| In this paper, the high temperature creep properties, the room temperature tensile properties and the high temperature tensile properties of GH4169 superalloy have been investigated. Through the optical microscope, SEM, EDS and XRD analysis, the 8 phase, which is controversial, has been examined profoundly.In order to evaluate the room temperature tensile properties of GH4169 superalloy, three different heat treatments were performed in this research. (1) 960℃×1h/AC+960℃× 10min/AC, namely, HT1. (2) 960℃×1h/AC+870℃×3h/AC+960℃×10min/AC, namely, HT2. (3) 960℃×1h/AC+870℃×6h/AC+960℃×10min/AC, namely, HT3. Because of the fact that the aging treatment was not performed in this experiment, the y" phase did not precipitate during the heat treatments. Thus, the research aims at the studying the effect of 8 phase on the room temperature tensile properties of GH4169 superalloy. Through SEM, it could be concluded that a great deal of 8 phases can be observed both inside the grains and at the boundaries of HT3 alloy. In HT2 alloy, the 8 phases only precipitated at the boundaries. In contrast, only a few 8 phases can be seen at the boundaries of HT1 alloy. Because of the precipitating of 8 phases, the Nb would be consumed seriously. The amounts of the incoherent interfaces between the 8 phases and bases would increase as well. Consequently, elasticity deformation tended to occur under the pressure. That is to say, with the extending of the holding at 870℃, the modulus of elasticity (MOE) and the yield strength of GH4169 alloy got decreased. On the other hand, with the extending of the plastic deformation, the grain boundaries would be curved by the rod-like 8 phases existing at the boundaries. As a result, the boundary slides were slowed down. Moreover, the dislocations accumulated in front of the 8 phases and the interaction raised by such 8 phases increased the resistance to the further deformation. To sum up, the tensile strength of the GH4169 alloy got increased with the precipitating of 8 phases. The Hollomon formula was often used to fit the true stress-true strain curve at the strain hardening stage. Compared to the original form (σ=kεn), the fitting results for corrected form (σ=Kx(ε0+ε)") are much more satisfactory. It could be seen from the fitting results were satisfactory, and the value of "n" got increased with the extending of the holding at 870℃. Through SEM, it could be concluded that the fracture modes of all the alloys came to the transgranular fracture.Experiments were conducted to evaluate the high temperature tensile properties of GH4169 superalloy. The two samples were taken from different places (rim and R/2 of the ingot). In addition, the samples were heat treated through ST i.e.,960℃×1h/AC+720℃×8h/furnace cooling (FC) at 55℃/h to 620℃+620℃×8h/AC. Through SEM, it can be concluded that the 8 phases can be seen in both of the samples. Moreover, the grain size of the rim sample was smaller than that of the R/2 sample. In addition, the amount of 8 phases in rim sample was also larger than that of R/2 sample. Under the condition of 950℃, the high temperature tensile experiments were conducted. The results showed that the yield strength and the tensile strength of the rim sample were little higher than those of the R/2 sample. Compared to the R/2 sample, the ductility of the rim sample is higher, which was as high as 85%. Under the high temperature, the existing of 8 phases was favorable to the elongation. The reasons could be summarized as follows: the Nb atoms involved in the 8 phases have great roles with the vacancy; under the high temperature, the 8 phases existing at the boundaries can change the form of the boundaries. As a result, the bending with small radius of curvature forms at the boundaries. Such bending can suppress the development of the cracks and increase the elongation. After heat treatment, more 8 phases precipitate inside rim sample. Compared to the R/2 sample, the rim sample exhibited higher elongation. After examining the fracture morphology of the two samples, it can be concluded that he fracture modes of the two alloys came to the transgranular fracture.Experiments were conducted to evaluate the effects of different heat treatments on the microstructures and creep properties of GH4169 alloy. In order to study the strengthening effect of the δ phase, three different heat treatments were performed in this research. (1) High standard treatment (namely, HST), i.e.,960℃×2h/air cooling (AC)+720℃×8h/furnace cooling (FC) at 55℃/h to 620℃+620℃×8h/AC. (2) Standard treatment (namely, ST), i.e.,960℃× 1h/AC+720℃×8h/furnace cooling (FC) at 55℃/h to 620℃+620℃×8h/AC. (3) Direct aging treatment (namely, DA), i.e.,720℃×8h/furnace cooling (FC) at 55℃/h to 620℃ +620℃×8h/AC. Through SEM, EDS and XRD, it could be concluded that a great deal of δ phases locate both inside the grains and at the boundaries of HST alloy. In ST alloy, the δ phases could only be observed at the boundaries. While, there hardly precipitate δ phase in DA alloy. Because the interfaces between the δ phases and bases are incoherently, some initial cavities must be exists at the interfaces. With the precipitating of the 8 phase, the Nb involved in the y" phase was consumed. As a result, the DA alloy that hardly contains 8 phase exhibited the longest creep life. On the other hand, the boundary δ phase could cause the "fixing effect", which is able to alleviate the harmful effect caused by the initial cavities. In addition, the precipitating of the intragranular 8 phases reduces the intragranular strength. As a result, the intragranular strength and the boundary strength matched well with each other. Thus, the creep life of HST alloy was longer than that of the ST alloy. The examination of the fractograph of the DA alloy indicated the intergranular fracture pattern. While, the ductile fracture (transgranular fracture) pattern could be observed in the fractographies of ST alloy and HST alloy. |
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