Study On The Low Cycle Fatigue Properties Of TC4ELI Alloy With Duplex Structure

TC4ELI alloy,as a kind of medium strength and high toughness titanium alloy,is widely used in the field of marine engineering,such as the pressure shell of deep submersibles.Due to the alternating pressure of sea water,the low cycle fatigue performance of deep submersibles is very demanding.Therefore,in this paper,the low cycle fatigue behavior of TC4 ELI alloy with duplex structure is studied systematically.Through the observation and analysis of its fatigue life,cyclic stress strain response and microstructure after fatigue under different cyclic strain amplitude,the mechanism of low cycle fatigue damage of TC4 ELI alloy with duplex structure is clarified,which aims to provide theoretical support for the extensive application of the alloy in the field of marine engineering.Through the fatigue test of TC4 ELI alloy at 0.5%,0.6%,0.7%,0.8%,0.9% and1.0% cyclic strain amplitude,it is found that with the increase of cyclic strain amplitude,the plastic strain energy increases gradually and the fatigue life decreases gradually.Based on the Coffin-Manson model,the fatigue life and plastic strain amplitude were fitted,and it is found that the plastic strain amplitude and fatigue life exhibit bilinear characteristics in a double logarithmic coordinate system.Through the analysis of the S-N curve,it is found that with the increase of number of cycles,material gradually softens,and with the increase of cyclic strain amplitude,the cyclic softening degree increases.In addition,through the decomposition of the friction stress and back stress of the hysteresis loop,it can be seen that the cyclic softening behavior under the high cyclic strain amplitude is a collective effect of softening of the friction stress and back stress,while the slight softening of the cyclic saturation under the low cyclic strain amplitude is caused by the mutual counteraction of the hardening of the back stress and the softening of the friction stress.The result shows that fatigue fracture of TC4 ELI alloy can be divided into three stages: crack initiation,crack propagation and crack instability fracture,by the observation of the fatigue fractures of the TC4 ELI alloy with dual structure at 0.5%,0.7% and 1.0% cyclic strain amplitudes.Although the crack sources are all located on the surface of the specimen when the strain amplitude is 0.5%,0.7% and 1.0%,with the increase of the cyclic strain amplitude,the number of crack sources on the surface of the specimen increases gradually.With the cyclic strain amplitude incresasing,the proportion of the area of the fatigue instantaneous fracture zone increases,and theproportion of the area of the crack growth zone decreases correspondingly.Through EBSD and TEM analysis of samples taken from fatigue fractures with different strain amplitudes,it is found that at low strain amplitudes,some of the grains of the sample have undergone plastic deformation.At high strain amplitudes(1.0%),the back stress softening of the sample is related to the deformation between grains tending to be homogenized,the destruction of ? phase and the appearance of multiple slips;at lower strain amplitudes(0.5%),the back stress hardening of the specimen is due to the dislocation pile-ups.