Precipitates in As-Cast and Heat-Treated ASTM F75 Co-Cr-Mo-C Alloys Containing Si and/or Mn

Abstract

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 alloys—Co-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 M23C6-type carbide, η phase (M6C-M12C–type carbide), and π phase (M2T3X-type carbide with a β-Mn structure), while in the case of the as-cast 1Mn alloy, M23C6-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 M23C6-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 M23C6-type carbide and metallic face-centered-cubic (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.