Deformation Behavior Of Ultra-Fine Grained Commercial Pure Titanium At Room Temperature

In order to expand the application range of the commercial pure titanium(CP Ti),it is necessary to improve its mechanical properties.In this work,ultra-fine grained commercial pure Ti(UFG CP Ti)with an average grain size of about 180 nm was produced by equal-channel angular pressing(ECAP)followed by cold rolling and rotary swaging,and then the UFG CP Ti was conducted to annealing treatment at 200 ? in 60 min.The average grain size of the annealed specimen was approximately 187 nm.The quasi-static compression,dynamic compression,fatigue and creep behavior and mechanism of UFG and annealed specimens at room temperature were studied.The main research results were as followings:The compression testing of the UFG and annealed specimens was carried out at the strain rate of lx 10-4x103 s-1.The results showed that flow stress curves exhibited an obvious steady state,and steady-state flow stress increased with increasing the strain rate.In addition,strain rate sensitivity index(m value)increased with the decrease of stress rate.The m values of both specimens were higher than that of the coarse pure Ti at the same deformation condition.Both specimens exhibited evident characteristics of adiabatic shear failure at high strain rate compression,and cracks nucleated and grew along adiabatic shear bands.The fatigue testing of the UFG and annealed specimens was carried out at room temperature with the symmetrical loading.The results showed that the fatigue strength of the smooth and notch UFG specimens was improved 56.5 and 42.6 MPa compared with the corresponding annealed specimens.The fatigue notch sensitivity index(q value)of the UFG and annealed specimens was 0.430 and 0.394,showing the low fatigue notch sensitivity.Furthermore,the fatigue equations of smooth and notch UFG specimens were lg ?a =-0.055lgNf + 2.89 and lg?a=-0.0941gNf +2.94,and the annealed specimens were lg?a =-0.0531gNf + 2.95 and lg?a ?-0.078lgNf +2.92,respectively.There was the linear relationship between the fatigue limit and ultimate tensile strength for both the UFG and annealed specimens regardless of the geometry of fatigue specimen.For instance,the correlation coefficients of the smooth and notch UFG specimens were 0.37 and 0.45,and the annealed specimens were 0.23 and 0.29,respectively.The room-temperature creep testing showed that there was obvious creep feature for both the UFG and annealed specimens.The steady-state creep rate was increased with increasing applied stress.The fracture mode of UFG specimen was transgranular rupture,while the annealed specimen did not fracture even at the high stress level of 650 MPa.Compared with the coarse Ti,both specimens possessed the higher creep property.In addition,the steady state constitutive equations of the UFG and annealed specimens were ? = 6.877E-19(?-640)5.9 and ? = 1.249E-14(?-547.6)2.8,respectively.Moreover,The UFG specimen possessed the higher creep property compared with the annealed specimen.Dislocation slip was the main creep mechanism.The nanometer indentation testing was also used to evaluate the creep property of the materials at room temperature.The results showed that the creep equations of the UFG and annealed specimens were ? = 12.92?10.15 and ? = 21.23?11.05.respectively.The creep dislocation was increased with the increase of the loading rate.Both specimens exhibited the high strain hardening capacity.The surfaces of the specimens had residual tensile stress.In addition,the creep rate sensitivity index of the annealed specimen was lower than that of the UFG specimen,showing the higher creep resistance.There was no relationship between the creep rate sensitivity index and loading rate.