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

Abstract

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 °C to 1300 °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 σ-phase and stabilized the face-centered cubic (fcc) metallic γ-phase. The precipitates observed in the as-cast and heat-treated Co-Cr-Mo-C-N alloys were of the M₂₃X₆ type, M₂X type, π-phase (M₂T₃X type with a β-Mn structure), and η-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 π-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.