Abstract
The Ni-based IN792 superalloy is widely applied as a component in industrial turbines and aircraft engines due to its good high-temperature properties and excellent corrosion resistance. Since these components have to suffer from cyclic thermal and mechanical stresses during service, the high-temperature fatigue failure becomes one of the major factors affecting their service lives. Grain refinement has been considered as an effective way to improve the mechanical performance of superalloys. However, due to the complexity of alloy composition, microstructure and service condition, there is no unified theory about the influence of grain refinement on the fatigue performance and fracture mechanism of superalloys. In the present research, the IN792 superalloy with different grain sizes was manufactured. Then, fully reversed, strain-controlled, low-cycle fatigue (LCF) tests with four different total strain amplitudes were carried out on the alloy at 700 °C and 800 °C to clarify the effects of grain refinement on its LCF behavior. The results show that grain refinement improved the fatigue life significantly, which is mainly attributed to increasing the grain boundary content and refining MC carbides, eutectic structures and dendritic structures. During fatigue test under lower strain amplitude, the alloy exhibits a pronounced initial fatigue hardening followed by a continuous well-defined stability stage, which is caused by the formation of dislocation networks and coarsening of primary γ’ phases. However, during fatigue test under higher strain amplitude, the alloy exhibits continuous hardening response because the dislocations could shear primary γ’ phases, which could give rise to resistance to dislocation movement. In addition, the fracture surface observation shows that the fatigue fracture mode is mainly affected by the total strain amplitude. Under lower total strain amplitude, the fatigue microcracks mainly initiate at the porosities near the specimen surface, while under higher total strain amplitude, the fatigue microcracks tend to form at the interior of the specimen.
Highlights
Ni-based polycrystalline cast superalloys have relatively low manufacturing costs, excellent mechanical properties and microstructure stability in the range of 600–900 ◦C, and they are widely used in aeroengines, gas turbines and automobiles components [1,2,3,4]
Wei [8] manufactured IN713LC superalloys with different grain sizes by adjusting the pouring temperature and the results show that the room temperature yield strength increases along with grain refinement
Under a low total strain amplitude, the above analysis has shown that a large number of dislocation networks are formed in the interface of γ/γ’ phases, which would decrease the coherence of the interface and result in fatigue softening
Summary
Ni-based polycrystalline cast superalloys have relatively low manufacturing costs, excellent mechanical properties and microstructure stability in the range of 600–900 ◦C, and they are widely used in aeroengines, gas turbines and automobiles components [1,2,3,4]. It has been regarded that the service performance of polycrystalline superalloys is closely related to their grain size, and the refining of grains is always considered as an effective means to improve the mechanical properties and retard their failure [5,6,7]. The authors have investigated the tensile properties of Ni-based K417G superalloys with different grain sizes [11], and found that grain refinement could improve the tensile strength but decrease the ductility at 700 ◦C. The creep performance of K417G superalloys with different grain sizes is studied by the authors of [20], who found that with the refining of grains, the creep life is improved under 760 ◦C/645 MPa. The creep life under 900 ◦C/315 MPa first increases and decreases, while the creep life under 950 ◦C/235 MPa continuously decreases
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