Fatigue Crack Growth Behavior Of Ultrafine Grained Pure Titanium Processed By ECAP + Rotary Swaging

In this paper,four kinds of ultrafine grained?UFG?pure titanium with different microstructures were fabricated by two passes of ECAP deformation with equivalent strain of 0.75,ECAP+rotary swaging?RS?with equivalent strain of 2.58,and annealing at 300°C and 400°C for 1hour after two passes of ECAP+RS.The microstructural evolution and mechanical properties of the four kinds of UFG pure titanium were analyzed,and the fatigue crack growth behaviors of the four kinds of UFG pure titanium were studied from crack growth rate,crack propagation path,and fatigue crack fracture mechanism.The main research results are as follows:After two passes of ECAP,the strength of pure titanium is significantly increased,and its microstructure is composed of bands and twins with a width of about 1?m,and the grain size is not uniform.Non-uniformity in microstructure and strength of pure titanium processed by ECAP are improved by subsequent RS.The grain size is refined to about 200 nm after RS.With the increase of annealing temperature,the grain size and the fracture elongation of pure titanium after rotary swaging increases and the dislocation density and strength decreases.The fatigue crack growth rate test results of the standard single edge notched tensile specimen?SENT?show that the microstructure has a significant effect on the fatigue crack growth of UFG pure titanium.There is a turning point in the fatigue crack growth rate curve,which divides crack growth region into near threshold region and stable propagation region.In the near threshold region,ECAP+RS deformed pure titanium has the highest?K_th,about 9.6 MPa·m^1/2,the fatigue crack growth rate of pure titanium processed by ECAP is higher than that of pure titanium processed by ECAP+RS.The fatigue crack growth rate of pure titanium processed by ECAP+RS increases with increasing annealing temperature.In the stable propagation region,the crack resistance of pure titanium annealed at 400°C for 1 hour after ECAP+RS is the strongest,followed by pure titanium processed by ECAP and ECAP+RS,and the crack resistance of pure titanium annealed at 300°C for 1 hour after ECAP+RS is the weakest.The crack paths of pure titanium processed by ECAP and ECAP+RS and annealed at 400°C are relatively tortuous,and the cross section fluctuates greatly.The crack path of pure titanium processed by ECAP+RS and annealed at 300°C are relatively straight.In the early stage of crack growth of pure titanium processed by ECAP+RS,many small cracks are observed and only one of which can become the main crack,making pure titanium processed by ECAP+RS have the highest crack initiation life.Furthermore,pure titanium processed by ECAP+RS has the highest fatigue life.The mechanisms of the fatigue fracture of four kinds of UFG pure titanium are gradually transformed from cleavage fracture to ductile fracture.The mechanism of the fatigue fracture of pure titanium processed by ECAP is mainly composed of transgranular and intergranular fracture,and the stable growth zone section has obvious crack closure contact trace.The fractures of pure titanium processed by ECAP+RS and annealed at300°C and 400°C are mainly intergranular fractures,and there are more secondary cracks on the fracture surface.Therefore,the differences in crack closure,crack deflection,secondary cracks and fracture modes are the reasons for the difference in crack growth rates of UFG pure titanium.The results of fatigue crack growth at different stress ratios?0.1,0.5?show that the stress ratio has a significant effect on the fatigue life and crack growth rate of UFG pure titanium.The fatigue life of pure titanium processed by ECAP and annealed at 400°C after ECAP+RS increases under the condition that the stress ratio is 0.5,while the fatigue life of pure titanium processed by ECAP+RS and annealed at 300°C decreases under the condition that the stress ratio is 0.5.The stress ratio increased from 0.1 to 0.5,With the increase of the stress ratio,?K_th is reduced,of which ECAP+RS deformed pure titanium has the largest decrease in?K_th,and the difference of?K_th is about 4.4 MPa·m^1/2.