【作者】 范志国；

【导师】 谢超英；

【作者基本信息】 上海交通大学 ， 材料学， 2009， 博士

【摘要】 纯Ti不含有害元素,具有良好的生物相容性以及优异的耐腐蚀性能,在生物医学领域显示出巨大应用潜力,但纯Ti由于较低的强度限制了它的广泛应用。近年来,通过大变形(Severe Plastic Deformation, SPD)法细化纯Ti的组织来提高其强度研究已经引起普遍关注。超细晶Ti及Ti合金的组织与力学性能已开展大量研究工作,但超细晶材料的相变行为及其相关性能研究开展较少。TiNi形状记忆合金具有丰富的相变现象、伴随马氏体相变及其逆相变呈现优异的形状记忆和超弹性性能,是目前已获得广泛应用的功能材料之一,超细晶TiNi合金相变行为及其对超弹性和形状记忆效应等性能影响研究已引起人们关注。本文选取3级工业纯Ti和富镍Ti-50.7.at%Ni形状记忆合金作为研究对象,采用中温(400℃～500℃)等径弯角挤压(Equal Channel Angular Extrusion, ECAE)工艺制备了大块体超细晶纯Ti及超细晶TiNi材料。并采用ECAE加液氮温度轧制两步大变形法制备高强度超细晶工业纯Ti材料。显微分析表明,纯Ti经400℃,8道次ECAE处理后,形成平均晶粒尺寸小于500 nm的亚微米晶组织。经8道次ECAE加液氮温度轧制两步大变形处理后,纯Ti中形成大量尺寸为100 nm～150 nm的位错胞状结构,同时产生明显(0002)织构。室温拉伸变形行为分析表明,与高纯0～2级工业纯Ti不同,具有较高杂质含量的3级工业纯Ti经4道次ECAE处理后,其真应力-真应变曲线上应变硬化阶段延长,其塑性变形呈均匀变形和塑性失稳两阶段,颈缩发生在第二阶段。超细晶3级工业纯Ti的抗拉强度和延伸率(ζb=816 MPa,δ=17.8%)均高于目前国际上文献报道ECAE处理超细晶0～2级工业纯Ti。8道次ECAE加液氮温度轧制两步大变形处理超细晶3级工业纯Ti的抗拉强度达1218 MPa,延伸率为12.6%。室温和液氮温度压缩试验表明,与粗晶纯Ti相比,8道次ECAE处理超细晶纯Ti的流变应力对温度和应变速率具有较低依赖性,应变速率从1×10-3增加到1×10-1,8道次ECAE处理超细晶纯Ti的应变速率敏感性因子(m)值为0.026,低于粗晶纯Ti的0.056。超细晶Ti-50.7.at%Ni合金微观组织研究表明,经500℃,8道次ECAE处理后,微观组织不均匀,除形成大量尺寸为200 nm～300 nm细小晶粒外,仍有少量宽度为100 nm～200 nm的拉长晶粒。超细晶TiNi合金组织较稳定,超细晶粒再结晶长大临界温度为550℃。室温轧制变形处理(累积变形量24%)使超细晶TiNi合金组织稳定性下降。透射电镜研究表明,富镍Ti-50.7.at%Ni合金在ECAE挤压前预热处理过程中析出Ti3Ni4相,但亚稳Ti3Ni4相在随后ECAE挤压过程中发生回溶。ECAE过程中引进大量位错缺陷提供富余Ni原子位置及中温Ni原子的热活性两个因素共同作用是导致Ti3Ni4相回溶的机理。Ti-50.7.at%Ni合金经ECAE处理后,B2?R相变被诱发,ECAE所诱发的B2?R相变开始温度(Rs)不随ECAE处理道次变化,而且高于同成分粗晶TiNi合金经500℃时效后的B2?R相变开始温度。B2?R相变发生在较宽的温度区间,DSC曲线上呈现馒头状峰。B2?R相变在超细晶粒中不是同时进行,在含有大量位错的超细晶粒内优先发生,不含位错的超细晶粒中不发生B2?R相变。ECAE处理富镍TiNi合金中B2?R相变由ECAE大变形引进内应力场所诱发。ECAE处理Ti-50.7.at%Ni合金马氏体相变(R?B19′)峰值温度(Mp)呈阶段性下降。1～2道次ECAE处理大变形累积使Mp温度急剧下降;3～8道次ECAE处理后,Mp温度下降速度减缓。Mp下降规律与ECAE过程累积变形量以及预热过程中发生回复密切相关。系统测试超细晶Ti-50.7.at%Ni合金室温超弹性结果表明,拉伸应变为1.5%,4道次ECAE处理超细晶TiNi合金应力-应变曲线呈现完全超弹性,10次加载-卸载循环中超弹性性能稳定,无残余应变;拉伸应变为4%,超细晶TiNi合金中残余应变随循环次数而增加但增大速度远低于固溶态粗晶TiNi合金,10次循环后超细晶TiNi合金累积残余应变仅为0.61%,低于粗晶TiNi合金的1.84%;拉伸应变为6%,超细晶和粗晶TiNi合金中残余应变随循环次数的增加速度几乎相同,10次循环后在超细晶TiNi合金和粗晶TiNi合金中分别有2.66%和2.75%的残余应变;拉伸应变增加到8%,4道次ECAE处理超细晶TiNi合金无室温超弹性。超细晶Ti-50.7.at%Ni合金室温完全超弹性最大应变量不超过4%。超细晶Ti-50.7.at%Ni合金形状记忆效应测试表明,4道次ECAE处理超细晶Ti-50.7.at%Ni合金在弯曲应变量<10%时呈现100%的单程形状记忆效应,相同应变量条件下,超细晶Ti-50.7.at%Ni合金记忆性能与固溶处理和500℃时效处理的粗晶Ti-50.7.at%Ni合金相同。更多还原

【Abstract】 Pure Ti is a promising material in biomedical fields, because of its better biocompatibility, resistant corrosion and without toxic element. But the application of pure Ti is limited for its low strength. In recent decade, improving its strength and refinement of its structure by Severe Plastic Deformation (SPD) has attracted much attention. Lots of investigations on microstructure and mechanical properties of ultrafine-grained (UFG) pure Ti and Ti alloys have been carried out. However phase transformation behavior and properties related to phase transformations of UFG materials are few reported. TiNi shape