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 σ phase was observed in the as-cast and heat-treated alloys without added carbon. In the carbon-added alloys, the M₂₃C₆-type, π-phase (M₂T₃X-type carbide with a β-Mn structure), η-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 °C). The formation of π-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 π 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.