Precipitates in Biomedical Co-Cr-Mo-C-Si-Mn Alloys

The phase and dissolution behavior of precipitates in biomedical ASTM F75 Co-Cr-Mo-C-Si-Mn alloys were investigated. Alloys of five different compositions, Co-28Cr-6Mo-0.25C-1Si, Co-28Cr-6Mo-0.25C-1Mn, Co-28Cr-6Mo-0.25C-1Si-1Mn, Co-28Cr-6Mo-0.15C-1Si, and Co-28Cr-6Mo-0.35C-1Si, were heat-treated from 1448 to 1548 K. The precipitates observed in the as-cast and heat-treated alloys were carbides (M23C6 type, eta-phase, and pi-phase) and an intermetallic compound (pi-phase). The main precipitates observed after heat treatment at high temperatures such as 1548 K were pi-phase and M23C6 type carbide. At these high temperatures, two types of starlike precipitates-dense and stripe-patterned-were observed. The starlike-dense precipitate was the pi-phase, and the starlike precipitate with a stripe pattern was identified as the M23C6 type carbide and metallic fcc gamma-phase. In the alloys heat-treated at 1448 to 1498 K, blocky-dense M23C6 type carbide was primarily observed. chi-phase was detected in the Co-28Cr-6Mo-0.15C-1Si alloy under as-cast condition and after heat treatment at 1448-1523 K for a short holding time. The addition of Si seemed to increase the holding time for complete precipitate dissolution because of the effects of Si on the promotion of pi-phase formation at high temperatures and the increased carbon activity in the metallic matrix.