Precipitates in biomedical Co-28Cr-6Mo-(0-0.41)C alloys heat-treated at 1473 K to 1623 K (1200 C to 1350 C)

The phase and formation/dissolution of biomedical Co-28Cr-6Mo-xC (x = 0, 0.12, 0.16, 0.24, 0.33, and 0.41, in mass pct) alloys were investigated before and after heat treatment at high temperatures. The heat-treatment temperatures and holding periods were varied from 1473 K to 1623 K (1200 C to 1350 C) and from 0 ks to 43.2 ks, respectively. The intermetallic r phase was observed in the as-cast and heat-treated alloys without added carbon. In the carbon-added alloys, the M23C6-type, p-phase (M2T3X-type carbide with a b-Mn structure), g-phase (M6CM12C-type), and M7C3-type carbides were observed depending on the carbon content and heattreatment conditions. A complete precipitate dissolution was achieved in the alloys with a carbon content of 0 to 0.24 mass pct. The holding time required for complete precipitate dissolution in the carbon-added alloys increased with increasing heat-treatment temperature above 1548 K (1275 C). The formation of p-phase and M7C3-type carbides was observed in the carbon-added alloys during heat treatment at high temperatures of 1548 K to 1623 K (1275 C to 1350 C). Two types of p phase with different lattice constants and chemical compositions were confirmed in the alloys with carbon contents of 0.24, 0.33, and 0.41 mass pct after heat treatment at 1573 K to 1623 K (1300 C to 1350 C). The M7C3-type carbide observed at high temperatures occurred in a starlike precipitate with a complicated microstructure.