memory alloy is characterized by multiple phase transformations, superior shape memory effect and super-elasticity associated with forward or reverse martensitic transformations, one of functional materials that have been widely utilized now. Phase transformation behaviors, super-elasticity and shape memory effect of UFG TiNi alloy have attracted much attention recently.Commercial pure Ti (Grade 3) and Ni-rich Ti-50.7.at%Ni alloy were selected, and bulk UFG Ti and UFG TiNi alloy were prepared by Equal Channel Angular Extrusion (ECAE) at 400℃and 500℃, respectively. High strength UFG Ti was manufactured by two-step SPD method i.e, ECAE and Cold Rolling (CR) processes at Liquid Nitrogen Temperature (LNT).Microscopic analysis indicates that submicron-grained microstructure is obtained in Ti subjected to eight passes ECAE at 400℃, less than 500 nm in size. After eight passes ECAE and CR at LNT, dislocation cell structures with a size of 100 nm～150 nm are formed in UFG Ti, and obvious (0002) texture revealed in XRD results.Tensile test at room temperature indicates that after four passes ECAE, strain hardening stage on its true stress-strain curves of UFG Ti (Grade-3) with high mass fraction of impurities is prolonged, and is much larger than that of pure Ti (Grade 0～2). There are two-stage plastic deformation, as uniform deformation and plastic instability on the engineering stress-strain curve, and necking occurred in the second stage. Both ultimate strength and elongation of UFG Ti (Grade 3) after four passes ECAE are higher than those of UFG Ti (Grade 0～2) reported in international articles. After eight passes ECAE and CR at LNT, the ultimate strength of UFG Ti (Grade 3) is improved to 1218 MPa, with an elongation of 12.6%.Compressive tests at RT or LNT reveal that the dependency of flow stress on temperature or strain rate of UFG Ti after eight passes ECAE is lower than that of coarse-grained (CG) Ti. At the strain rate range of 1×10-3～1×10-1/s, the strain rate sensitivity (m) of UFG Ti after eight passes ECAE is 0.026, lower than 0.056 of CG Ti.Microstructure observation of UFG Ti-50.7.at%Ni alloy indicates that microstructure after eight passes ECAE at 500℃is inhomogeneous, many smaller grains with a size of 200 nm～300 nm and some elongated grains with a size of 100 nm～200 nm were observed. Microstructure of UFG TiNi alloy after eight passes ECAE is stable, and the critical temperature for grains growth is determined as 550℃. Further CR treatment at RT, with an accumulative strain of 24%, the structure stability of UFG TiNi alloy decreased.TEM observation reveals that Ti3Ni4 phase precipitated in Ni-rich Ti-50.7.at%Ni alloy during the preheating treatment before each ECAE pass, but metastable Ti3Ni4 phase re-dissolved during sequent ECAE processes. It is suggested