Changes in microstructure of biomedical Co-Cr-Mo alloys during aging at 973 to 1373 K

Changes in the microstructure and hardness of biomedical Co-Cr-Mo forged alloys (Co-27Cr-6Mo-0.77Si-0.64Mn-0.17N-0.06C (mass%)) during aging were investigated with a focus on precipitation. After solution treatment at 1523 K for 1.8 ks, the alloys were subjected to aging at temperatures between 973 and 1373 K for up to 86.4 ks. η-phase (M6X-M12X type), M23X6-type and π-phase (A2T3X type) precipitates were detected after aging. The formation of π-phase precipitates was detected in alloys aged between 1023 and 1123 K for holding times ≥21.6 ks, with a nose temperature between 1073 and 1123 K. The presence of π-phase precipitates in the aged Co-Cr-Mo alloy is reported for the first time; N contained within the alloy is believed to contribute to formation of the π-phase. The formation of a hexagonal-close-packed Co-based metallic phase (ϵ-phase) was observed between 1023 and 1123 K. The Vickers hardness increased in alloys aged at 1023 and 1073 K. This increase in hardness is attributed to the presence of π-phase and ϵ-phase and their wide-area precipitation.