【作者】 江鸿；

【导师】 谢超英；

【作者基本信息】 上海交通大学 ， 材料学， 2010， 博士

【摘要】 纯钛作为生物医用材料,具有良好的生物相容性,不含有毒化学元素,受到国内外研究者的关注。但是因强度较低、耐磨损性能较差,纯钛在人体承受较大载荷部位的应用受到限制。因此,寻求一种新型工艺制备具有良好综合性能的纯钛材料,已成为生物医用材料的一个重要攻关方向。近年来,运用等通道弯角挤压（Equal channel angular extrusion, ECAE）法细化纯钛组织、提高材料强度等力学性能的研究已经引起研究者的关注。本文选用3级工业纯钛（CP-Ti）为研究对象,通过在400℃沿B_C路径八道次ECAE处理加冷轧两步大变形法制备出大块体超细晶CP-Ti。利用三维有限元数值模拟方法计算分析了多道次ECAE处理CP-Ti的变形过程,并研究了变形热及初始挤压温度对变形行为的影响;借助极图和ODF函数系统地研究了CP-Ti在两步大变形过程中的织构特征,以及织构对其力学性能和变形机制的影响;并采用动电位极化技术和电化学阻抗谱（EIS）研究了超细晶CP-Ti在Ringer’s（林格式）人体模拟体液中的耐腐蚀性能。400℃沿Bc路径四道次ECAE处理CP-Ti过程的三维有限元模拟结果表明, ECAE处理后试样在长度、宽度、厚度三个方向上的变形不均匀,应变分布梯度较大;但试样中间部分呈现稳定均匀变形区（试样最大应变值1.124出现在该变形区内）,均匀变形区尺寸及应变量随着挤压道次增加而增大;四道次ECAE处理后均匀变形区最大应变值达3.723,均匀变形区尺寸约占试样总长的50%。CP-Ti试样最大挤压载荷峰值随着挤压道次增加而增大。400℃沿Bc路径两道次ECAE处理后的CP-Ti试样中,损伤参数C_N （Cockcroft–Latham断裂准则）最大值达0.33,分布在靠近试样尾部的上表面边沿区域,大于临界C_N^*值（约0.31）,预示试样上表面相应处有裂纹产生。实验观察与FEM损伤模拟结果预测裂纹基本吻合。采用热-机耦合的刚塑性有限元分析了400℃一道次处理CP-Ti过程中的温度分布,结果表明:试样在剪切变形区温度骤升,温度增加值ΔT最大达110℃;试样未变形部分受到变形部分和模具传递的热量而在更高温度发生塑性变形。采用质点跟踪法揭示了在挤压过程中CP-Ti试样沿长度方向体中心线上和表面各质点的温度变化规律。一道次ECAE处理CP-Ti试样中温度分布不均匀,最高温度分布在试样剪切变形区的上表面。当初始挤压温度升高时,试样变形更加均匀,温差变小,且模具内温差也变小。多道次ECAE处理CP-Ti的织构分析表明,退火粗晶CP-Ti的织构组分在400℃一道次ECAE处理后不再保留。400℃沿B_C路径多道次ECAE处理CP-Ti的织构最大强度在四道次后逐渐稳定。四道次ECAE处理后,主要织构组分的欧拉角位置为{φ₁=315°,Φ=66°,φ₂=12°}（Bunge符号系统）,其相应的密勒指数约为（（1|-）03（2|-）2）[10（1|-）2];较弱织构的欧拉角位置为{φ₁=165°,Φ=72°,φ₂=48°},即（12（3|-）2）[（2|-）111]。八道次ECAE处理后,主要织构组分为（01（1|-）1）[（1|-）013],其欧拉角位置为{φ₁=120°,Φ=60°,φ₂=30°}。织构和Schmid因子分析表明,一道次ECAE处理CP-Ti过程中,{10（1|-）1}孪生为主要变形方式,同时伴随锥面-<c+a>滑移;二道次ECAE处理过程中,柱面-<a>滑移为主要的变形方式。在拉伸过程中,四道次ECAE处理后CP-Ti织构（（1|-）03（2|-）2）[10（1|-）2]和（12（3|-）2）[（2|-）111]在柱面-<a>滑移系上的Schmid因子明显增加,更有利于柱面-<a>滑移开动,因而在比较小的应力下就能达到滑移所需要的临界分切应力值,导致屈服强度降低。八道次ECAE处理后获得的超细晶CP-Ti经室温76%冷轧后的织构分布比较分散,织构组分主要有（01（1|-）5）[1（1|-）01]、（01（1|-）5）[2（2|-）01]、（0001）[（1|-）（3|-）43]和（01（1|-）3）[2（1|-）（1|-）1];经液氮温度74%冷轧后织构主要组分为棱锥型织构（1（2|-）14）[21（3|-）（1|-）]及较弱（01（1|-）3）[20（2|-）1]织构。超细晶CP-Ti的动电位极化曲线和电化学阻抗谱EIS测试表明:在Ringer’s溶液中,随着挤压道次增加,CP-Ti的破裂电位Etp明显增加,超细晶CP-Ti的耐点蚀能力较粗晶CP-Ti有显著提高。超细晶CP-Ti的极化电阻Ep较粗晶CP-Ti亦有明显提高,表明超细晶CP-Ti表面形成的氧化层更稳定。更多还原

【Abstract】 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-<c+a> slip during one-pass ECAE. The crystallographic relationship in one-pass ECAEed billet induces the prismatic-<a> slip during two-pass ECAE process.The prismatic-<a> 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-<a> slip is obviously enhanced in the four-pass ECAEed billet. Besides, a lower stress is needed for the prismatic-<a> slip due to the smallest critical stress for prismatic-<a> 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.更多还原