Probing plastic deformation-related properties in static recrystallized Co–Cr–Fe–Ni–Mo alloy for biomedical applications

Abstract

40Co–20Cr–16Fe–15Ni–7Mo (mass%, Co–Cr–Fe–Ni–Mo) alloys are currently used in biomedical implants such as balloon-expandable stents. Materials used as balloon-expandable stents are statically recrystallized and subsequently utilized within the body in a plastically deformed state. This study describes the microstructures and the mechanical and corrosive properties related to the plastic deformation of statically recrystallized Co–Cr–Fe–Ni–Mo alloys. The η-phase (M6X-M12X type, M: metallic elements; X: C and/or N) and M23X6-type precipitates were observed. The grain sizes of the recrystallized alloys were 6–92 μm. The mechanical properties were dependent on the grain size: the 0.2% proof strength and ultimate tensile strength increased and plastic elongation decreased with an increase in grain refinement. The stacking fault energy (SFE) of the Co–Cr–Fe–Ni–Mo alloy was determined by the width of partial dislocations, measured as 16 ± 3 mJ⋅m−2. Corresponding to this SFE, the formation of the ε-phase associated with strain-induced martensitic transformation and deformation twins were observed after plastic deformation. This suggests that the Co–Cr–Fe–Ni–Mo alloy exhibited a twinning-induced plasticity-like work-hardening mechanism. The polarization curves of the alloys in a simulated body fluid were not affected by the formation of precipitates (η-phase and M23X6-type) or plastic deformation, thereby suggesting that heat treatment does not deteriorate the corrosive property. The recrystallized alloy with a grain size of 36 μm had the ultimate tensile strength of more than 1000 MPa, 0.2% proof strength of less than 500 MPa, and plastic elongation greater than 70%. This alloy is expected to be applied in low-yield-strength-type balloon-expandable stents.