Impact of minor alloying with C and Si on the precipitation behavior and mechanical properties of N-doped Co–Cr alloy dental castings

The addition of carbon and silicon as minor alloying elements was examined as a means to improve the mechanical properties of novel nitrogen-doped Co–Cr-based alloy dental castings. Samples of Co–32Cr–9W–Si–0.25N–C (mass%) alloys were prepared using a dental-casting machine. Microstructural analysis was performed on the alloys using scanning electron microscopy, electron-backscatter diffraction, electron-probe microanalysis, and X-ray diffraction, with a particular focus on the precipitation behavior. The findings were compared with thermodynamic predictions and examined in relation to the tensile properties and Vickers hardness at room temperature. All of the prepared alloys had a face-centered-cubic γ-phase matrix, with grains measuring a few millimeters in diameter and consisting of dendritic substructures. The precipitation of the intermetallic σ-phase, which occurred in the interdendritic regions with solidification segregation of Cr and W, was replaced with M₂₃C₆ through the addition of carbon. This significantly increased the ultimate tensile strength of the alloys without severe loss of ductility, although the 0.2% proof stress did not change. The addition of silicon, on the contrary, promoted the formation of the precipitates, which included M₆C and the σ-phase, making the alloys brittle. The results of this study highlight the role of minor alloying elements, such as carbon and silicon, on the microstructural and mechanical properties; the findings also shed light on the significance of precipitation control in dental castings of Co–Cr alloys, which should aid the design of novel dental alloys.