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 σ-phase, M₂₃X₆-type phase, η-phase (M₆X-M₁₂X type), π-phase (M ₂T₃X type), χ-phase, M₇X₃-type phase, or M₂X-type phase (M and T refer to metallic elements, and X refers to carbon and/or nitrogen); the σ- and χ-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 π-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 M ₇X₃-type precipitates. Nitrogen stabilizes the M ₂X-type, η-phase, and π-phase precipitates, and Si stabilizes the η-phase and χ-phase precipitates. The balance between the minor alloying element abundances also affects the constitution of the precipitates in Co-Cr alloys.