Fatigue behavior in Co–Cr–Ni–Mo medical wires drawn with different drawing practices

Abstract

35%Co–20%Cr–35%Ni–10%Mo alloy (MP35N) with a low titanium (LT) content is commonly used as the material for the manufacture of leads in medical applications such as Cardiac Rhythmic Management (CRM), neurostimulation and Deep Brain Stimulation (DBS). The material is drawn to thin wires, and this involves subjecting the material to extensive plastic deformation to obtain the desired mechanical and fatigue properties. The critical part in the medical device design requires understanding the relationship between the processing method, microstructure and their impact on the cyclic performance. In this study, the influence of drawing practices namely Full Die Drawing (FDD) and Half Die Drawing (HDD) on the cyclic performance of MP35NLT wires have been investigated, by differing the amount of plastic strain applied on the material. The as-drawn wires were subjected to rotary beam fatigue tests (R = −1) with varying stress amplitudes, and the microstructural factors controlling the Low Cycle Fatigue (LCF) and High Cycle Fatigue (HCF) performance were determined through post fatigued TEM investigations. The results conclude that there is a significant impact on the drawing techniques on the fatigue performance of MP35NLT wires, with the FDD wires having a higher endurance limit than the HDD drawn wire until 75% CW, but there was a slight drop in the fatigue performance when the CW was increased to 95%. This was attributed to the difference in grain sizes, dislocation structures, twin density, twin spacing, and grain boundary orientations observed between the two wires which contributed to contrasting cyclic behavior. The drop-in fatigue performance of the FDD wire upon an increased strain to 95% has been attributed to the shear band formations in the material, which increased the stress localization in the material bestowing to drop-in fatigue performance.