Improvement of mechanical properties of Co–Cr–W–Ni alloy tube suitable for balloon-expandable stent applications through heat treatment

Abstract

This study focuses on the changes in the microstructure and mechanical properties of the Co–20Cr–15W–10Ni (CCWN, mass%) alloy tube, which, with heat treatment, is used as a balloon-expandable stent. To the best of our knowledge, this is the first report on the improvement of mechanical properties by microstructure control in CCWN alloy tubes. In addition, optimized heat treatment conditions are proposed for the development of next-generation small-diameter stents. The cold-worked CCWN alloy tube for stents was statically recrystallized by heat treatment at temperatures of 1373–1473 K for holding times of 60–900 s. The recrystallized alloy tubes exhibited a microstructure with a γ(fcc)-phase matrix and an average grain size of 5.8–34.1 μm. The strength and plastic elongation increased with decreasing grain size. In the coarse-grained structure, a large plate-like ε-phase formed along the twin boundary and stacking fault. The ε-phase grew to the grain boundaries, such that large cracks could form, resulting in early fracture. In the fine-grained structure, a small ε-phase formed by strain-induced martensitic transformation was uniformly dispersed, and the formation of large cracks was suppressed, leading to high ductility. The effect of low-temperature heat treatment (LTHT) at 873 K after recrystallization depended on the grain size of the alloy tube. In the alloy tube with a grain size of >15 μm, ε(hcp)-phase formation during plastic deformation was suppressed through the decrease in stacking fault density during LTHT, which resulted in improved ductility. In contrast, in the alloy tube with a grain size of <15 μm, the amount of precipitates increased during LTHT, resulting in reduced ductility.