Ultrafine Grain Refinement of Biomedical Co-29Cr-6Mo Alloy during Conventional Hot-Compression Deformation
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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 treatments.[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) 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: [email protected] Manuscript submitted January 30, 2009. Article published online June 20, 2009 1980—VOLUME 40A, AUGUST 2009
alloys have been produced not by microstructure control achieved using conventional thermomechanical processing but by severe plastic deformation (SPD).[2–8] Various SPD techniques have been proposed, including equalchannel angular pressing (ECAP),[2–4] high-pressure torsion,[2] accumulative roll-bonding,[5,6] and multidirectional forging.[7,8] Although grain refinement is an effective strengthening method that is independent of alloy design, there have been no reports of the application of SPD techniques to Co-Cr-Mo alloys. This is because Co-Cr-Mo alloy forms a dual-phase microstructure 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 refinement. Therefore, when plast
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