-Ray diffraction analysis. As a result, it appears that the phase fraction of austenite expands when the time of an annealing at 600° C is increased. However, a saturation can be highlighted after an annealing of 24 hours. The resulting microstructure presents austenite grains that grow from the martensite laths and prior austenite grain when the annealing time at 600° C is increased. On the other hand, performing a thermal cycling on the alloy accelerates the reversion onto austenite and presents an austenite phase fraction as high as the one resulting from an homogenization followed by the same annealing treatment. In addition, the resulting microstructure is finer and composed as well of martensite and austenite. Moreover, the mechanical properties of the alloy increase as well when the annealing time increases. Again, there is a saturation of the improvements of the mechanical properties after an annealing time of 24 hours. Furthermore, very high corrosion rates have been detected at the beginning of the immersions. In addition, the corrosion rates increase globally when the annealing time increases. However, when an annealing of 24 hours is performed, the corrosion rates decrease. Moreover, the corrosion of all the samples was localised. Specifically, the edges of the sample constituted favourable sites for corrosion. The energy dispersive X-ray spectroscopy revealed that there were large amounts of oxygen, iron and carbon that could highlight the presence of iron oxide. In addition, small amounts of calcium and phosphorus on the attacked area of the samples could be distinguished.