Effects of processing temperatures on FGH4097 superalloy fabricated by hot isostatic pressing: Microstructure evolution, mechanical properties and fracture mechanism

A newly-developed γ′-hardenable powder metallurgy (P/M) nickel based surperalloy FGH4097 developed by China was fabricated by hot isostatic pressing (HIP) with different processing temperatures. The microstructure evolution, tensile properties and fracture mechanism of the FGH4097 superalloy were systematically studied. Grains size, strengthening γ′ phase, prior particle boundaries and carbide precipitations, were characterized by optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), meanwhile the mechanical properties were assessed by hardness measurements and tensile tests at room temperature (RT) and 650 °C. A dendrite structure inherited from the pre-alloy powder was partially reserved in the as-sintered alloy processed at 1100 °C. It transformed into a cellular structure when the HIP temperature was increased to 1200 °C. With increasing the HIP temperature, the contents of γ′ phase forming element decreased, accompanied by the decrease of γ′ lattice parameters and volume fraction. Due to a promoted atomic diffusion at higher temperatures, MC carbide and two types of HfO2 precipitations formed in the γ matrix that were not present in the pre-alloy powder. Besides, the grain size increased with increasing HIP temperatures, and the average grain sizes of the sintered samples were 5.8 µm (1100 °C), 17.8 µm (1200 °C) and 74.7 µm (1300 °C), respectively. Compared with two other processing temperatures, samples fabricated with at 1200 °C had the highest ultimate tensile strengths (1410 MPa at room temperature and 1236 MPa at 650 °C), along with a superior elongation (33.0 at room temperature and 31.4 at 650 °C), which was attributed to an appropriate microstructure and mediate volume fraction of γ′ phase. Owing to the incomplete recrystallization and prior particle boundaries, the yield strength and ultimate tensile strength at 1100 °C exhibited a bit less than that of at 1200 °C, but showed obvious difference in elongation. However, the tensile strengths decreased significantly at 1300 °C as a result of coarse grains. With increasing the HIP temperature, the fracture appearances indicated that the fracture changed from dimple ductile mode to brittle mode, because of the formation of coarsening MC carbide at grain boundaries.