title: Strengthening of biomedical Ni-free Co-Cr-Mo alloy by multipass "low-strain-per-pass" thermomechanical processing.
abstract: Further strengthening of biomedical Co-Cr-Mo alloys is desired, owing to the demand for improvements to their durability in applications such as artificial hip joints, spinal rods, bone plates, and screws. Here, we present a strategy-multipass "low-strain-per-pass" thermomechanical processing-for achieving high-strength biomedical Co-Cr-Mo alloys with sufficient ductility. The process primarily consists of multipass hot deformation, which involves repeated introduction of relatively small amounts of strain to the alloy at elevated temperatures. The concept was verified by performing hot rolling of a Co-28 Cr-6 Mo-0.13N (mass%) alloy and its strengthening mechanisms were examined. Strength increased monotonically with hot-rolling reduction, eventually reaching 1,400 MPa in 0.2% proof stress, an exceptionally high value. Synchrotron X-ray diffraction (XRD) line-profile analysis revealed a drastic increase in the dislocation density with an increase in hot-rolling reduction and proposed that the significant strengthening was primarily driven by the increased dislocation density, while the contributions of grain refinement were minor. In addition, extra strengthening, which originates from contributions of planar defects (stacking faults/deformation twins), became apparent for greater hot-rolling reductions. The results obtained in this work help in reconsidering the existing strengthening strategy for the alloys, and thus, a novel feasible manufacturing route using conventional hot deformation processing, such as forging, rolling, swaging, and drawing, is realized.
  
  
  CONCLUSIONS
  The results obtained in this work suggested a novel microstructural design concept/feasible manufacturing route of high-strength Co-Cr-Mo alloys using conventional hot deformation processing. The present strategy focuses on the strengthening due to the introduction of a high density of lattice defects rather than grain refinement using dynamic recrystallization (DRX). The hot-rolled samples obtained by our process exhibited exceptional strength, which is comparable to the highest strength reported for biomedical Co-Cr-Mo alloys. It was also found that the acceptable ductility can be obtained even in such highly distorted Co-Cr-Mo alloys. We described the strengthening mechanisms in detail; this will be helpful for further investigations or industrial realization of the proposed strategy.
authors.affiliation: Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan; Department of Materials and Environmental Engineering, Sendai National College of Technology, 48 Nodayama, Medeshima-Shiote, Natori 981-1239, Japan.
authors.forename: Manami
authors.initials: M
authors.lastname: Mori
authors.affiliation: Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan. Electronic address: k_yamanaka@imr.tohoku.ac.jp.
authors.forename: Kenta
authors.initials: K
authors.lastname: Yamanaka
authors.affiliation: Graduate School of Science and Engineering, Ibaraki University, 4-12-1 Nakanarusawa, Hitachi 316-8511, Japan.
authors.forename: Shigeo
authors.initials: S
authors.lastname: Sato
authors.affiliation: Faculty of Engineering, Tokyo City University, 1-28-1 Tamazutsumi, Setagaya, Tokyo 158-8557, Japan.
authors.forename: Shinki
authors.initials: S
authors.lastname: Tsubaki
authors.affiliation: Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan.
authors.forename: Kozue
authors.initials: K
authors.lastname: Satoh
authors.affiliation: Faculty of Engineering, Tokyo City University, 1-28-1 Tamazutsumi, Setagaya, Tokyo 158-8557, Japan.
authors.forename: Masayoshi
authors.initials: M
authors.lastname: Kumagai
authors.affiliation: Faculty of Engineering, Tokyo City University, 1-28-1 Tamazutsumi, Setagaya, Tokyo 158-8557, Japan.
authors.forename: Muneyuki
authors.initials: M
authors.lastname: Imafuku
authors.affiliation: Japan Atomic Energy Agency, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5148, Japan.
authors.forename: Takahisa
authors.initials: T
authors.lastname: Shobu
authors.affiliation: Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan.
authors.forename: Akihiko
authors.initials: A
authors.lastname: Chiba
mesh_terms: D000497:Alloys; D008670:Metals / Q000737:chemistry*; D017356:Synchrotrons; D013718:Tensile Strength; D014961:X-Ray Diffraction
chemical_list: D000497:Alloys; D008670:Metals
keywords: Biomedical Co−Cr−Mo alloy; Dislocation structures; Materials design; Strengthening mechanism; Thermomechanical processing