Co-Cr-Ni-Mo wires for medical applications : processing, mechanical, microstructure and fatigue characterization

• Main Author: Gvk, Sai Srikanth
• Other Authors: Tan Ming Jen
• Format: Theses and Dissertations
• Language: English
• Published: 2019
• Subjects: Engineering::Mechanical engineering
• Online Access: https://hdl.handle.net/10356/105610; http://hdl.handle.net/10220/50147
• Institution: Nanyang Technological University

MP35N (35% Co-35% Ni-20% Cr-10% Mo) is a cobalt-based face-centered cubic (FCC) superalloy which is commonly used as conductor in cardiac and neurostimulation leads due to its good combination of strength, ductility, fatigue life, and corrosion resistance. Thin wires for the conductor leads are manufactured from this material using the cold drawing process which involves plastic deformation of the material and subsequent heat treatments to get the desired properties. As the process involves substantial to severe plastic deformation, it is necessary and very critical to understand the relationship between the process and the microstructure and to correlate them to mechanical and fatigue performance. The crystallographic texture is the link correlating the deformation and thermal treatments to their properties and performance. In the present study, an attempt was made to understand the influence of the drawing practices i.e. Full Die Drawing (FDD) and Half Die Drawing (HDD) on their mechanical and electrical properties, deformation homogeneity, plastic instability, strain rate sensitivity, strain rate hardening and cyclic fatigue behavior of MP35N wires, for different cold work (CW) reductions. The difference in properties observed, are associated and compared to its microstructure, which has been characterized by Field Emission Scanning Electron Microscope (FESEM), Electron Beam Scattered Diffraction (EBSD), and Transmission Electron Microscope (TEM). The results showed that wires drawn with FDD practice demonstrated a higher yield strength (σ_YS) ultimate tensile strength (σ_UTS) higher work hardening rate and different work hardening regimes when compared to the HDD wires for a similar amount of CW. It was observed that FDD wires had a smaller grain size, more homogenous texture and a higher number of twins and dislocations, whereas the HDD wires exhibited coarse grains, non-uniform texture and a lower amount of lattice defects. The Inhomogeneous Factor (IF) and electrical conductivity was measured on the wires, and the results concluded that the FDD drawn wires exhibited homogenous deformation, uniform microstructural and hardness gradient across the wire when compared to HDD wires. The electrical conductivity of the HDD drawn wires was higher than the FDD wires, and the level of inhomogeneity and the variation of conductivity decreased with the increase of CW. Plastic instability (PI) in MP35N wires, was determined from the logarithmic plots of the true stress and strain curves and the strain hardening exponent (γ) and strain rate sensitivity (SRS) (m) values were computed. It was found that wires drawn with FDD practice had a lower instability until 75% CW, after which the PI increased significantly when drawn to 95% CW. The lower instability in the FDD wires at 75% CW, was attributed to the smaller grain size, higher dislocation density and lower twin spacing which contributed to higher γ and m, as characterized by TEM. Several shear bands were observed in the 95% CW FDD wires, which led to plastic strain localization and increased instability. The influence of strain rate (SR) on the SRS, strain rate work hardening (SRWH) in MP35N wires, was investigated by subjecting the wires to uniaxial tensile tests, and by varying the SR from 10-6s-1 to 10-2s-1. The experimental results illustrate that the strength, SRWH, and the m values were observed to be higher in the FDD drawn wire. The increase in strength and hardening rate of the FDD drawn wire with the rise in SR was ascribed to increased dislocation density and reduced twin thickness, and the increased SRS and ductility at low SR were attributed to the increased grain boundary (GB) activities. An abnormal SRWH was observed in the HDD drawn wire tested to an SR of 10-2s-1, where a Stage II hardening peak was observed at a very high strain, due to the solute segregation of the Mo atoms to the GB. Finally, the fatigue behavior of the wires was compared, and the results demonstrated that the FDD drawn wires had a higher endurance limit 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 different grain sizes, dislocation structures, fracture surfaces and crack morphologies observed between the two wires. It can be concluded from the current study that the drawing process and its relationship to texture are highly significant and the selection of the appropriate process should be determined based on the functional requirements and application of the product.