Improvement of Mechanical Properties by Microstructural Evolution of Biomedical Co–Cr–W–Ni Alloys with the Addition of Mn and Si

Abstract

We investigated changes in the microstructure and mechanical properties of biomedical Co20Cr15W10Ni alloys (mass%) containing 8 mass% Mn and 03 mass% Si due to hot forging, solution treatment, cold swaging, and static recrystallization. The ©-phase (M 6 XM 12 X type cubic structure, M: metallic elements, X: C and/or N, space group: Fd-3m ( 227)) and CoWSi type Laves phase (C14 MgZn 2 type hexagonal structure, space group: P63/mmc (194)) were confirmed as precipitates in the as-cast and as-forged alloys. To the best of our knowledge, this is the first report that reveals the formation of CoWSi type Laves phase precipitates in CoCrWNi-based alloys. The addition of Si promoted the formation of precipitates of both ©-phase and CoWSi type Laves phase. The solution-treated 8Mn+(0, 1)Si-added alloys exhibited TWIP-like plastic deformation behavior with an increasing work-hardening rate during the early to middle stages of plastic deformation. This plastic deformation behavior is effective in achieving both the low yield stress and high strength required to develop a high-performance balloonexpandable stent. The 8Mn+2Si-added alloy retained the CoWSi type Laves phase even after solution treatment, such that the ductility decreased but the strength improved. Additions of Mn and Si are effective in improving the ductility and strength of the CoCrWNi alloy, respectively.

Development of Low-Yield Stress Co–Cr–W–Ni Alloy by Adding 6 Mass Pct Mn for Balloon-Expandable Stents

“…In the coarse grain structure, a reduction in the yield stress by the addition of 6 mass pct Mn to CCWN alloy was not observed. On the other hand, in the 8 mass pct Mn-added alloy, [26] the yield stress was lower than that of the Base alloy even in the coarse grain structure. Lee et al reported that grain coarsening reduces apparent SFE and facilitates the formation of stacking faults.…”

“…In our previous study, 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] Furthermore, 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 not only acts as a c-stabilizing element, [28][29][30] but also increases twinnability [29] and suppresses precipitate formation [31] in Co-based alloys.…”

“…[26] Furthermore, 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 not only acts as a c-stabilizing element, [28][29][30] but also increases twinnability [29] and suppresses precipitate formation [31] in Co-based alloys. In addition, Mn is also an essential trace element for the human body and is suitable as an alloying element for biomedical metallic materials.…”

“…Therefore, in this study, we focused on developing a new alloy based on CCWN alloy to produce a ''low-yield stress type'' stent material. [26] Twinning induced plasticity (TWIP) steels contain Mn, which is an austenite stabilizing element in Fe, resulting in a stacking fault energy (SFE) at room temperature of approximately 20 to 40 mJ m À2 . [27] Therefore, in the fcc alloy, it is presumed that the composition should be controlled such that the SFE is in the range of 20 to 40 mJ m À2 to exhibit TWIP-like plastic deformation behavior.…”

“…In addition, Mn is also an essential trace element for the human body and is suitable as an alloying element for biomedical metallic materials. However, the addition of 8 mass pct Mn reduces not only the yield stress, but also the ultimate tensile strength of the ASTM F90 composition alloy [26] ; hence, it is necessary to optimize the alloy composition. In addition, considering that the grain size in current balloon-expandable stents is approximately 30 lm or smaller, [5] it is important to clarify the grain size effect on the mechanical properties of the Mn-added CCWN alloy in the range of around 20 to 40 lm.…”