Precipitates in As-Cast and Heat-Treated ASTM F75 Co-Cr-Mo-C Alloys Containing Si and/or Mn
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cobalt-chromium-molybdenum (Co-Cr-Mo) alloys are used in the preparation of alloy castings by the investment casting technique.[1,2] These alloy castings are widely applied in surgical implants owing to their excellent corrosion and wear resistance and mechanical properties.[3–5] The alloy components prepared by investment casting exhibit a microstructure comprising a Co-rich dendrite matrix and interdendritic and grain boundary precipitates.[6–10] In the as-cast ASTM F75 Co-Cr-Mo alloys, the main precipitate was consistently identified as the M23C6-type carbide, wherein M refers to metallic elements such as Cr. Other phases such as the M6C-M12C–type carbide (η phase)[11–14] and intermetallic σ phase (Co(Cr,Mo)) were identified as minor precipitates. The possible precipitation of M7C3type carbide in the ASTM F75 Co-Cr-Mo alloys during ALFIRANO and SHINGO MINETA, Graduate Students, KYOSUKE UEDA, Assistant Professor, and TAKAYUKI NARUSHIMA, Professor, are with the Department of Materials Processing, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan. Contact e-mail: [email protected]. tohoku.ac.jp SHIGENOBU NAMBA, Senior Research Metallurgist, is with the Materials Research Laboratory, Kobe Steel, Ltd., Kobe 651-2271, Japan. TAKASHI YONEDA, President, is with Yoneda Advanced Casting Co., Ltd., Takaoka 933-0951, Japan. Manuscript submitted August 30, 2010. Article published online February 2, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
solidification was also suggested, but its presence was not confirmed.[9] As-cast Co-Cr-Mo alloys are typically subjected to heat treatment in order to eliminate casting defects and to improve their mechanical properties. The behavior of carbides in Co-Cr-Mo alloys during heat treatment was previously investigated by several research groups, including the present authors.[10,15–20] It is known that the addition of alloying elements to biomedical Co-Cr-Mo alloys affects the phases of precipitates. For example, the tendency toward σ-phase formation increases with the Fe content in Co-Cr-Mo(5 to 20 mass pct)Fe alloys.[21] The presence of blocky MC-type carbides comprising Zr and carbon has been demonstrated in Co-Cr-Mo alloys when 0.05 to 0.1 mass pct Zr and 0.22 to 0.23 mass pct C were added.[22] Moreover, note that nitrogen is known to suppress the formation of the σ phase.[4] In a prior study, the present authors investigated the effect of the addition of carbon on the phase and morphology of precipitates in as-cast and heat-treated ASTM F75 Co-28 mass pct Cr-6 mass pct Mo-(0.12 to 0.35)mass pct C alloys.[10] We detected a π phase (M2T3X-type carbide with a β-Mn structure;[23–26] M and T: metallic elements; X: carbon) in the as-cast alloy with 0.15 mass pct C and in the alloys with (0.15 to 0.35) mass pct C heat treated at 1548 K (1275 °C) for a short holding time of less than 1.8 ks. In this study, Si and Mn were used as the alloying elements in biomedical Co-Cr-Mo-C alloys. The ASTM F75 standard permits the addition of Si and Mn up to 1 VOLUME 42A, JULY 2011—1
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