The Effect of Cooling Rate on the Microstructure and Hardness of As-Cast Co-28Cr-6Mo Alloy Used as Biomedical Knee Implant

Co-based alloys are currently the most suitable material used for hip and knee joint replacements due to their good biocompatibility, high strength, wear resistance and high corrosion resistance. In this study, the effects of cooling rate on the microstructure and hardness in different regions of a commercial knee implant (tibial implant) of the as-cast Co-28Cr-6Mo alloy (ASTM F75) processed under industrial conditions were evaluated. The X-ray diffraction (XRD) patterns showed that the microstructure of as-cast Co-28Cr-6Mo alloy consists of two main phases: γ (FCC) and ε (HCP) phases. Scanning electron microscopy (SEM) together with Energy dispersive spectroscopy (EDS) analysis also revealed the presence of carbides with composition of (Cr, Co)C 6 which was found precipitated in the grain boundaries and in interdendritic regions. Porosity and a small fraction of oxides localized in regions of high free energy such as grain boundaries and carbides were also observed. Casting numerical simulation applied to the implant showed that the regions of implant with turbulent flow of the liquid metal were more susceptible to the presence of porosity and oxides. However, larger and more concentrated defects can lead to brittle failure of the tibial implant. The regions of the implant with high cooling rates presented grain refinement and, consequently, high hardness values. An increase in cooling rates from 2.2 to 30 °C/s promotes an increase of 48.5% in hardness. Besides, the rise in cooling rate did not affect the fraction of carbides, but promoted the formation of more refined carbides. Graphical Abstract