Precipitates in biomedical co-cr-mo-c-n-si-mn alloys

The microstructures of biomedical ASTM F 75/F 799 Co-28Cr-6Mo-0.25C-0.175N- (0 to 1)Si-(0 to 1)Mo alloys (mass pct) were investigated before and after heat treatment, with special attention paid to the effect of nitrogen on the phases and the dissolution of precipitates. The heat treatment temperatures and holding periods employed ranged from 1448 to 1548 K (1175 to 1275 °C) and 0 to 43.2 ks, respectively. A blocky-dense π-phase precipitate and a lamellar cellular colony, which consisted of an M ₂X type precipitate and a Γ phase, were mainly detected in the as-cast alloys with and without added Si, respectively. The addition of nitrogen caused cellular precipitation, while the addition of Si suppressed it and enhanced the formation of the π phase. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) analyses suggested that a discontinuous reaction, i.e., Γ1 → Γ2 + M ₂X, might be a possible formation mechanism for the lamellar cellular colony. Nitrogen was enriched in the M ₂X type, η-phase, and π-phase precipitates, but was excluded from the M ₂₃X ₆ type precipitate. Complete precipitate dissolution was observed in all of the alloys under varied heat treatment conditions depending on the alloy composition. The addition of nitrogen decreased the time required for complete precipitate dissolution at low heat-treatment temperatures. At high temperatures, i.e., 1548 K (1275 °C), complete precipitate dissolution was delayed by the partial melting that accompanied the formation of the precipitates such as the π phase resulting in the boundary between the complete and incomplete precipitate dissolution regions in having a C-curved shape.