Microstructural evolution during solution treatment of Co–Cr–Mo–C biocompatible alloys

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Three different Co–Cr–Mo–C alloys conforming to ASTM F75 standard were poured in an industrial environment and subjected to a conventional solution treatment at 1225 °C for several time intervals. The microstructural changes and transformations were studied in each case in order to evaluate the way in which treatment time influences the secondary phase fraction and clarify the microstructural changes that could occur. To assess how treatment time affects microstructure, optical microscopy and image analyzer software, scanning electron microscopy and energy dispersion spectrometry analysis were employed. The main phases detected in the as-cast state were: σ-phase, M6C, and M23C6 carbides. The latter presented two different morphologies, blocky type and lamellar type. Despite being considered the most detrimental feature to mechanical properties, σ-phase and lamellar carbides dissolution took place in the early stages of solution treatment. M23C6 carbides featured two different behaviors. In the alloy obtained by melting an appropriate quantity of alloyed commercial materials, a decrease in size, spheroidization and transformation into M6C carbides were simultaneously observed. In the commercial ASTM F75 alloy, in turn, despite being the same phase, only a marked decrease in precipitates size was noticed. These different behaviors could be ascribed to the initial presence of other phases in the alloy obtained from alloyed materials, such as σ-phase and “pearlitic” carbides, or to the initial precipitate size which was much larger in the first than in the commercial ASTM F75 alloy studied. M6C carbides dissolved directly in the matrix as they could not be detected in samples solution-treated for 15 min.