【作者】 袁宝国；

【导师】 李春峰； 孙东立；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2010， 博士

【摘要】 钛及其合金具有比强度高、高温性能好以及防腐蚀能力强等一系列优异特性,在航空航天、船舶、海洋等领域得到了广泛的应用,是一种理想的金属结构材料。然而,大部分钛合金在室温下塑性低,冷成形容易开裂,限制了钛合金的冷态工艺性。由于室温塑性成形所生产的零件机械性能好、精度高、表面质量好、生产效率高,是制造钛合金零件的最经济手段。所以,有必要进一步开展钛合金室温塑性成形技术的基础研究工作,尽快实现钛合金室温塑性成形技术的应用。钛合金热氢处理技术可以改善钛合金的力学性能,但是氢对钛合金室温塑性变形行为的影响及其机理一直缺乏系统的研究,本文主要针对这个问题进行了深入研究。利用光学金相显微镜、X射线衍射仪、扫描电子显微镜、透射电子显微镜等材料分析设备研究了氢含量对Ti–6Al–4V合金微观组织的影响,并对置氢Ti–6Al–4V合金室温压缩变形过程中微观组织的演变进行了研究。结果表明:置氢后,Ti–6Al–4V合金中发现马氏体和氢化物等相。随着氢含量的增加,β相、α’’马氏体以及氢化物等相的含量逐渐增加,且氢化物优先沿晶界或相界析出。氢促进了位错的增殖。利用INSTRON–5569型材料试验机对合金进行了室温拉伸试验,研究了氢含量对Ti–6Al–4V合金室温拉伸性能的影响。结果表明:置氢后,合金的室温拉伸性能逐渐恶化,表明置氢不利于钛合金的拉伸变形。为了揭示氢对Ti–6Al–4V合金拉伸断裂行为的影响及机理,本文利用原位拉伸试验对合金拉伸变形过程中裂纹的萌生、扩展及断裂的全过程进行实时观察和录像,并利用有限元模拟技术对合金的拉伸过程进行了模拟,分析了拉伸变形过程中合金的应力应变分布规律。利用ZWICK/Z100型材料试验机对合金进行了室温静态压缩试验,并利用电磁成形机对合金进行了高速压缩试验,系统研究了氢含量和应变速率对合金室温压缩性能的影响。结果表明:Ti–6Al–4V合金无论是在静态下压缩变形,还是在高速率下压缩变形,合适的氢含量对Ti–6Al–4V合金的压缩性能均存在有益的影响,可以显著提高合金的极限变形率(极限变形率的最大增幅达56.3%),降低对成形设备及模具的要求。揭示了置氢钛合金室温塑性变形的改性机理,并建立了理论模型。氢对钛合金室温拉伸和压缩性能的不同影响规律是由合金的氢含量及其受力状态决定的。氢含量的影响可分为由固溶态氢和氢致相变引起的影响。拉应力会加速晶界处裂纹的萌生和扩展,并促进晶间变形。随着氢含量的增加,晶界或相界处氢化物的含量逐渐增加,导致晶界或相界逐渐变弱,使其力学性能逐渐恶化。而压应力可以抑制或削弱裂纹的萌生和扩展,减少合金的晶间变形。当氢含量较低时,固溶态氢和氢化物对压缩性能影响较小,影响压缩性能的主要因素是晶内变形,塑性β相含量的增加导致合金的塑性提高。随着氢含量的增加,氢化物的含量逐渐增加,并聚集于晶界处,导致晶间变形的作用逐渐增加,使合金的塑性下降,此时脆性的氢化物相对合金的压缩性能所起的作用逐渐增强。根据热重试验结果制定了置氢Ti–6Al–4V合金的除氢规范,对置氢Ti–6Al–4V合金进行了真空除氢处理,并对除氢合金的微观组织及室温力学性能等进行了研究。结果表明:除氢过程中,置氢Ti–6Al–4V合金中亚稳定相发生分解,转变为稳定的α相和β相,固溶氢及氢化物中的氢完全从合金中逸出,导致组织细化,但是晶粒形貌没有恢复,导致合金的室温力学性能有所恢复,但没有完全恢复。利用M–200型摩擦磨损试验机于室温大气中对合金进行干滑动摩擦磨损试验,以研究氢对Ti–6Al–4V合金摩擦磨损性能的影响。利用SEM及其EDS等材料分析方法对磨损试验后的销试样及对磨盘的微观形貌和成分进行了观察和分析,以揭示合金的磨损机理。结果表明:置氢后,合金的磨损率变大。除氢合金的磨损率低于相应的置氢合金的磨损率,但仍高于未置氢合金的磨损率。合金的磨损率是由合金的性质(主要是合金的硬度以及热传导率)决定的。未置氢合金主要呈现氧化磨损特征,置氢合金的磨损以磨粒磨损为主,除氢合金的磨损机理主要是氧化磨损和磨粒磨损。结果表明当除氢合金应用于摩擦磨损领域时,仍需进行表面处理,以提高其抗磨性。根据本文的试验结果,制定了置氢Ti–6Al–4V合金的最佳室温塑性成形条件。当利用热氢处理技术使Ti–6Al–4V合金室温塑性成形时,应选择在压应力的作用下塑性成形的方法,而不是拉应力。在本文的试验条件下,当置氢Ti–6Al–4V合金在静态下室温压缩成形时,合金的最佳氢含量为0.6wt.%~0.8wt.%。当利用磁脉冲成形等高速率成形方法时,合金的最佳氢含量为0.1wt.%,放电电压为1.1kV。更多还原

