Characterization And Interactive Influence Of Multi-Factors On Stable Fatigue Crack Propagation

The harsh operating condition of turbine disk nickel based superalloys puts forward strict requirements on high damage tolerance design, so it is vital that the Paris region has larger proportion of the whole fatigue crack propagation process. Characterize and evaluate the stability of fatigue crack propagation, and characterize the level of interrelation between the multi-microstructure characteristic and environmental factors have not been solved yet. For this purpose, the propagation characteristic in Paris zone under high temperature was studied in this paper. Regularity of single, multi-characteristic factors and environmental factors was lucubrated analysis, which can offer the thinking and direction of high damage tolerance design under high temperature.With analyzing the fatigue crack growth curve of power metallurgy nickel superalloys (FGH4096, FGH4097, FGH4098) and wrought superalloy GH4738 and the fatigue crack propagation mechanism in the Paris zone under high temperature, the results shown that a visible slow and fast fatigue crack growth zones existed in the Paris region. This phenomenon revealed that the slow fatigue crack growth mode of main transgranular fatigue crack, which propagation direction vertical to the loading direction, turned to a fast fatigue crack growth mode which intergranular main fatigue crack with many intergranular secondary fatigue cracks. Based on this results, the conception of stable propagation toughness ?KT, stable propagation life coefficient Ds and the stable propagation characteristic pattern built by ?KT-Ds were proposed to characterize the stability of fatigue crack propagation under high temperature.The grain size, grain boundary carbide and y'phase characteristic and effects to fatigue crack propagation rate, life of different steps and propagation stability were studied systematically. The results show that methods of grain size enlargement, increasing grain boundary carbide and decreasing the secondary ?' phase all can improve the fatigue crack growth life, increasing the stable propagation toughness ?KTand stable propagation life coefficient Ds effectively. Meanwhile the multi-microstructure interaction model was built to gain the weight of different microstructure, which can offer choosing microstructure guide for high stable fatigue crack propagation alloy design.The fatigue crack propagation regularity under different temperature and load of GH4738, GH4720Li and FGH4098 was analyzed and the temperature or stress sensitivity was observed in a critical temperature or stress. And then the essence of decline in fatigue crack propagation life under sensitive temperature is caused by the decline in fatigue crack propagation rate and stable propagation toughness ?KT. And temperature effects on grain boundary and phase interface of primary y' can change the stable propagation toughness ?KT obviously, and the essence of stress sensitivity is that the increase the AK can decline the relative stable propagation toughness ?KT (?KT-?K), resulting in the fatigue crack access the fast fatigue crack growth zone after shorter propagation distance.In conclusion, based on the large amount of typical nickel superalloy fatigue crack propagation behavior analyses, the stable propagation characteristic pattern of secondary division to Paris zone was proposed, and effect of multi-microstructure and environment regularity was expounded systematically, which can offer the theoretical and experimental basis and feasible study methods for designing a high stability fatigue crack propagation microstructure and operating condition.