The Effect of Carbon and Silicon Variation on the Microstructure and Mechanical Properties of a Cobalt-Based Superalloy

Abstract

The performance of cast Co-based superalloys is strongly influenced by the nature, stability, and distribution of carbide phases, which depend on C and Si content. This study examines the effect of C (0.25, 0.5 wt pct) and Si (1, 4 wt pct) on the microstructure and tensile behavior of a cast Co-based superalloy. As-cast microstructures were dominated by M23C6 in low-Si alloys and M12C in high-Si alloys, with χ phase formed in the low-C, high-Si alloy. Thermal exposures at 800 °C and 1000 °C (100 h) promoted carbide transformation and intragranular precipitation, whereas 1200 °C (50 h) accelerated transformation and coarsening. At room temperature, high-Si contents increased yield strength but significantly reduced ductility, with crack susceptibility increasing from M12C to M23C6 to χ. Embrittlement was exacerbated after 800 °C aging due to needle-like intragranular σ and M12C precipitation in high-Si alloys. Furthermore, 1000 °C tensile testing of as-cast alloys revealed a performance inversion: the M23C6-reinforced low-Si alloy achieved superior strength (UTS ~ 196 MPa) compared to high-Si variants. These findings establish mechanistic links between chemistry, carbide evolution, and tensile response, underscoring the need to limit Si and balancing C to optimize ductility and strength in Co-based superalloys.