The Effect of Rapid Solidification and Heat Treatment on Microstructure and Electrochemical Properties of Advanced Biomaterial Co-Cr-Mo-C Alloy

Abstract

Co-Cr-Mo-C alloys have been used for implant materials for decades because of their strength, corrosion resistance, wear resistance, and biocompatibility. To develop durable low-friction joint replacement implants, it is important to understand the solidification structure and properties of these materials. While HCP phase is the thermodynamically stable phase in these alloys, they maintain their high-temperature γ-FCC matrix and coarse dendritic structure on cooling to room temperature when cast by conventional methods. In this research, a wedgeshaped copper chill mold was used to examine the effect of cooling rates from 10 K/s to 450 K/s on the microstructure, HCP fraction, and corrosion properties of ASTM F75 alloy. The effect of laser surface modification, with an effective cooling rate of ~8900 K/s, was also investigated. In addition, the effect of isothermal aging (750 – 900°C, 3 – 25 hours) on the γ-FCC to eta-HCP transformation was studied. Cast specimens showed a columnar dendritic structure, with the dendrite arm spacing and carbide size decreasing and fraction of eta-HCP increasing as cooling rate increased. On the other hand, laser surface modification resulted in a fully cellular structure with 100% γ-FCC. Rapid solidification was found to result in an improvement in corrosion resistance, which was attributed to a more uniform structure and distribution of alloying elements. In addition, a time temperature transformation (TTT) diagram was developed for isothermal aging of this alloy; the γ-FCC to -HCP transformation was found to be most rapid at 800°C.