The Superplastic Deformation Mechanism And Behavior Of Fine-grained TC4 Titanium Alloy

The superplastic behavior of fine-grained TC4 titanium alloys with different initial grain size(2μm,8μm,18μm)has been studied.Uniaxial tensile tests and microstructure observation with OM,SEM and TEM are carried out to evaluate the superplastie mechanical behavior,microstruetures,deformation and fracture mechanism.The results show that there is a significant effect of deformation parameters(temperature,strain rate,grain size)on superplastic behavior and deformation mechanism for TC4 titanium alloy.The results of superplastic tensile tests invest that TC4 alloy have good superplasticity at the temperature range 860℃-950℃and the strain rate range 5×10^-4-5×10^-3s^-1.At experiment conditions,the elongation decrease with strain rate improved as well as initial grain size increased,moreover,the elongation raise firstly with temperature improved,and then decrease.At the optimum deformation condition(890℃,5×10^-4s^-1),the maximum elongation for the TC4 titanium alloys(2μm,8μm,18μm)is respectively 1290%,910%and 450%.After superplastie deformation,in the fine-grained(2μm～8μm)TC4 alloys,the grains are equiaxed,moreover,primaryα-grains grow and when the temperature is higher,the strain rate is quicker,the initial grain size is larger and the grain growth is more obvious.In the coarse-grained(18μm)TC4 alloys,the microstrueture has little equiaxedα-grains and lots of primaryα-grains size decrease.The main superplastie deforming mechanism of the TC4 alloys is due to grain boundary sliding(GBS)and grain rotating.Under the optimum deformation condition,in the TC4 alloy with 2μm grain size,the GBS is accommodated by the climbing and gliding of dislocations in the vicinity of the grain boundaries;in the TC4 alloy with 8μm grain size,the GBS is accommodated by the movement of dislocations in grains and dynamic recovery,reerystallization.In the TC4 alloy with 18μm grain size deformation is controlled by GBS,grain rotating and the movement of dislocations which are accommodated by dynamic recovery and recrystallization.The larger of initial grain size,the greater contribution of dislocations movement to superplasite deformation.Quick strain rate and large initial grain size tend to generate dynamic recovery and reerystallization more easily during superplasitc deformation.The superplastie tensile fracture surface of TC4 titanium alloys are analyzed by SEM. The results indicate that the fraetographies present many dimples,the feature of intergranular sliding is obvious.There is no cavity in the fracture for the specimen with a grain size of 2～8μm,but the specimen with a grain size of 18μm fracture appear cavity nucleation and growth and linkage and intergranular fracture feature.