Isothermal Phase Transformation in Biomedical Co-29Cr-6Mo Alloy without Addition of Carbon or Nitrogen
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INTRODUCTION
COBALT-CHROMIUM-MOLYBDENUM (Co-CrMo) alloys have been used for surgical implants, such as hip and knee implant prostheses, because of their excellent mechanical properties, corrosion resistance, wear resistance, and biocompatibility.[1–4] However, particularly for hip and knee joint replacements, even higher reliability and safety levels are required because these procedures are performed on younger patients, significantly increasing the implant service lifetime. Further improvement of the mechanical properties of Co-Cr-Mo alloys should be achieved by microstructural control, such as can be achieved by thermomechanical or heat treatment. It is now well known that the tensile properties and hardness of Co-Cr-Mo alloys with high carbon content (0.15 to 0.35 mass pct) can be improved significantly by aging treatment at temperatures between 923 K and 1173 K (650 C and 900 C), since a lamellar structure with a high M23C6 carbide content can be obtained.[5–9] However, it has been reported that these carbides increase the rate of polyethylene wear debris in metal-on-polyethylene (MOP) joint replacements, which leads to osteolysis and in some cases to loosening of the components.[10] In order to solve this problem, metalon-metal (MOM) compositions employing Co-Cr-Mo SHINGO KUROSU, Postdoctoral Researcher, 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 June 25, 2009. Article published online June 25, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
alloys are often used for hip and knee joint replacements instead of MOP. The wear results for Co-Cr-Mo alloy combinations depend on the carbon content. Medley et al.[11] reported the wear behavior of MOM combinations using a hip simulator, in which a low-carbon wrought Co-Cr-Mo alloy exhibited a lower wear than either a high-carbon cast alloy or a low-carbon commercial alloy. Chiba et al.[12] also reported that a low-carbon forged Co-Cr-Mo alloy had lower wear than a high-carbon cast Co-Cr-Mo alloy. However, there are a number of reports[13,14] suggesting that high-carbon forged Co-Cr-Mo alloys show superior wear properties, due to the presence of distributed fine carbides at the surface rather than the widely spaced coarse carbide of the cast alloy. Previous research suggested that there are at least two possibilities for coupling Co-Cr-Mo/Co-Cr-Mo MOM combinations with excellent wear resistance: one is the combination of low-carbon forged Co-Cr-Mo alloys, and the other is that of high-carbon forged Co-Cr-Mo alloys. Controversy remains over which microstructure exhibits the optimal wear resistance in the combination of MOM articulation. The ASTM standard alloy for artificial hip or knee joint material, Co-(26-30)Cr-(5-7)Mo-(0-0.35)C-(0-0.25)N, i.e., ASTM F75, consists mainly of the c phase (fcc) and athermal e (hcp) martensite formed due to rapid cooling, but also contains minor phases and M23C
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