Creep-fatigue Mechanism Research Of Directionally Solidified Nickel-base Superalloy DZ125

Aero-engines undergo start-up,slow-moving,cruising,and afterburning during long-term service,therefore turbine rotor blades are subjected to complex multi-field stresses(such as centrifugal force,aero-dynamic forces,thermal stress,etc.).These stresses change periodically during the service of the engine.Deeply,the corresponding load spectrum appears as a coupling between static load and alternating load.In this complex and demanding service condition,fatigue,creep and creep-fatigue interactions are important modes of turbine blade failure.In this study,the nickel-based directional solidification super-alloy DZ125 was taken as the research target.The mechanical properties,failure mechanism life model of fatigue and creep behavior under various load conditions were studied.Firstly,the experimental results under the stress(strain)control conditions of various materials are reviewed.While the lateral comparison is carried out to try to find the general law of creep-fatigue(in terms of experimental data and microscopic analysis).Then,based on the experimental results: it was found that cyclic loading has an important influence on the creep properties of DZ125.Such as the introduction of the hold time reduces the creep life of DZ125 super-alloy.In the short-term load,the cycle acceleration effect first increases and then decreases Otherwise,the long-lasting life decreases first and then rises.In the long-term load,the load-increasing cycle acceleration effect does not change and the longevity life is basically the same.Further,microscopic analysis: the creep fracture morphology of DZ125 super-alloy is macroscopically characterized as ductile fracture.What's more the overall morphology of creep fracture central fiber zone,outward crack propagation zone and outermost shear lip zone can be observed.As for crack initiation,they are mainly caused by carbides between dendrites and eutectic structure.Furthermore,the creep-fatigue of short-time load retention has typical fatigue morphology characteristics.But for long-time load retention exhibits more creep properties,there are many creep pores in the middle part of the fracture.In addition,the surface oxidation near the fracture is more severe.The creepfatigue crack mainly originates from the eutectic structure at the grain boundary and carbide in the crystal.Due to the different stress fields,the crack propagation mode is also different.The cracks at the grain boundary can be crystallized along the crystal.However the cracks in the crystal are generally intergranular and then crystallized.Finally,comparing the prediction results with Miner's linear cumulative damage quasi,the power function is used to fit the dwell time with the creep-fatigue cycle life to further predict the durable life under different load-holding history.A better prediction of the durable life of DZ125 super-alloy under stress-controled was forcasted at elevated temperature 850? and 980?,which lifetime dispersion band is within 2 times.