Effects of the Solution Treatment on Microstructural Evolution, Mechanical Properties, and Fracture Mechanism of Nickel-Based GH4099 Superalloy

Grain growth, mechanical properties, and fracture mechanism of nickel-based GH4099 superalloy are investigated using heat treatments, tensile tests, optical microscopy (OM), and scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS). The OM observation shows that the matrix grains (γ-grains) undergo an apparent growth during the solution treatment. The grain size diameter increases from 100 to 174 μm when the solution temperature rises from 1100℃ to 1160℃ for 30 min. When the holding time increases from 15 to 60 min at 1140℃, the grain size diameter increases from 140 to 176 μm, indicating that the γ-grain growth is more sensitive to temperature than time. Standard deviation, Sv, and the grain size distribution are utilized to characterize the microstructural uniformity. To predict the grain size more accurately, we develop the grain growth kinetics and find that the growth index is close to 5. The yield strength (Rp0.2), tensile strength (Rm), and ductility (Af) are also measured. It is found that the effect decreases in the order cooling rate, solution temperature, time. Rp0.2 reduces by 47% with the increase in the cooling rate from 1℃ to 8000℃/min, while both strength and ductility exhibit little changes with time. The SEM results show that the fracture surfaces have typical mixed brittle and ductile characteristics when specimens are subjected to water quenching and air cooling. However, a complete brittle fracture occurs under furnace cooling conditions. The EDS analysis indicates that the brittle γ' (Ni3Ti) phase precipitates around the γ-grain boundary during the slow cooling process, which is the main factor yielding the complete brittle fracture. Finally, the optimal solution treatment scheme for the GH4099 superalloy is proposed—a temperature of 1140℃ for 30 min followed by air cooling.