Abstract
This is the first report presenting the development of a Co–Cr–W–Ni–Mn alloy by adding 6 mass pct Mn to ASTM F90 Co–20Cr–15W–10Ni (CCWN, mass pct) alloy for use as balloon-expandable stents with an excellent balance of mechanical properties and corrosion resistance. The effects of Mn addition on the microstructures as well as the mechanical and corrosion properties were investigated after hot forging, solution treatment, swaging, and static recrystallization. The Mn-added alloy with a grain size of ~ 20 µm (recrystallization condition: 1523 K, 150 seconds) exhibited an ultimate tensile strength of 1131 MPa, 0.2 pct proof stress of 535 MPa, and plastic elongation of 66 pct. Additionally, it exhibited higher ductility and lower yield stress while maintaining high strength compared to the ASTM F90 CCWN alloy. The formation of intersecting stacking faults was suppressed by increasing the stacking fault energy (SFE) with Mn addition, resulting in a lower yield stress. The low-yield stress is effective in suppressing stent recoil. In addition, strain-induced martensitic transformation during plastic deformation was suppressed by increasing the SFE, thereby improving the ductility. The Mn-added alloys also exhibited good corrosion resistance, similar to the ASTM F90 CCWN alloy. Mn-added Co–Cr–W–Ni alloys are suitable for use as balloon-expandable stents.
Highlights
INTRODUCTIONManuscript submitted April 16, 2021; accepted June 25, 2021. Article published online July 7, 2021
MINIMALLY invasive medical treatment approaches are in high demand for stent surgery to minimize operative trauma in patients
High ductility and low yield stress were achieved in Mn-added alloys while maintaining high strength
Summary
Manuscript submitted April 16, 2021; accepted June 25, 2021. Article published online July 7, 2021. An alloy with 8 mass pct Mn added to the ASTM F90 CCWN alloy (average grain size: 129 lm) achieved high ductility (75 pct of plastic elongation) and exhibited an increased work hardening rate during plastic deformation.[26] the 8 mass pct Mn-added CCWN alloy showed a lower yield stress than that of the ASTM F90 composition alloy with an average grain size of 207 lm.[26] Mn was chosen as the alloying element for the CCWN alloy because it acts as a c-stabilizing element,[28,29,30] and increases twinnability[29] and suppresses precipitate formation[31] in Co-based alloys. We developed a Co–Cr–W–Ni–Mn alloy by adding 6 mass pct Mn to ASTM F90 CCWN alloy to obtain ‘‘low-yield stress type’’ stent materials. (mass pct, VDM Metals, DEU, Lot 2) alloy forged bar Both were based on ASTM F90 standard chemical composition. The raw materials used were Co–20.12Cr–14.94W– 9.77Ni–1.86Fe–1.58Mn–0.079C–0.033N (mass pct, Carpenter Technology Corporation, Lot 1) alloy forged bar and Co–20.43Cr–15.14W–10.19Ni–2.61Fe–1.81Mn–0.084C
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