Low Cycle Fatigue Behavior Of Ultra-fine Grained Pure Titanium Processed By ECAP And Rotary Swaging

Pure titanium has good comprehensive properties.As a structural material,its application is limited due to low strength.In order to improve the strength and fatigue life of pure titanium,ultra-fine grained?UFG?pure Ti was obtained by ECAP following with rotary swaging.The low cycle fatigue behaviors of coarse grained?CG?and UFG pure Ti at room temperature were investigated by transmission electron microscopy?TEM?,scanning electron microscope?SEM?,tensile test and low cycle fatigue test which total amplitude is in range of 0.5%¹.1%at room temperature.The main results of the research are as follows.The CG Ti with equiaxed ultra-fined grains?an average grain size of about 200nm?was obtained by four passes of ECAP and rotary swaging at room temperature.The yield strength of UFG pure Ti is 758 MPa.The strain controlled low cycle fatigue test was carried out on the UFG CG pure Ti at room temperature.The results show that the low cycle fatigue life of the UFG pure Ti is 2³ times longer than that of the CG pure Ti at room temperature.With the increase of strain amplitude,the fatigue lives of UFG and CG pure Ti decrease,and the plastic strain energy increases.The relationship between the total strain amplitude and the number of fatigue failure reversing follows the Manson-Coffin?M-C?formula.The fatigue ductility coefficient of UFG pure Ti is greater than that of CG pure Ti.When the total strain amplitude is 0.5%,the UFG pure Ti presents cyclic hardening,the UFG pure Ti exhibits cyclic softening when the total strain amplitude is0.6%¹.1%during the low cycle fatigue deformation at room temperature.In the process of low cycle fatigue,the relationship between the cyclic stress and strain of UFG pure Ti is no longer linear,which is in line with the general characteristics of low cycle fatigue because of the local macro plastic deformation.The specific expression of cyclic stress-strain of UFG pure Ti is:??/2?28?656.577?7???_p/2?8?^0.08672The low cycle fatigue deformation mechanism of UFG pure Ti is dislocation slip.Under the low total strain amplitude,the type of fatigue fracture is quasi-cleavage fracture.Under the high total amplitude,the fracture mode is cleavage fracture.While the total strain amplitude of the CG pure Ti is small,its low cycle fatigue deformation mechanism is dominated by dislocation slip.When the total strain amplitude is high,the low cycle fatigue deformation mechanism is dominated by twin deformation.The type of fatigue fracture is toughened.The type of fatigue fracture of CG pure Ti is quasi-cleavage fracture.