High Temperature Low Cycle Fatigue Behavior And Fracture Mechanism Of GH4698 Nickel-based Alloy

GH4698 nickel-base superalloy possesses good high-temperature strength, excellent resistance to oxidation and outstanding resistance to corrosion at high temperatures, being the key material for high-temperature components such as aviation engine turbine disk at present. In actual service process, aero-engine is prone to produce high temperature low cycle fatigue damage. Therefore, the research of high temperature low cycle fatigue properties is of crucial significance. As at present, the research on low cycle fatigue properties of GH4698 alloy has seldom been reported. In particular, the influence of microstructure on low cycle fatigue properties for GH4698 alloy has not yet been reported. Study on effect of microstructure on low cycle fatigue properties for GH4698 alloy has an important guiding significance for its the actual service.Two kinds of heat treatment system were adopted. The heat treatment system 1 is as follows: 1050 °C/8h, AC+1000 °C/4h, AC +775 °C/16 h, AC +700 °C/16 h, AC(AC means air cooling). The heat treatment system 2 is as follows: 1100 °C/8h, AC+1000 °C/4h, AC+775 °C/16 h, AC+700 °C/16 h, AC. The causes of grain growth in the process of heat treatment were analyzed by the dissolution and precipitation of γ’ phase. Total strain-controlled low cycle fatigue tests on MTS 810 fatigue testing machine were conducted out for GH4698 alloy at 650°C. The effects of microstructure and strain amplitude on low cycle fatigue properties of the alloy were investigated.The cyclic deformation behaviors of GH4698 alloy during the low cycle fatigue were studied through the analysis of cyclic stress response curve and fatigue hysteresis loop. The results showed that peak stress, fatigue life, cyclic hardening and softening behavior of the alloy are closely related to the total strain amplitudes. Under the condition of the same strain amplitude, absorbed irreversible deformation power and fatigue life of the alloy obtained by heat treatment system 1(recorded as alloy A) are higher than the alloy obtained by heat treatment system 2(recorded as alloy B). It indicates that cyclic toughness and low cycle fatigue property of alloy A is better than those of alloy B. A new fatigue life prediction model is established considering the fatigue limit and the grain size. The prediction accuracy of the fatigue life prediction model considering the effect of the grain size is higher than that of Ostergren energy method life model and Manson-Coffin life model, which possesses general applicability.The changes of metallographic microstructure and γ’ phase were analyzed so as to investigate the microstructure evolution mechanism of GH4698 alloy after low cycle fatigue. Grain boundary and twin boundary can easily become fatigue crack initiation site due to their pinning role in dislocation motion. Under the condition of the same strain amplitude, the number of γ’ phase in alloy A sheared by dislocations is lower than that of alloy B. In addition, the size of γ’ phase in alloy A is smaller than that of alloy B. It indicates that alloy A possesses better fatigue property than that of alloy B. The microstructure evolution mechanism of the alloy during low cycle fatigue is as follows: with the increase of the total strain amplitudes, on the one hand, absorbed strain energy increasing results in crystal stored energy increasing; on the other hand, owing to the increasing dislocation density, dislocations severely shearing γ’ phase causes the decreasing pinning effect of γ’ phase on grain boundary. These two factors lead to grain grouth of the alloy after low cycle fatigue.In order to investigate the fatigue fracture mechanism of GH4698 alloy, slip bands, dislocation configurations and fatigue fracture morphologies were observed and analyzed after low cycle fatigue. Fracture analysis shows that low cycle fatigue fracture modes of the alloy are mainly brittle fracture at low strain amplitudes and toughness fracture at high strain amplitudes. Microstructure analysis indicates that low cycle fatigue deformation mechanism of GH4698 alloy includes plane deformation and non-planar deformation. With the increase of the total strain amplitudes, the densities of slip bands and the dislocations increase significantly. Slip bands and dislocations seriously shear γ’ phases, leading to the rapidly decreasing strengthening role of γ’ phases on the matrix. In addition, dislocations pile up at the grain boundaries or twin boundaries. The occurance of stress concentration leads to the cracking of the grain boundaries or twin boundaries. Then microcracks grow and expand, eventually leading to fatigue fracture of the alloy.