Research On Superplasticity And Fracture Of Fine-grained TC21 Alloy

In this study,the tensile tests of fine-grained TC21 alloy with grain size 2μm,4μm and 7μm have been performed.And the deformed microstructure,fracture and deformed mechanism also have been examined to provide scientific support for the application.The superplastic tensile tests have been performed at various temperature(860℃～950℃) and strain rate(1×10^-3s^-1～5×10^-4s^-1).The results show that fine-grained TC21 alloy has excellent superplasticity.On the optimal superplatic deformation condition(890℃,5×10^-4s^-1), the maximum of elongation for TC21 alloy is up to 1400%.The strain rate sensitive index m is more than 0.5 for TC21 alloy with grain size 2μm and 4μm,and the maximum ofm is 0.87, while m is between 0.3 and 0.5 for the alloy with grain size 0.7μm.The microstructure of fine-grained TC21 alloy as a function of deformed temperature and strain rate was examined.Deformation primarily occurred alongβ/βorβ/αgrain boundary.During the initial stage of superplastic deformation,grain group coordinate deformation to establish the equilibrium state.So the microstructure varies significantly,and static grain growth is dominant.When equilibrium state is established,the microstructure is highly stable,thus ensue excellent superplaticity of fine-grained TC21 alloy.Strain inducing gain connection and growth,which is clearly observed during superplastic deformation.The SEM morphology of the fracture surfaces is composed of various dimples.The macroscopic feature is punctiform.The alloy with grain size 2μm and 4μm submits to cavities rupture mechanism.The macroscopic feature of alloy with 7μm is knifepoint.The SEM morphology indicates the microstructure because of cleavage fracture and gliding fracture.The dominant deformed mechanism of TC21 alloy with grain size 2μm and 4μm is grain boundary gliding.In the condition of low strain rate,coordination mechanism is dislocation creep controlled by diffusion through atoms at grain boundaries.As the strain rate increase or temperature decrease,the diffusion of atoms is obviously weakened,thus dislocation creep becomes the dominant coordination mechanism.And a lot of dislocation wall arranged regularly are observed in deformedα.The deformation of TC21 alloy with grain size 7μm is non-superplastic,just high temperature plastic deformation.