π-phase and ᵡ-phase: New precipitates in biomedical Co-Cr-Mo alloys

Abstract

The phases of precipitates in ASTM F 75 and F 799 Co-Cr-Mo alloys are reviewed with a focus on new precipitates of the π- and ᵡ-phases. The π-phase carbide was observed at temperatures of 1,548-1,623 K where partial melting in Co-Cr-Mo-C system alloys occurred. Si was slightly enriched in the π-phase precipitate and nitrogen substituted carbon in the octahedral sites of the π-phase. It is suggested that the formation and stability in the partial melting region and the transformation of the π-phase take part in the final constitution of the precipitates in ASTM F 75 and F 799 alloys. A ᵡ-phase has been detected in the as-cast and heat- treated Co-Cr-Mo alloy with a midlevel carbon content and the addition of Si. The ᵡ-phase precipitate is an intermetallic compound. The conditions under which the ᵡ-phase is formed are limited by the heat-treatment temperature and chemical composition.