| Turbine disk is an important load-bearing component of aero-engine. Its service temperature is slightly lower than that of turbine blade, while it must work at a very complicted stress state. The structure is of some complexity to the turbine disk of aero-engine, and the turbine disk must work at a state of high speed rotation, which results in the temperature, stress state, degrees of corrosion sustained by the components, such as the hub, the web, the rim, and the serration of turbine disk, are different from each other. Therefore, the materials used for the turbine disk should possess the mechanical peoperties as follows: higher yield strength, tensile strength and sufficient plasticity; higher persistent strength, creep and plasticity; excellent fatigue resistance; excellent corrosion resistance and the stability of microstructure.The nickel-base GH4133B superalloy is modified from a GH4133 superalloy by means of microalloying of adding amount of magnesium and zirconium elements. Comparing with the GH4133 superalloy, the GH4133B superalloy eliminates the notch sensitivity in GH4133 at temperature above 750oC, which results in a doubling service life of material, and an increasing endurance strength and plasticity. Therefore, in the aviation industry, the GH4133B superalloy is selected as the material to produce the turbine disk of aero-engine. In this work, the reliability and stability of mechanical property parameters, fatigue damage mechanism, fatigue crack propagation behavior of GH4133B superalloy at ambient temperature is studied in the way of theoretic analysis and tests. The main contents and conclusions are as follows.1. The fatigue behaviors of metallic materials are introduced. The various periods of fatigue crack of metallic materials, and the corresponding macroscopic and microscopic morphological properties are analyzed. The advances on the researches of the fatigue of metallic materials are reviewed, and the characterization methods in the macroscopic and microscopic forms for the fatigue damage, the basic models for the fatigue damage, and the morphological analysis for the fatigue fracture surfaces of metallic materials as well, are summarized.2. The reliability and stability of materials is the base for the structural safety assessment. The uniaxial tensile tests, impact tests and Brinell hardness tests for the specimens of GH4133B superalloy are carried out at ambient temperature, and 622 sets of experimental data on the mechanical properties for GH4133B superalloy are obtianed. Then, the mechanical property parameters of GH4133B superalloy are analyzed using the statistical analysis technique. The mechanical property parameters, such as tensile strengthσb, yield strengthσ0.2, percentage elongationδ5, reduction of cross-section areaψ, Brinell hardness HB and impact toughnessαk, are statistically analyzed using the normal probability density function, and D'Agostino test procedure is adopted to detect the statistical results. The mean-range chart is introduced to examine the periodical stability of GH4133B superalloy at different test stages. The global stability and reliability of mechanical property parameters of GH4133B superalloy are investigated using two-parameter and three-parameter Weibull distrubution models. The results demonstrate that the mechanical parameters of GH4133B superalloy, whose values possess the property of periodical stability, are fitted with the normal probability density function approximately. The three-parameter Weibull distribution is fit for the reliability assessment of GH4133B superalloy. The global stability of GH4133B superalloy and reliability as well, are all well match with the requirement of practical services and applications.3. Fatigue is the main reason for fracture of engineering structures or components. The fatigue damage and fracture mechanism of GH4133B superalloy used in turbine disk of aero-engine are studied. The fatigue limit of GH4133B superalloy is measured, and the relation between the ratio of resistance change and the fatigue cycle is investigated at ambient temperature. The experiment data are processed using the regression analysis and the probabilistic method, the theoretical fatigue threshold and P-S-N representation are achieved, and the fatigue damage evolution equation is deduced. The results illustrate that the theoretical fatigue threshold agrees well with the experimental one, and the modified Chaboche model can precisely predict the accumulated damage of GH4133B superalloy. The fracture surfaces of GH4133B superalloy are analyzed using scanniing electron microscopy (SEM). It can be found from the fracture surface morphologies that in the initiation region, the GH4133B superalloy exhibits a mixed fracture mode and many secondary cracks emerge. In the propagation region, a series of fatigue striations can be observed, especially at lower stress amplitude, some brittle fatigue striations can be found. In the final rupture region, the fracture surface appears as a typical semi-brittle fracture mode, furthermore, the dimensions and counts of dimple decrease with increasing stress amplitude.4. Fatigue life is composed of crack initiation life and growth life. The fatigue crack propagation tests of GH4133B superalloy used in turbine disk of aero-engine are carried out under different stress ratios at ambient temperature, and the fatigue crack growth threshold values are measured. It is shown from the regression analysis by Paris formula that the crack growth rate increases with increasing stress intensity factor and stress ratio, the modified Paris formula considering threshold value can describe the fatigue crack growth behavior accurately, and the general representation with stress ratio can state the law of the fatigue crack growth at relative small stress intensity factor range. The crack growth path is observed by an optical microscopy, and the fracture surface morphologies are investigated using scanning electron microscope. It is shown that with increasing stress intensity factor, the crack growth path changes from a straight line to a curve. In the crack initiation region, steady growth region and rapid growth region, the fracture surface individually exhibits a cleavage fracture mode, a series of fatigue striations and a mixed intergranular dimple frature mode. Finally, the crack growth equations and loads are derived using the inverse educing method. It is suggested that the inverse equations can predict the crack growth behavior, and the predict results can effecitvely prevent the occurrence of fatigue fracture. |
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