Effect of Aging on Microstructure and Tensile Properties of ASTM F75 Co-based Alloy Containing Different Carbon Contents

This study aims to investigate the effect of aging on microstructural evolution and mechanical properties of two ASTM F75 alloys, one without the carbon additive and the other one containing 0.21 wt.% C. Aging on the samples solution-annealed at 1225 °C for 1 hour was performed at 720, 760, and 800°C for 5 hours. The evaluation of the microstructure of both samples after aging showed that the γ→ε transformation led to the formation of two various morphologies of the precipitates in the regions of the ε-phase. At the first stages of aging, the precipitates appeared as a series of straight bands in both samples, while with an increase in the temperature, pearlite-like regions were found at the grain boundaries of only the carbon-containing sample. In the sample without the carbon additive, the microstructure consisted of the σ-phase. In the carbon-containing sample, lamellar M23C6 carbides were formed during aging in the vicinity of the remaining blocky M23C6 carbides. The carbon-containing sample consisted of fine M23C6 carbides after aging at 720°C, which were distributed inside the grains and at the grain boundaries. The appropriate precipitation rate and their optimum distribution in the structure led to an increase in the yield strength by up to 15%, along with an increase in the ultimate tensile strength by up to 6% compared with the as-cast state. However, no significant improvement was found in the ductility of the alloy.