Carbides and their Role in Advanced Mechanical Properties of L605 Alloy: Implications for Medical Devices

Abstract

L605 (ASTM F90), a cobalt-chromium-tungsten alloy with excellent mechanical properties and high radiopacity, has been widely accepted as a suitable alloy for stent applications. The presence of carbides in this alloy, primary carbides and secondary carbides, leads to difficulties in controlling mechanical performances and therefore in optimizing stent size and performances. This work is thus to investigate the carbides and their role in advanced mechanical properties of L605 alloy for stent fabrication. Herein, the nature, nucleation, distribution and dissolution of the carbides were investigated in a series of recrystallized L605 tubes from hard-drawn (HD) state. The mechanical properties corresponding to each carbide state were examined by tensile tests and microhardness measurements. The results indicate important relationships among carbide precipitation, grain size and mechanical behaviors, as a function of annealing temperature and duration. The intergranular secondary carbides, induced at the onset of the recrystallization of L605 matrix, were preferentially precipitated at grain boundaries. The nucleation of such particulate phase leads to a pinning effect on grain coarsening, resulting in a strengthening effect of the material. However, the further growth of the secondary carbides brings about considerable reduction of ductility, which is inacceptable for stent application. Therefore, an optimization protocol on carbides controlling was developed to maintain the strengthening effect without losing ductility and small grain size.