3D Finite Element Analysis Of Deformation Behavior Of CP-Ti During Equal Channel Angular Extrusion Process And The Texture And Corrosion Resistance Characterization Of Ultrafine-grained CP-Ti

Pure Ti attracted much attention in medical application because of better biocompatibility, corrosion resistance and without toxic element. The application of pure Ti as medical implant materials, especially as implant components to sustain heavy load, is limited due to its lower strength and bad abrasion resistance. Developing a new process to increase the strength of pure Ti with good ductility is what to go after for medical implant materials researchers. In recent decades, it has attracted much attention that improving the strength of pure Ti through ultrafine-grained structures after equal channel angular extrusion (ECAE).In the present paper, bulk ultrafine-grained commercial pure Ti (UFG CP-Ti) was prepared by multi-pass ECAE at 400℃via route BC and cold rolling (CR). The deformation behavior of CP-Ti during four-pass ECAE process at 400℃via route BC was simulated by three-dimension (3D) finite element method (FEM). The deformation heat and the effect of initial extrusion temperature on deformation behaviors of CP-Ti during ECAE were analyzed. The texture evolution of CP-Ti during multi-pass ECAE and CR process were investigated by pole figures and ODF. The effect of texture on mechanical properties and deformation mechanism were also analyzed. The electrochemical properties of UFG CP-Ti in Ringer's solution were investigated by electrochemical techniques (potentiodynamic polarization test and electrochemical impedance spectroscopy (EIS) measurement).3D FEM simulation of four-pass ECAE process indicates that the deformation of CP-Ti during ECAE process is inhomogeneity in the directions of the length, the width and thickness along the billet. But there is a steady-state deformation region in the middle part of the billet after one-pass ECAE, with a maximum effective strain is 1.124. The strain and the size of the steady-state region are increased with the number of ECAE process. The strain reaches to 3.723 and the steady-state strain region takes about 50% length of the total billet after four-pass. The maximum value of extrusion load increases with the number of ECAE. A maximum damage factor value C_N of 0.33 is obtained on the top surface at a length of 16.356mm to the tail of CP-Ti billet processed after two-pass ECAE at 400℃via route B_C, which is larger than the critical value C_N^* of 0.31. Crack is predicted to appear from this area at the top surface of the billet. The experimental observation is consistence with the FEM prediction.The simulation of temperature distribution in CP-Ti billet during one-pass ECAE at 400℃by rigid-plastic FEM shows that abrupt temperature rise occurs within the shear zone during one-pass ECAE and the maximum temperature riseΔT is 110℃. The undeformed part of the billet gains heat from the deformed part and die and is deformed at a higher temperature. The temperature responses at different locations are generated by point tracking method. The temperature distributes imhomogenously in CP-Ti billet during one-pass ECAE and the maximum temperature locates on the top surface of billet where closes to the shear deformation zone. When the initial extrusion temperature increases, the deformation of billet is more homogenously and the temperature difference in both the billet and die become smaller.The analysis of texture in CP-Ti processed by multi-pass ECAE indicates that the texture components of coarse-grained (CG) CP-Ti are not retained in the billet after multi-pass ECAE. The maximum value of texture intensity is stabilized after four-pass ECAE. The main texture component of CP-Ti billet after four-pass ECAE is centered at a Euler angle position with {φ₁ 1=315°,Φ=66°,φ₂=12°} (in the Bunge notation system), which corresponds to ((1|-)3(2|-)2)[10(1|-)2] and secondary texture component is (12(3|-)2)[(2|-)111], i.e. {φ₁=165°,Φ=72°,φ₂=48°}. The texture component of CP-Ti billet after eight-pass ECAE is (01(1|-)1)[(1|-)013]. The analysis of texture and Schmid factors shows that {10(1|-)1} twinning is accompanied by pyramidal- slip during one-pass ECAE. The crystallographic relationship in one-pass ECAEed billet induces the prismatic- slip during two-pass ECAE process.The prismatic- slip is favored by the ((1|-)3(2|-)2)[10(1|-)2] and (12(3|-)2)[(2|-)111] textures in the tensile deformation of four-pass ECAEed billet, since Schmid factor of ((1|-)03(2|-)2)[10(1|-)2] and (12(3|-)2)[(2|-)111] textures at the prismatic- slip is obviously enhanced in the four-pass ECAEed billet. Besides, a lower stress is needed for the prismatic- slip due to the smallest critical stress for prismatic- slip, which results in the decrease of the yield strength.After cold rolling at room temperature with an accumulative strain of 76%, the textures of UFG CP-Ti processed by eight-pass ECAE are spreaded and distributed along the line ofΦ=18°. The main texture components are (01(1|-)5)[1(1|-)01], (01(1|-)5)[2(2|-)01], (0001)[(1|-)(3|-)43] and (01(1|-)3)[2(1|-)(1|-)1]. The textures of UFG CP-Ti after CR at liquid nitrogen temperature with an accumulative strain of 74% are mainly pyramidal-texture (1(2|-)14)[21(3|-)(1|-)] and weakly (01(1|-)3)[20(2|-)1] texture.The results of potentiodynamic polarization test and electrochemical impedance spectroscopy (EIS) measurement show that the breakdown potential Etp of CP-Ti increases remarkably with the ECAE pass number in Ringer's solution, which reveals an improvement of pit corrosion resistance. The polarization resistance (RP) of UFG CP-Ti is improved, which indicates that the oxidizing layer formed on the surface of UFG CP-Ti is more stabilized.