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SHINGO MINETA, SHIGENOBU NAMBA, TAKASHI YONEDA, KYOSUKE UEDA, and TAKAYUKI NARUSHIMA The microstructures of as-cast and heat-treated biomedical Co-Cr-Mo (ASTM F75) alloys with four different carbon contents were investigated. The as-cast alloys were solution treated at 1473 to 1548 K for 0 to 43.2 ks. The precipitates in the matrix were electrolytically extracted from the as-cast and heat-treated alloys. An M 23 C 6 type carbide and an intermetallic r phase (Co(Cr,Mo)) were detected as precipitates in the as-cast Co-28Cr-6Mo-0.12C alloy; an M 23 C 6 type carbide, a r phase, an g phase (M 6 C-M 12 C type carbide), and a p phase (M 2 T 3 X type carbide with a b-manganese structure) were detected in the as-cast Co-28Cr-6Mo-0.15C alloy; and an M 23 C 6 type carbide and an g phase were detected in the as-cast Co-28Cr-6Mo-0.25C and Co-28Cr-6Mo-0.35C alloys. After solution treatment, complete precipitate dissolution occurred in all four alloys. Under incomplete precipitate dissolution conditions, the phase and shape of precipitates depended on the heat-treatment conditions and the carbon content in the alloys. The p phase was detected in the alloys with carbon contents of 0.15, 0.25, and 0.35 mass pct after heat treatment at high temperature such as 1548 K for a short holding time of less than 1.8 ks. The presence of the p phase in the Co-Cr-Mo alloys has been revealed in this study for the first time.

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Precipitates in As-Cast and Heat-Treated ASTM F75 Co-Cr-Mo-C Alloys Containing Si and/or Mn ALFIRANO, SHINGO MINETA, SHIGENOBU NAMBA, TAKASHI YONEDA, KYOSUKE UEDA, and TAKAYUKI NARUSHIMA The effect of the addition of Si or Mn to ASTM F75 Co-28Cr-6Mo-0.25C alloys on precipitate formation as well as dissolution during solution treatment was investigated. Three alloysCo-28Cr-6Mo-0.25C-1Si (1Si), Co-28Cr-6Mo-0.25C-1Mn (1Mn), and Co-28Cr-6Mo-0.25C-1Si-1Mn (1Si1Mn)-were heat treated from 1448 K to 1548 K (1175°C to 1275°C) for a holding time of up to 43.2 ks. In the case of the as-cast 1Si and 1Si1Mn alloys, the precipitates were M 23 C 6 -type carbide, η phase (M 6 C-M 12 C-type carbide), and π phase (M 2 T 3 X-type carbide with a β-Mn structure), while in the case of the as-cast 1Mn alloy, M 23 C 6 -type carbide and η phase were detected. The 1Si and 1Si1Mn alloys required longer heat-treatment times for complete precipitate dissolution than did the 1Mn alloys. During the solution treatment, blocky dense M 23 C 6 -type carbide was observed in all the alloys over the temperature range of 1448 K to 1498 K (1175°C to 1225°C). At the heat-treatment temperature of 1523 K (1250°C), starlike precipitates with stripe patterns-comprising M 23 C 6 -type carbide and metallic face-centeredcubic (fcc) γ phase-were detected in the 1Si and 1Si1Mn alloys. A π phase was observed in the 1Si and 1Si1Mn alloys heat treated at 1523 K and 1548 K (1250°C and 1275°C) and in the 1Mn alloy heat treated at 1548 K (1275°C); its morphology was starlike-dense. The addition of Si appeared to promote the formation of the π phase in Co-28Cr-6Mo-0.25C alloys at 1523 K and 1548 K (1250°C and 1275°C). Thus, the addition of Si and Mn affects the phase and morphology of the carbide precipitates in biomedical Co-Cr-Mo alloys.