Microstructure Evolution of K6509 Cobalt-base Superalloy for Over-temperature

Abstract In this study, the K6509 cobalt-base superalloy were heat treated at different temperatures and the microstructure was detected by scanning electrical microscopy and electrical probe micro-analyzer. In as-cast K6509, the MC carbides are present as a discrete, blocky dispersion with a well-distributed Chinese script morphology and the eutectics (γ+M23C6) and (γ+M7C3) can be seen. With the temperature increasing, the MC arise degeneration. Below 1150 o C, the script-like MC is dominant; reaching 1200 o C, the particle and block of MC are dominant; reaching 1250 o C, few script-like MC is present. In as-cast alloy, the M23C6 and M7C3 carbides are lamellar eutectic with matrix. When temperature is up to 1150 o C, the eutectic (γ+M23C6) disappears; however, much eutectic (γ+M7C3) can be found. When the temperature reaches 1200 o C, a large amount of eutectic M7C3 is dissolved, and some pores form in the matrix, and some zones rich in Cr form around the pores. When the temperature is above 1250 o C, many pores form in the matrix, the cause of pores is need to be studied further.