Microstructure and creep behavior of FGH95 nickel-base superalloy

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Abstract

By means of the measurement of creep curves and the microstructure observations, the influence of the solution temperatures on the creep behavior of FGH95 nickel-base superalloy is investigated. Results show that, after solution treated at 1150 °C, some coarser γ′ precipitates are distributed in wider boundary regions where no fine γ′ phase is precipitated. As the solution temperature is raised to 1165 °C, the grain size of the alloy increases obviously, and the carbide is continuously precipitated to form the film along the boundaries. When the alloy is solution treated at 1160 °C, the coarser γ′ phase in the alloy is fully dissolved, the fine γ′ phase with higher volume fraction is dispersedly distributed within the grains, and some particles of (Nb, Ti)C are precipitated along the grain boundaries, which can effectively hinder the grain boundary slipping and dislocation moving. Thereby, the alloy displays a better creep resistance under the applied stress of 1034 MPa at 650 °C. The deformation mechanism of the alloy during creep is twinning, dislocation shearing or bypassing the γ′ phase, and the 〈1 1 0〉 super-dislocation which shears into the γ′ phase may be decomposed to form the configuration of (1/3)〈1 1 2〉 super-Shockleys partial and stacking fault. In the later stage of creep, the deformation features of the alloy are the single and double orientations slipping of dislocations activated in the alloy. As the creep goes on, some dislocations piled up in the regions near the boundaries may bring the stress concentration to promote the initiation and propagation of micro-cracks, which is thought to be the fracture mechanism of the alloy during creep.

Research highlights

▶ Investigating the influence of solution temperatures on the microstructure and creep property of the alloy. ▶ Investigating the deformation mechanism and fracture features of alloy during creep. ▶ Discussing the effect factors of creep resistance for the alloy. ▶ Discussing on the deformation and fracture mechanisms of the alloy during creep.

Introduction

With the development of aerospace and ground transportation industry, the properties of aerospace turbines are increasingly required to be improved, and especially, the turbine disks of aeroengines are required to be of high temperature tolerance and creep resistance [1]. Traditional wrought superalloys can hardly meet the requirements of aerospace turbine disks for their poor temperature tolerance and loading capacity resulted from their serious composition segregation in ingots and poor hot processability [2], [3], [4], [5], especially the weaker cohesive force of grain boundaries [6], [7], [8].

FGH95 alloy is a nickel-base superalloy with higher alloying degree and volume fraction of γ′-phase [6], [9]. Compared with wrought superalloys, FGH95 superalloy has the capabilities of higher temperature tolerance and loading tolerance, due to their excellent characteristics, such as uniform chemical composition and fine grain size. Therefore, the alloy is an excellent material used for preparing the advanced aerospace turbine disks with high thrust-weight ratio [10], [11].

The microstructure of FGH95 superalloy consists of γ matrix, γ′ and carbide phases. Various size, morphology and distribution of γ′ phase can be obtained in the alloy by different heat treatment regimes [12], [13]. The deformation mechanism of the polycrystalline Ni-base superalloys during creep includes twinning, dislocations by-passing or shearing into the γ′ phase [14], [15], [16]. Actually, the mechanical properties and creep behaviors of the alloy are related to the quantities, morphology and distribution of γ′-phase, and especially, the configuration of the boundary and carbides have an important effect on the creep resistance of the alloy [17]. For example, after the alloy is solution treated for cooling in molten salt, the total number and size of secondary γ′ phase increase, which can effectively improve the plasticity of the alloy at high temperature [18], [19]. Because the various microstructures in the alloy may be obtained by different heat treatment regimes, it is very important to understand the influence of heat treatment regimes on the microstructure and creep resistance of the alloy. Although some literatures report the creep behaviors and deformation features of the powder superalloys, the influence of heat treatment regimes on the configuration and distribution of γ′-phase and carbide in FGH95 superalloy is still not clear.

In this paper, the alloy is solution treated at different temperatures, and then the creep properties are measured and the microstructure is observed by using SEM and TEM for investigating the influences of the solution temperatures on the boundary morphologies and distribution of the γ′ and carbide phases. Additionally, the deformation mechanism and fracture feature of the alloy during creep are briefly discussed.

Section snippets

Experimental procedure

FGH95 powder particles of the nickel-base superalloy with the size about 150 mesh were put into a stainless steel can for pretreating at 1050 °C for 4 h. The can containing FGH95 alloy powders was hot isostatic pressing treated for 4 h under the conditions of 1120 °C and 120 MPa. The various solution temperatures are selected for investigating the influence of solution temperatures on the microstructure and creep properties of the alloy, and the cooled rate of the specimen in molten salt is measured

Influence of solution temperature on the microstructure

After the alloy was solution treated at 1150 °C and twice aged, the microstructure of the alloy consists of the γ′ and γ phases, and the average grain size of the alloy is about 10–20 μm, as shown in Fig. 1(a), indicating that some coarser γ′-particles are precipitated in the wider boundary regions, and the average size of the coarser γ′ phase is about 1–2.5 μm. The magnified morphology of the alloy is shown in Fig. 1(b), indicating that significant amount of the fine γ′ particles is dispersedly

Analysis on deformation features of the alloy during creep

Significant amount of the fine γ′ particles is precipitated within the grains, which may effectively hinder the dislocation movement. When the deformed dislocations move over the γ′ phase during creep, the dislocation loops are kept around the γ′ particles as shown in Fig. 7(a), which suggests that the deformation feature of the alloy during creep is the dislocations moving over the γ′-phase by Orowan bypasses mechanism. It is reasonable consideration that the various spaces between the γ′

Discussion

When the solution temperature increases from 1150 °C to 1160 °C, the average grain sizes of FGH95 alloy grow up from 10–20 μm to 15–25 μm, as shown in Table 3. Meanwhile, under the applied stress of 1034 MPa at 650 °C, the strain rate of the alloy during steady state creep decreases from 0.0102%/h to 0.00367%/h, the creep lifetime of the alloy increases from 67 h to 104 h, as shown in Fig. 5 and Table 4. This indicates that the coarser grain size can improve the creep resistance of the alloy. The

Conclusion

  • (1)

    After solution treated at 1150 °C, some coarser γ′ precipitates are distributed in the wider boundary regions where appears the depleted zone of the fine γ′-phase. After solution temperature rises to 1160 °C, the coarser γ′ phase in the alloy is fully dissolved, the fine secondary γ′ phase with high volume fraction is dispersedly distributed within the grains, and the particles of (Nb, Ti)C carbide are precipitated along the boundaries. When the alloy is solution treated at 1165 °C, the size of

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