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THE Co-Cr-Mo alloys were registered in ASTM F 75 and F 799 standards as surgical implant materials for castings and forgings, respectively.[1,2] Precipitates in biomedical Co-Cr-Mo alloys are known to affect the wear and corrosion-resistant properties of castings[3–5] as well as the workability of forgings.[6] As a result, precipitates in biomedical Co-Cr-Mo alloys were studied by several research groups including ours.[7–16] We reported on the phases and dissolution behavior of precipitates in Co-Cr-Mo-C[7] and Co-Cr-Mo-C-SiMn[8,9] system alloys under as-cast condition and after heat treatment at 1448 to 1548 K (1175 to 1275 °C). The precipitate evaluation in this temperature range proves useful for selecting the hot working conditions and controlling the as-cast microstructure of biomedical Co-Cr-Mo alloys. In our previous studies, two types of precipitates, i.e., a π phase (M2T3X-type carbide with a β-Mn structure, where M and T are metallic elements and X is carbon)[7] and a χ phase (intermetallic ALFIRANO and SHINGO MINETA, Graduate Students, KYOSUKE UEDA, Assistant Professor, and TAKAYUKI NARUSHIMA, Professor, are with the Department of Materials Processing, Tohoku University, Sendai 980-8579, Japan. Contact e-mail: [email protected] 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 June 30, 2011. Article published online January 24, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A

compound with an α-Mn structure),[9] were detected for the first time in biomedical Co-Cr-Mo alloys. It was reported that nitrogen in Co-Cr-Mo alloys stabilizes a face-centered-cubic (fcc) Co-based metallic phase (γ phase) and improves the hot-workability[4] and mechanical properties[17] of the alloys. In biomedical Co-Cr-Mo alloys containing nitrogen, the formation of M23C6 type,[17,18] η-phase (M6C),[17] and Cr2X[19,20] precipitates was reported. However, the effect of nitrogen on the phases and dissolution of precipitates in ASTM F 75/F 799 Co-Cr-Mo alloys has not yet been clarified in detail. In this study, the effect of nitrogen, Si, and Mn on the phases and dissolution of the precipitates in ASTM F 75/F 799 Co-Cr-Mo alloys was investigated based on the observation of the microstructures of as-cast and heat-treated Co-Cr-Mo-C-N-Si-Mn alloys with a focus on the precipitates.

II.

MATERIALS AND METHODS

A. Specimens Table I gives the chemical composition of the alloys used in this study. The nitrogen content was controlled to the level of 0.175 ± 0.025 mass pct. The carbon content was approximately 0.25 mass pct, and the contents of Si and Mn were 0 or 1 mass pct. The chemical composition of the alloys is denoted by mass percent, although the notation “mass pct” is omitted. VOLUME 43A, JUNE 2012—2125

Table I. Chemical Composition of the Co-Cr-Mo-C-N-Si-Mn Alloys Used in This Study (Mass Percent) Abbreviation 1Si0Mn0.175N 0Si1