Study On Mechanism Of Microstructure Evolution And Casting Defects Of Nickel-based Superalloys On Creep & Fatigue Properties

Nickel-based alloys are extensively used in the manufacture of hot-end structure parts because of their excellent mechanical properties such as creep,fatigue and oxidation resistance at elevated temperatures.In the working environment of high temperature and complex stress,the microstructure evolution and casting defects of the material have great influence on the creep and fatigue life of the structure.The paper uses the combination of experimental observation and finite element analysis to systematically study the microstructure evolution and casting defects of nickel-based superalloys on mechanical properties such as creep and fatigue.An anisotropic creep constitutive model and an anisotropic elastoplastic model are established based on crystal plasticity theory.The impact of temperatures,crystal orientations,defects and stress states are considered for creep and fatigue life.The ABAQUS subroutine was compiled to realize the simulation work of finite element.Creep tests of three different orientations of the single crystal superalloy were carried out under different temperatures.The microstructure evolution law of rafting and dislocation motion is studied,especially the slip system actuation mode and rafting morphology under specific orientation,indicated that the creep failure mechanism of single crystal alloy is caused by the deterioration of material express as the evolution of microstructure and crack initiation caused by dislocation motion,showing anisotropic mechanical properties.The microstructure evolution mechanism of creep deformation and failure of nickel-based single crystal alloy is proposed.Based on the crystal plasticity theory,the creep constitutive model and creep damage model were established with Orowan effect and dislocation effect considered,meanwhile,the model parameters were fitted according to the creep curve obtainedfrom the test.Moreover,the finite element simulation results of the model and creep fracture morphology of monocrystalline materials mutually confirm and explain the anisotropic behavior of monocrystalline creep.Based on the quantitative description of the microstructure evolution during the creep process,the residual life prediction model of single crystal superalloy was established,which provides guidance for engineering applications.A creep experiment method combining bicrystal bars with I-shaped samples cutting from bicrystal blades is proposed,and the degradation mechanism of grain boundary defects on creep mechanical properties is studied.Studies have shown that the angle between the grain boundary and the tensile direction determines whether the specimen will break along the grain boundary,and the grain boundary angle determines the development of dislocation density near the grain boundary during creep.The creep damage model considering the grain boundary angle and the tensile direction is used to describe the weakening of the grain boundaries,and the creep life of bicrystal can be accurately predicted.The creep tests of the I-shaped samples cutting from bicrystal blades indicate that the anisotropic crystal plastic creep constitutive equation can be applied to the blade structure containing grain boundaries.Low cycle fatigue experiments of Nickel-based polycrystal superalloy samples were carried out at high temperature.The geometric models of the polycrystal alloy were established based on the observation of its microscopic structure under the scanning electron microscopy,while its constitutive model was built based on the crystal plasticity theory.Both the fatigue experiments and the finite element analysis indicated that the HIP process could dramatically reduce the stress concentration and plastic deformation of K403 alloy by eliminating the internal casting defects effectively so as to extend its fatigue life.Moreover,the microstructure could be largely optimized,which guarantees the improvement of the alloy fatigue property.