Effect of cooling rate on microstructure and tensile properties of powder metallurgy Ni-based superalloy

The effects of size distribution, morphology and volume fraction of gamma' phase and grain size on tensile properties of powder processed Ni-based superalloy were investigated by using two different quenching methods. Oil quenching and air cooling were adopted with cooling rate of 183 degrees C/s and 4-15 degrees C/s, respectively. The experimental results show that the average size of the secondary gamma' after oil quenching is 24.5 nm compared with 49.8 nm under air cooling, and corresponding volume fractions of gamma' are 29% and 34%, respectively. Meanwhile, the average grain size remains nearly equivalent from both oil-quenching and air-cooling specimens. The tensile strength at room temperature is higher for the oil-quenched specimen than the equivalent from the air-cooled specimen, but the difference approaches each other as the temperature increases to 650 degrees C. The fractography clearly demonstrates that transgranular fracture governs the failure process at ambient temperature, in contrast to the intergranular fracture at 650 degrees C or even higher temperature. These two mechanical responses indicate the strengthening effects of gamma' precipitates and grain boundary for polycrystalline Ni-based superalloys at different temperatures.