Phase and Formation/Dissolution of Precipitates in Biomedical Co-Cr-Mo Alloys with Nitrogen Addition

The precipitates in as-cast and heat-treated biomedical Co-28Cr-6Mo-(0 to 0.35)C-(0.15 to 0.25) N alloys (mass pct) were investigated. Heat treatment was carried out at temperatures of 1473 K to 1573 K (1200 degrees C to 1300 degrees C) for holding periods of 0 to 43.2 ks. In the as-cast and heat-treated Co-Cr-Mo-N alloys, no precipitates were detected; nitrogen effectively inhibited the formation of the sigma-phase and stabilized the face-centered cubic (fcc) metallic gamma-phase. The precipitates observed in the as-cast and heat-treated Co-Cr-Mo-C-N alloys were of the M23X6 type, M2X type, pi-phase (M2T3X type with a beta-Mn structure), and eta-phase (M6X-M12X type). Complete precipitate dissolution was detected in the alloys with carbon contents of less than 0.3 mass pct regardless of the nitrogen content. The main precipitates were of the M2X and M23X6 types after heat treatment for 1.8 to 43.2 ks. The pi-phase precipitate was detected in the early stage of heat treatment at high temperatures. The formation of the M2X-type precipitate was enhanced by the addition of nitrogen, although the constitution of the precipitates depended on the balance between the nitrogen and carbon contents and the heat-treatment conditions. DOI: 10.1007/s11661-012-1399-7 (C) The Minerals, Metals & Materials Society and ASM International 2012