Improvement of strength and ductility by combining static recrystallization and unique heat treatment in Co-20Cr-15W-10Ni alloy for stent application

The Co-20Cr-15W-10Ni (CCWN, mass%) alloy has excellent corrosion resistance and strength-ductility balance and is applied in almost all balloon-expandable stent platforms. To further reduce the invasiveness of stent placement, it is necessary to reduce the diameter of the stent. That is, both high strength and high ductility should be achieved while maintaining a low yield stress. In our previous studies, it was discovered that low-temperature heat-treatment (LTHT) at 873 K improves the elongation of the CCWN alloy. In this study, we focused on the grain refinement by swaging and static recrystallization to improve the strength of the alloy. The as-swaged alloy was recrystallized at 1373-1473 K for 100-300 s, followed by LTHT. A fine grain structure with an average grain size of 3-17 µm was obtained by static recrystallization. The η-phase (M₁₂X-M₆X type precipitates, M: metallic elements, X: C and/or N) formed during the recrystallization at 1373-1448 K. The alloys recrystallized at 1448 and 1473 K had a homogeneous structure with a small variation in the grain size. On the other hand, the alloys recrystallized at 1373 and 1423 K had an inhomogeneous structure in which fine and coarse grains were mixed. Both the strength and ductility of the CCWN alloy were improved by combining high-temperature short-time recrystallization and LTHT.