Precipitation during γ-ε phase transformation in biomedical co-cr-mo alloys fabricated by electron beam melting

We studied the precipitates that were induced during γ-ε phase transformation in biomedical Co-28Cr-6Mo (mass%) alloys that were fabricated by electron beam melting with carbon contents of 0.184 and 0.018 mass%. In the high-C as-built alloy, M₂₃X₆-type, π-phase (M₃T₂X-type), and η-phase (M₆X-M₁₂X-type) precipitates were observed (M and T: metallic elements, X: C and/or N). σ-phase (Co(Cr,Mo)), π-phase, and Co₃Mo₂Si-type precipitates were observed in the low-C as-built alloy. This is the first report that shows the presence and chemical composition of this precipitate, as the Co₃Mo₂Si-type precipitate has not been detected in biomedical Co-Cr-Mo alloys before. After aging in the ε-phase stability region, the high-C and low-C alloys both contained a single ε-phase matrix, and the amount of π-phase precipitates increased. Conversely, the amount of π-phase precipitates in both alloys decreased when a reverse transformation treatment was applied in the γ-phase stability region after aging. In the low-C alloy, the amount of Co₃Mo₂Si-type precipitates increased after reverse transformation treatment. These results indicate that the dissolution of π-phase precipitates and the formation of Co₃Mo₂Si-type precipitates during reverse transformation promote the formation of fine γ-phase grains at the precipitate/ε-phase matrix interface, because the formation and dissolution of these precipitates affect the γ-phase stability of the matrix.