Evaluation of the Effect of Carbon and Solution Treatment on the ‎Microstructure and Mechanical Properties of ASTM F75 Cast Co ‎Based Alloy

The medical cast CoCrMo ASTM F75 alloy is used in orthopedic implants such as artificial toggle and knee. In this alloy, due to the existence of defects such as chemical inhomogeneities and large grain size, heat treatment is performed. In this research the effect of solution annealing heat treatment on the microstructural evolution and mechanical properties of two ASTM F75 alloy (the free of carbon specimen and the specimen containing 0.21 wt% carbon) was investigated. The solution treatment done at 1175, 1225 and 1275oC for 0.5, 1, 2 and 4 hours. Subsequently the specimens quenched in water. The results show that in the specimen without carbon, the σ precipitates were formed. However, in the specimen containing carbon, the M23C6 carbides and σ were formed. Besides, by adding carbon the volume fraction of carbides increased and reached to 6.55 % leads to increase in strength. By increasing annealing temperature from 1175 to 1275 oC and the time from 0.5 to 4 hours, the volume fraction and the size of carbides have been decreased. In the specimen containing carbon, by annealing at 1225oC for 1 hours the distribution of precipitates became homogenous in the microstructure. Besides, most of carbides at the grain boundaries in the cast condition were disappeared by heat treatment. Finally, in the specimen containing carbon in the heat-treated condition, the mechanical properties such as the ultimate tensile strength raised up to 19% and the ductility increased twice in comparison to the one in the cast condition