The Effect of Precipitation Hardening on Microstructural Characteristics of ‎Directionally Solidified Nickel-Based Superalloy GTD-111‎

The Ni-based superalloy GTD-111 possesses excellent hot corrosion resistance, oxidation resistance and high strength at elevated temperature; accordingly, the alloy is used in manufacturing of the first stage blades of powerful gas turbines. In this study, the effect of homogenization treatment, partial dissolution with two different cooling rates (air and water) along with aging on the microstructure and micro hardness of directionally solidified GTD-111 were studied. To this end, cylindrical specimen (diameter of 10 mm and height of 40 mm) with columnar-grained structure were produced under growth rate of 6mm/min using Bridgman technique. The macro-structural studies of the DS GTD-111 specimen show that the columnar grains along the longitudinal direction of specimen were provided by temperature gradient in liquid metal. Microstructural analyses illustrate that homogenization treatment led to dissolution of a portion of primary γʹ precipitates. Furthermore, the primary γʹ precipitates in the specimen that was cooled in water after partial solution treatment, were spherical and cubic with curved corner. However, the primary γʹ precipitates were angular cubic in the specimen which was cooled in air after partial solution treatment. Since the γ matrix became supersaturated during the homogenization and partial solution treatment, aging process was effective on nucleation and growth of secondary γʹ precipitates. This was especially more substantial in the specimen which was cooled in water after partial solution. Hardness of the DS specimen was reduced after homogenization. While, aging resulted in increasing the hardness of the DS specimen due to the improvement of morphology, size and distribution of γʹ.