Effect of Prior Thermomechanical Treatment on Annealed Microstructure and Microhardness in Cobalt-Based Superalloy Co-20Cr-15W-10Ni

Microstructure evolution in Co-20Cr-15W-10Ni alloy with hot deformation and subsequent annealing has been studied using hot isothermal compression at different temperatures and strain rates followed by annealing at 1,423 K for 1 h. In the as-deformed condition, optical microstructure reveals deformation bands in specimens deformed at low temperature (1,323 K) and high strain rate (10 s(-1)). In deformation beyond 1,423 K, very fine recrystallized grains are observed at all the strain rates, which is seen to be fully developed at 1,473 K onwards especially at lower strain rates (0.1 and 0.01 s(-1)). EBSD maps reveal recrystallized grains and presence of deformation twins. Grain growth is observed at strain rate lower than 0.1 s(-1) and temperature more than 1,473 K. After annealing of deformed samples, fully recrystallized grains and annealing twins have been observed. Samples subjected to prior deformation at higher strain rates showed mixed grains with small banding of recrystallized structure, whereas more uniform recrystallized microstructure is observed at slow strain rates. Microhardness decreases with increase in deformation temperature. However, annealed hardness is found to be increasing marginally with decreasing prior deformation temperature (especially for slower strain rates), attributing to relatively uniform and fine microstructure after annealing.