Fatigue-Resistant CoCrMoNi Medium Entropy Alloys for Medical Devices

Aiming for the application to medical devices, Co_50-<i>x</i>Cr₂₀Mo₁₀Ni_20+<i>x</i> (<i>x</i> = 0<b>–</b>15) medium entopy alloys were designed based on the FCC-HCP phase stability. Room temperature tensile tests revealed markedly high work-hardening rate and good strength<b>–</b>elongation balance of the alloys. X-ray diffraction (XRD) and Electron Backscatterd Diffraction (EBSD) analysis revealed that the dominant deformation modes gradually shift from the FCC-HCP transformation to deformation twinning with the Ni content, <i>x</i>. The observed good strength<b>–</b>elongation balance may be due to the TRIP and TWIP. In the tensile deformed samples of the <i>x</i> = 3<b>–</b>9 alloys, HCP phase and twins coexist.<br/>The low cycle fatigue (LCF) behavior was investigated with a total strain amplitude of 0.02. The highest LCF lives were observed for the <i>x</i> = 6 alloy (6537 cycles). They are much higher than those of alloys currently used for coronary stents. These findings suggests that the formation of epsilon-twin bundles may be an important factor to obtain high LCF resistance.<br/>Microstructures and mechanical properties of tubes and wires made of the MEA alloys as well as the property of a proto type stent will be also reported.<br/><br/>Thus work was partially supported by a Grant-in-Aid for Scientific Research on Innovative Area,“High-Entropy Alloys-Science of New Class of Materials Based on Elemental Multiplicity and Heterogeneity” through MEXT, Japan (contract no. 18H05451), by JSPS KAKENHI Grant Number 23HD1719, and by AMED under Grant Number JP24ym0126104s0102.