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

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.Yanagihara S., Ueki K., Ueda K., et al. Development of Low-Yield Stress Co–Cr–W–Ni Alloy by Adding 6 Mass Pct Mn for Balloon-Expandable Stents. Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science, 52, 9, 4137. https://doi.org/10.1007/s11661-021-06374-7

Improvement of mechanical properties by microstructural evolution of biomedical Co-Cr-W-Ni alloys with the addition of Mn and Si

We investigated changes in the microstructure and mechanical properties of biomedical Co-20Cr-15W-10Ni alloys (mass%) containing 8 mass% Mn and 0-3 mass% Si due to hot forging, solution treatment, cold swaging, and static recrystallization. The η-phase (M₆X-M₁₂X type cubic structure, M: metallic elements, X: C and/or N, space group: Fd-3m (227)) and CoWSi type Laves phase (C14 MgZn2 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 Co-Cr-W-Ni-based alloys. The addition of Si promoted the formation of precipitates of both η-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 balloon-expandable 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 Co-Cr-W-Ni alloy, respectively.Ueki K., Yanagihara S., Ueda K., et al. Improvement of mechanical properties by microstructural evolution of biomedical Co-Cr-W-Ni alloys with the addition of Mn and Si. Materials Transactions 62, 229 (2021); https://doi.org/10.2320/matertrans.MT-M2020300

Overcoming the strength-ductility trade-off by the combination of static recrystallization and low-temperature heat-treatment in Co-Cr-W-Ni alloy for stent application

A process combining swaging, static recrystallization, and heat treatment at 873 K (low-temperature heat-treatment, LTHT) was developed for achieving both high ultimate strength and high ductility in Co-20Cr-15W-10Ni (mass%, CCWN) alloy for stent application. The alloys swaged to a sectional area reduction rate of 58.3% were annealed at 1373–1473 K for 30–300 s. Under annealing at 1373 K for 300 s, a fine grain structure with an average grain size of ~6 μm formed, while under annealing at 1473 K, a structure with an average grain size of 12 μm formed after 120 s. In the alloys annealed at 1373–1448 K, the formation of η-phase precipitates (M6X-M12X type, M: metallic elements, X: C and/or N) was observed, while no precipitates were observed in the alloys annealed at 1473 K. The improvement in ultimate strength by grain refinement was confirmed. Alloys annealed at 1473 K showed higher ductility compared to those annealed at 1373–1448 K even if the grain size was similar. It is considered that the η-phase precipitates deteriorated the ductility of the annealed alloys. LTHT suppressed the strain-induced martensitic γ-to-ε transformation to improve the ductility of the fine-grained as well as coarse-grained alloys. Thus, regardless of the grain size, it is newly evidenced that LTHT effectively improves ductility in CCWN alloy. By combining high-temperature short-time annealing and LTHT, both the ultimate strength and ductility of Co-20Cr-15W-10Ni (mass%) alloy improved, and it was possible to provide properties suitable for next-generation balloon-expandable stents with Co-20Cr-15W-10Ni (mass%) alloy.Ueki K., Yanagihara S., Ueda K., et al. Overcoming the strength-ductility trade-off by the combination of static recrystallization and low-temperature heat-treatment in Co-Cr-W-Ni alloy for stent application. Materials Science and Engineering A, 766, 138400. https://doi.org/10.1016/j.msea.2019.138400

The definition of healthcare-associated pneumonia (HCAP) is insufficient for the medical environment in Japan: a comparison of HCAP and nursing and healthcare-associated pneumonia (NHCAP)

Healthcare-associated pneumonia (HCAP) is a new concept of pneumonia, which was proposed in the ATS/IDSA guidelines. The guidelines explain that HCAP patients should be treated with broad-spectrum antimicrobial drugs directed at multidrug-resistant pathogens. However, in Japan, there are many elderly people who received in-home care service. These patients seemed to be consistent with the concept of HCAP, but they did not meet the definition of HCAP. Therefore, the Japanese Respiratory Society modified the definition of HCAP according to the medical environmental in Japan. We retrospectively observed HCAP patients and nursing home and healthcare-associated pneumonia (NHCAP) patients who were hospitalized during 24 months at the Japanese Red Cross Nagasaki Genbaku Hospital (Nagasaki, Japan). Patient background, disease severity, identified pathogens, initial antibiotic regimens, and outcomes were compared. A total of 108 patients (77 HCAP and 31 NHCAP except HCAP patients) were evaluated. Of NHCAP except HCAP patients, 27 (87.1 %) were above 3 in the ECOG PS score. There were almost no significant differences between the two groups in characteristics, pneumonia severity, identified bacteria, initial antibiotic regimens, and response rate of initial antibiotic therapy. Although the in-hospital mortality of HCAP patients and NHCAP except HCAP patients was 9.1 % and 19.4 %, respectively, this difference did not reach statistical significance (P > 0.05). Our study suggested that, in the criteria of HCAP, some Japanese patients, who were consistent with the concept of HCAP, were classified as community-acquired pneumonia (CAP). Therefore, there is a need to change the definition of HCAP according to the medical environment in Japan

Evaluation of FilmArray respiratory panel multiplex polymerase chain reaction assay for detection of pathogens in adult outpatients with acute respiratory tract infection

Although viruses are the major pathogen that causes upper respiratory tract infection (URTI) and acute bronchitis, antibiotics have been prescribed. This was a prospective observational study in influenza epidemics that enrolled adult outpatients who visited a hospital with respiratory tract infection symptoms. In this study, we evaluated the usefulness of FilmArray respiratory panel (RP). Fifty patients were enrolled. FilmArray RP detected the pathogens in 28 patients. The common pathogens were influenza virus (n = 14), respiratory syncytial virus (n = 6), and human rhinovirus (n = 6). Of the 14 patients with influenza virus, 6 were negative for the antigen test. The physicians diagnosed and treated the patients without the result of FilmArray in this study. Of the patients with positive FilmArray RP, 9 were treated with antibiotics; however, bacteria were detected in only 3 patients. By implementing FilmArray RP, URTI and acute bronchitis would be precisely diagnosed, and inappropriate use of antibiotics can be reduced

宇宙往還機形状の空力特性推定のための飛行試験法に関する研究

University of Tokyo (東京大学