【Abstract】 Titanium and its alloys are ideal structural materials and widely used in aviation, aerospace, marine and ocean industries because of their specific strength, good hot workability and good corrosion resistance. However, their plasticity is low at room temperature and they are easy to crack during their cold deformation, which restrict their applications. It need study further about the fundamental research on cold deformation of titanium alloys and its application, because cold deformation is one of the most economic methods to form titanium alloys, and the parts deformed at room temperature have good mechanical properties, high accuracy, good surface quality and high efficient. Thermohydrogen processing (THP) can enhance the mechanical properties of titanium alloys. However, the influence of hydrogen on the deformation behavior of titanium alloys at room temperature and its mechanism still lack systematic study. This paper systematically studied the problem.The effects of hydrogen content on microstructural evolution of Ti–6Al–4V alloy were investigated by optical microscope (OM), X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM). Microstructural evolution of hydrogenated Ti–6Al–4V alloy during compressive deformation was also studied. Results show that martensites and hydride appear after hydrogenation. The amounts ofβphase,α’’ martensite and hydride phase increase with an increase in the hydrogen content, and hydride prefers to precipitate along grain/phase boundaries. Hydrogen can promote the increment of dislocation.Tensile tests were carried out at room temperature through INSTRON-5569 matrials tester to study the influence of hydrogen content on the tensile properties of Ti–6Al–4V alloy at room temperature. Results show that hydrogen deteriorates the tensile properties, indicating hydrogen has disadvantage on the tensile deformation of titanium alloys at room temperature. In order to investigate the influence of hydrogen on fracture behavior of Ti–6Al–4V alloy and its mechanism, the whole process of crack initiation, propagation and failure during tensile deformation was observed and recorded in real time by in-situ tensile tests, and the distributions of stress and strain during tensile deformation were analysed through finite element method (FEM).In order to study the influence of hydrogen content and strain rate on the compressive properties of Ti–6Al–4V alloy at room temperature, compressive tests were carried out at room temperature through ZWICK-Z100 matrials tester and electromagnetic forming (EMF). Results show that hydrogen has favorable effets on the compressive proerties of Ti–6Al–4V alloy, can enhance the ultimate compression (the maximum increases 56.3%) under quasi-static compression and EMF, and can reduce the demand for equipment and die.The modification mechanisms about the effects of hydrogen content on the tensile and compressive properties were disclosured, and its model was built. The dissimilar effects of hydrogen content on the tensile and compressive properties are caused by the hydrogen content and stress state. The effects of hydrogen content include hydrogen in solid solution and hydrogen-induced phase transformation. As hydrogen content increases, the tensile properties decrease gradually. Intergranular deformation dominates at the tensile state, which is caused by the increased hydrogen atoms in solid solution and hydrides at grain/phase boundaries. While at the compressive state, intragranular deformation dominates at lower hydrogen content. The increased amount of plasticβphase improves the ultimate compression with the increasing hydrogen content. The intergranular deformation plays an increasing role during compressive deformation with the increasing hydrogen content because of the increased amounts of hydrogen atoms in solid solution and hydrides and leads to the degradation of compressive properties.The dehydrogenation procedure was determined according to the results of TG test of hydrogenated Ti–6Al–4V alloys. The hydrogenated Ti–6Al–4V alloys were dehydrogenated, and their microstructure and mechanical properties at room temperature were studied. Results show that the metastable phases decompose to stableαandβphases during the procedure of dehydrogenation, hydrogen in solid solution and hydride are removed, leading to the refinement of microstructures, but the grain can not be refined because of the heredity of titanium alloys. The mechanical properties can be restored, but can not be fully restored after dehydrogenation.The dry sliding wear properties of non-hydrogenated, hydrogenated and dehydrogenated Ti–6Al–4V alloys sliding against GCr15 steel were investigated using an M-200 pin-on-disk tribometer in air at room temperature. The wear mechanisms were investigated by studying the morphology and chemical element of pins and steel using SEM and EDS. Results show that wear rate increases after hydrogenation. Wear rates of dehydrogenated Ti–6Al–4V alloys are higher than those of non-hydrogenated Ti–6Al–4V alloys, although they are lower than those of hydrogenated Ti–6Al–4V alloys. The wear rates are attributed to their hardness and thermoconductivity. The non-hydrogenated Ti–6Al–4V alloy is controlled by oxidative mechanism, hydrogenated Ti–6Al–4V alloys by abrasive mechanism, and dehydrogenated Ti–6Al–4V alloys by oxidative and abrasive mechanisms. Results indicate that the dehydrogenated Ti–6Al–4V alloys should be treated to increase abrasion resistance before they are used.The optimal hydrogen content was determined for the cold deformation of hydrogenated Ti–6Al–4V alloys according to the experimental results. The alloy should be formed under compressive stress when hydrogen is applied on its cold deformation, while not under tensile sress. The optimum hydrogen content is the range of 0.6 wt.%~0.8 wt.% when the alloy is deformed under quasi-static compression. While the optimal hydrogen content is 0.1 wt.% when deformed under EMF, and the discharging voltage is 1.1 kV.更多还原