Precipitates in Biomedical Co-Cr Alloys

Herein, precipitates in biomedical Co-Cr-Mo and Co-Cr-W-Ni alloys are reviewed with a focus on their phase, chemical composition, morphology, and formation/dissolution during heat treatment. The effects of the heat-treatment conditions and the addition of minor alloying elements such as carbon, nitrogen, Si, and Mn on the precipitates are also discussed. Mostly, the precipitates in the alloys are of the sigma-phase, M23X6-type phase, eta-phase (M6X-M12X type), pi-phase (M2T3X type), chi-phase, M7X3-type phase, or M2X-type phase (M and T refer to metallic elements, and X refers to carbon and/or nitrogen); the sigma- and chi-phases are intermetallic compounds, and the others are carbides, nitrides, and carbonitrides. The dissolution of the precipitates during solution treatment is delayed by the formation of the pi-phase at temperatures where partial melting occurs in the alloys. In addition, the stability of the precipitates depends on the content of minor alloying elements. For example, the addition of carbon enhances the formation of M23X6-type and M7X3-type precipitates. Nitrogen stabilizes the M2X-type, eta-phase, and pi-phase precipitates, and Si stabilizes the eta-phase and chi-phase precipitates. The balance between the minor alloying element abundances also affects the constitution of the precipitates in Co-Cr alloys.