Quasi-static and dynamic impact tests of hat shaped specimen with three different widths of shear zone (800μm, 400μm, 50μm) were performed by electronic universal testing machine and Split Hopkinson Pressure Bar (SHPB) respectively.The size effect phenomenon of shear stress and failure strain with different width of shear zone was investigated.The flow stress and failure strain of material increases with the decrease of the width of shear zone.And the size effect weakens with the increase of strain rate.Ductile-brittle-ductile transition was investigated with the change of strain rate.A smooth failure surface may be generated during impact at the specified strain rate.A modified Johnson-Cook constitutive model was employed to fit the plastic rise part and the fitted curves by modified Johnson-Cook models showed good agreement with the experimental curves.