that defects and dislocations induced by severe plastic deformation of ECAE process supplies position for surplus Ni atoms and relatively high thermal-activity of Ni atoms at mediate temperature result in the re-dissolution of Ti3Ni4 precipitate.B2?R transformation is induced by ECAE process in ECAE treated Ti-50.7.at%Ni alloy. The B2?R transformation starting temperature (Rs) keeps almost unchanged with the increase of ECAE pass number, and higher than that of coarse-grained (CG) TiNi specimen aged at 500℃for 1 hour. After ECAE processes, B2?R transformation with steamed-bread shape exothermal peak took place within a larger temperature range. The appearance of R phase was not accordant in all ultrafine grains. It is observed in some grains where there are lots of dislocations, in contrast, no R phase observed in the grains with few dislocations. It is suggested that B2?R transformation of ECAE processed Ni-rich TiNi alloy is induced by internal stress fields formed during ECAE processes.Martensitic transformation (R?B19′) peak temperature (Mp) of Ti-50.7.at%Ni alloy decreases with the pass number of ECAE process in two stages. After 1～2 passes ECAE, the Mp is dramatically lowered for the accumulation of strain. During 3～8 passes ECAE, the Mp decreases slowly with the pass number of ECAE process, which is closely related to accumulative deformation strain imposed by ECAE and recovery or dynamic recovery caused by preheating process before ECAE or during ECAE.Super-elasticity behavior test shows that at room temperature, when the tensile strain is 1.5%, UFG Ti-50.7.at%Ni alloy after four passes ECAE shows complete super-elasticity behavior and the super-elasticity keeps stable during loading-unloading cycling for 10 times, no residual strain observed. Tensile strain increases to 4%, the residual strain in UFG TiNi alloy increases with the loading-unloading cycling number, but the increase rate is much lower than that of CG TiNi alloy. After 10 cycles, the residual strain is only 0.61% for UFG TiNi alloy, but 1.84% for CG TiNi alloy. Tensile strain increases to 6%, the increase rate of residual strain with the cycling number is almost the same for UFG TiNi and CG TiNi alloy. After 10 cycles, the residual strains in UFG and CG TiNi alloy are 2.66% and 2.75%, respectively. When the tensile strain increases to 8%, UFG TiNi alloy reveals no super-elasticity behavior. The maximum super elastic strain of UFG Ti-50.7.at%Ni alloy at room temperature is less than 4%. Shape memory effect test shows that when the bending deformation strain is smaller than 10%, UFG Ti-50.7.at%Ni alloy after four passes ECAE showes 100% one-way shape memory effect. The shape memory property is comparable to that of CG Ti-50.7.at%Ni alloy solution-treated and aged at 500℃at a same prestrain.更多还原