the effects of solid solution temperature on microstructure and properties in extruded and forged FGH95 superalloys are studied in this work. The superalloys with different micrtstructures are prepared through solid solution heat treatment at different temperature, and the microstructures were analyzed by SEM and EBSD. The results show that following with the increase of solid solution temperature, the grain size increase and the amount of primary γ′-phase located at the grain boundary decrease. When the solid solution temperature is higher than the solvus, the primary γ′-phase disappear completely, and as a result, the grain size grow significantly. The orientation of the FGH95 superalloys would not be influenced by the solid solution temperature, but the amount of twins increases following with the increase of solid solution temperature. The tensile and creep property is also studied in this work, the results show that the sub-solid solution heat treatment is suitable for Extruded and Forged FGH95 superalloys.
Thermal stabilities of microstructure and mechanical properties during long-term aging of 550°C/1500h, 650°C/1500h and 700°C/500h have been investigated on superalloy FGH95, in order to characterize the long-term properties of this alloy on service temperature. The results showed that the secondary and tertiary gamma-prime maintained good thermal stability at 550°C and 650°C with aging time up to 1500h. However, during 700°C aging for 500h, the secondary and tertiary γ′ both grew obviously, besides, the shape of secondary γ′ has turned from circular to square. The hardness, high-temperature tensile and creep residual strain had good stability at 550°C and 650°C with aging time up to 1500h. However, the creep residual strain apparently increased when the alloy aged at 700°C for 500h, which means that the FGH95 is suitable to be used for long time below 650°C, and short time at 700°C.
The oxidation behavior of FGH720Li(P/M Udimet720Li) superalloy was investigated under static atmosphere in temperature ranging from 600°C to 730°C. The oxidation kinetics, composition and morphology of the oxidation layers were characterized by means of isothermal oxidation tests, X-ray diffraction(XRD), scanning electron microscopy(SEM)and energy dispersive X-ray spectroscopy(EDS). The results showed that the oxidation kinetics curves of FGH720Li superalloy followed the parabolic law. The results of cross-sectional morphology and elemental distribution indicated that the oxidation layer could be divided into three parts:porous Cr2O3 outer layer, dense Cr2O3 medium layer and oxidation affected zone with nail-like Al2O3 inner layer. The oxidation process was primarily controlled by the diffusion of chromium and oxygen through the oxide scale.
The hot deformation behavior of FGH720Li superalloy was investigated by hot compressive tests on Gleeble-1500D thermal simulation test machine in different temperatures and strain rates. The true stress-strain curves were obtained, and based on the deformation data, the constitutive equation of FGH720Li superalloy was built. The Deformation Active Energy of FGH720Li was determined to be Q=787.6KJ/mol. The main deformation modes were dislocation glide and twinning. At the beginning of the deformation, a large number of dislocations generated, glided and scrambled in the alloy, then entwisted to form dislocation cells, which were the recrystallization nucleus. At the later period of the deformation, the dislocation would rotate to easy glide direction through twinning, inducing that the deformation of the alloy ensued. At the same time, the deformation had significant effect on grain refinement and the crushed of the primary particle boundary.
Low cycle fatigue (LCF) properties of a powder metallurgy(PM) nickel base superalloy FGH720Li were systematically studied in this work, including smooth LCF and notched LCF tested at various temperatures and different stress. The relationship between the fatigue life and applied stress was analyzed both for smooth fatigue and notch fatigue tests. The effects of loading frequency and stress ratio on LCF behavior were also studied. As an important influencing factor of the fatigue life in powder metallurgy superalloy, the effect of inclusions on LCF life was also investigated. The results showed that the fatigue properties of FGH720Li alloy was excellent, when tested at the temperature of 450°C and applied stress of 1230MPa, the fatigue life could exceed 5×104 cycles. When tested at 650°C and 1150MPa, the average fatigue life was still beyond 2×105 cycles.
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