Ultrafine Grain Refinement of Biomedical Co-29Cr-6Mo Alloy during Conventional Hot-Compression Deformation KENTA YAMANAKA, MANAMI MORI, SHINGO KUROSU, HIROAKI MATSUMOTO, and AKIHIKO CHIBA In order to examine the microstructural evolution during hot-compression deformation of the biomedical Co-29Cr-6Mo (weight percent) alloy without the addition of Ni, hot-compression tests have been conducted at deformation temperatures ranging from 1050 °C to 1200 °C at various strain rates of 10 3 to 10 s 1 . The grain refinement due to dynamic recrystallization (DRX) was identified under all deformation conditions by means of field-emission scanning electron microscopy/electron backscattered diffraction (FESEM/EBSD) and transmission electron microscopy (TEM) observations. Although the DRX grain size (d) of the deformed specimens considerably decreased with an increasing Zener–Hollomon (Z) parameter at strain rates ranging from 10 3 to 0.1 s 1 , a grain size coarser than that predicted from the d-Z relation was obtained at strain rates of 1.0 and 10 s 1 . An ultrafine-grained microstructure with a grain size of approximately 0.6 lm was obtained under deformation at 1050 °C at 0.1 s 1 , from an initial grain size of 40 lm. The grain refinement to a submicron scale of biomedical Co-Cr-Mo alloys has been achieved with hot deformation by ~60 pct due to DRX, in which the bulging mechanism is not operative. The ultrafine grains obtained due to DRX without bulging is closely related to the considerably low stacking-fault energy (SFE) of the Co-Cr-Mo alloy at deformation temperatures. DOI: 10.1007/s11661-009-9879-0 Ó The Minerals, Metals & Materials Society and ASM International 2009 I. INTRODUCTION ALLOYS of Co-Cr-Mo have been extensively used in biomedical implants such as artificial hip and knee joints because of their excellent corrosion and wear resistance as well as their good mechanical properties. Although Co-Cr-Mo alloys are frequently used in the as-cast condition, they exhibit a coarse dendritic microstructure with many flaws, such as interdendritic microvoids, precipitations, and segregation of solute atoms, which are formed during solidification. Therefore, the strength of these alloys in the as-cast condition is extremely low. It is thus important to enhance their strength for practical applications by eliminating inherent defects through the application of thermomechanical treat- ments. [1] Grain refinement is being investigated for various metals and alloys, because it enhances strength without sacrificing toughness. In particular, ultrafine grains smaller than 1 lm and nanocrystalline (d < 100 nm) alloys have been produced not by microstructure control achieved using conventional thermomechanical process- ing but by severe plastic deformation (SPD). [2–8] Various SPD techniques have been proposed, including equal- channel angular pressing (ECAP), [2–4] high-pressure torsion, [2] accumulative roll-bonding, [5,6] and multidi- rectional forging. [7,8] Although grain refinement is an effective strengthening method that is independent of alloy design, there have been no reports of the applica- tion of SPD techniques to Co-Cr-Mo alloys. This is because Co-Cr-Mo alloy forms a dual-phase micro- structure due to a martensitic transformation during cooling to room temperature and straining. This dual phase consists of a metastable fcc c phase and an hcp e phase and it suffers from poor ductility. In addition, the application of SPD is also limited by the high work hardening generated by the strain-induced martensitic transformation during plastic deformation caused by the instability of the c phase. Moreover, according to the Co-Cr-Mo ternary phase diagram, [4] when Co-Cr-Mo alloy is held at high temperatures, there is a high probability that it will precipitate the r phase (CoCr: P42/mnm). This can act as a starting point of fracture, making it difficult to simultaneously suppress the r phase, using microstructure control in conjunction with static recrystallization (SRX), and perform grain refine- ment. Therefore, when plastically forming the alloy, a large amount of Ni (10 to 37 mass pct) is generally added to Co-Cr-Mo alloys to improve their ductility by stabilizing the c phase (the high-temperature phase) at room temperature. However, using Ni in biomaterials is problematic, because it can cause allergies or cancer in KENTA YAMANAKA, Graduate Student, formerly with the Department of Materials Processing, Graduate School of Engineering, Tohoku University, is with Kobe Steel, Ltd., Kobe 651-8585, Japan. MANAMI MORI, Graduate Student, formerly with the Department of Materials Processing, Graduate School of Engineering, Tohoku University, is with Nissan Arc, Ltd., Yokosuka 237-0061, Japan. SHINGO KUROSU, Postdoctoral Student, HIROAKI MATSUMOTO, Assistant Professor, and AKIHIKO CHIBA, Professor, are with the Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan. Contact e-mail: a.chiba@imr.tohoku.ac.jp Manuscript submitted January 30, 2009. METALLURGICAL AND MATERIALS TRANSACTIONS A