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 A degrees C to 1350 A degrees C) and from 0 ks to 43.2 ks, respectively. The intermetallic sigma phase was observed in the as-cast and heat-treated alloys without added carbon. In the carbon-added alloys, the M23C6-type, pi-phase (M2T3X-type carbide with a beta-Mn structure), eta-phase (M6C-M12C-type), and M7C3-type carbides were observed depending on the carbon content and heat-treatment 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 A degrees C). The formation of pi-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 A degrees C to 1350 A degrees C). Two types of pi 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 A degrees C to 1350 A degrees C). The M7C3-type carbide observed at high temperatures occurred in a starlike precipitate with a complicated microstructure.