Effect Of Geometric Discontinuities On The Fatigue Behavior Of Single Crystal Superalloy

The areas with geometric discontinuities of nickel-based single crystal super alloy turbine blades have a high incidence of failure, such as the transition zone between blade and shroud of shrouded blade. Currently, the researches on the areas with geometric discontinuities are in most cases based on the phenomenology while ignoring the effect of the particularity of the areas. Actually, the microstructure of the metal existed heterogeneity in the areas during the casting process. The geometric discontinuities make great influences on the mechanical behavior and the fatigue life of the single crystal superalloy. On one hand, the microstructure of single crystal superalloy is no longer a crystal with single orientation in the areas with geometric discontinuities. On the other hand, geometric discontinuities result stress concentration in the area. In consideration of the reasons above, the following work is carried out. 1. A new yield criterion is built based on the macroscopic phenomenological model and the tension-compression asymmetry orthogonal yield criteria, in which the coupling effect of tension and shearing and the tension-compression asymmetry are considered. Because of the existence of stray grains in the areas with geometric discontinuities, a new bicrystal plate model is used to analyze the influence of the stray grains. 2. In consideration of the factor of asymmetricloading1 k and the factor of crystalorientation1 f, a new low-cycle fatigue model of nickel-based single crystal superalloy is built. At the same time, based on the theory of equivalent strain of nickel-based single crystal superalloy, a low-cycled fatigue model is raised too. The relevant fatigue tests are carried out. 3. Based on the work above, several low-cycled fatigue models for notched components are built, in which local stress-strain approaches, strain gradient approaches and critical distance approaches are applied. The fatigue tests of nickel-based single crystal superalloy with notched specimens are carried out and compared with the theoretical evaluations. 4. The characteristics of fracture surfaces of nickel-based single crystal superalloy are analyzed.