title: Cold-rolling behavior of biomedical Ni-free Co-Cr-Mo alloys: Role of strain-induced ε martensite and its intersecting phenomena.
abstract: Ni-free Co-Cr-Mo alloys are some of the most difficult-to-work metallic materials used commonly in biomedical applications. Since the difficulty in plastically deforming them limits their use, an in-depth understanding of their plastic deformability is of crucial importance for both academic and practical purposes. In this study, the microstructural evolution of a Co-29Cr-6Mo-0.2N (mass%) alloy during cold rolling was investigated. Further, its work-hardening behavior is discussed while focusing on the strain-induced face-centered cubic (fcc) γ→hexagonal close-packed (hcp) ε martensitic transformation (SIMT). The planar dislocation slip and subsequent SIMT occurred even in the initial stage of the deformation process owing to the low stability of the γ-phase and contributed to the work hardening behavior. However, the amount of the SIMTed ε-phase did not explain the overall variation in work hardening during cold rolling. It was found that the intersecting of the SIMTed ε-plates enhanced local strain evolution and then produced fine domain-like deformation microstructures at the intersections. Consequently, the degree of work hardening was reduced during subsequent plastic deformation, resulting in the alloy exhibiting a two-stage work hardening behavior. The results obtained in this study suggest that the interaction between ε-martensites, and ultimately its relaxation mechanism, is of significant importance; therefore, this aspect should be addressed in detail; the atomic structures of the γ-matrix/ε-martensite interfaces, the phenomenon of slip transfer at the interfaces, and the slipping behavior of the ε-phase itself are needed to be elucidated for further increasing the cold deformability of such alloys.
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: 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: D001672:Biocompatible Materials / Q000737:chemistry*; D003080:Cold Temperature; D003722:Dental Alloys / Q000737:chemistry*; D008422:Materials Testing; D009584:Nitrogen / Q000737:chemistry; D013314:Stress, Mechanical; D014800:Vitallium / Q000737:chemistry*
chemical_list: D001672:Biocompatible Materials; D003722:Dental Alloys; C093824:Martensite; D014800:Vitallium; D009584:Nitrogen
keywords: Biomedical Co–Cr–Mo alloy; Cold rolling; Deformation microstructures; Martensitic transformation; Work hardening