【作者】 范承亮；

【导师】 杨冠军；

【作者基本信息】 西安建筑科技大学 ， 材料加工工程， 2004， 硕士

【摘要】 α钛合金在低温下具有比强度高、导热率低、热膨胀系数小、介质相容性好、抗氢脆等特点。这些性能特点使得α钛合金能很好的满足液体燃料储箱和低温管道的使用要求。研究的两种近α钛合金CT20和CT20A是西北有色金属研究院为航天应用的低温构件而设计的。开展显微组织以及间隙元素含量对近α钛合金低温性能影响的研究，对促进我国自创的低温钛合金系列材料在低温工程中的推广和应用具有重要意义。 为了研究显微组织和间隙元素含量变化对两种合金低温塑韧性的影响，研究中主要作了如下工作：对CT20合金管材进行了六种温度的退火处理，测试了不同组织试样的室温和20K拉伸性能，对拉伸试样取样进行SEM和TEM分析；制备出了CT20A合金三种氧当量实验样品，测试了相应试样的室温和20K拉伸性能以及室温和77K冲击性能，并取样进行SEM和TEM分析；在以上工作的基础上，对近α钛合金的低温塑韧性机理进行了探讨。 研究表明： CT20合金冷轧态管材通过热处理可以获得三种典型的组织类型，即等轴组织(T1、T2和T3制度)、双态组织(T4和T5制度)和片状组织(T6制度)；显微组织对CT20合金的室温性能影响不大，而对20K下性能的影响则较明显。20K下各组织强度显著升高，比室温时高约一倍，延伸率则明显下降，片状组织的延伸率最好。CT20合金室温拉伸断口以塑性韧窝为主要特征，20K拉伸时断口上出现了一些孔洞和解理台阶，表现出一定的低温脆断特征。CT20合金20K拉伸变形时，等轴组织主要是位错的滑移起主导作用，双态组织除了位错滑移外还有少量的变形孪晶产生，片状组织中则能观察到数量较多的孪晶。与组织中变形孪晶增多相对应，合金20K的塑性增加。西安建筑科技大学硕士学位论文 间隙元素含量变化(氧当量变化为0.15%一0.23%)对CT20A合金的显微组织没有明显的影响，但却强烈影响合金在低温下的性能。随间隙元素含量增加合金强度升高而塑性降低，低间隙元素含量的合金在低温下有较好的塑韧性。氧当量为0.15%的CTZoA合金试样拉伸断口显示为大量韧窝，合金塑性较好;氧当量为0.23%的试样断口上韧窝大小不均，出现局部的准解理特征，合金塑性较差。随氧当量的增加，合金的冲击韧性下降，77K下尤为显著。CT20A合金低氧当量拉伸试样的TEM组织中有相当数量的{11几}型孪晶存在，孪晶在形貌上表现为穿过几个片的细条。高氧当量试样中没有观察到变形孪晶的存在。 近以钦合金的室温变形主要由位错滑移控制，20K下的变形则由位错滑移和孪生变形共同控制。在本文研究的CT20合金低温下变形的组织中观察到了三种类型的孪晶，即{一l丁l}、{20了2}和{一l玄2}型孪晶;eTZoA合金中目前只观察到了{11丁2}型孪晶。不同类型的形变孪晶对近a钦合金低温下变形的作用分为直接作用和间接作用两方面。直接作用主要是主要表现在:孪生变形自身能产生一定的应变，对增加塑性有一定贡献，但这种作用对合金塑性贡献较小，当孪生变形能够普遍发生时其作用也是不容忽视的;孪生变形对合金低温变形的贡献主要表现在其间接作用方面，即:①调整晶粒取向，缓解应力集中，延缓裂纹的萌生;②促进应变滑移，改变变形局部化倾向，使变形变得较均匀。更多还原

【Abstract】 a titanium alloys possess characteristics such as high specific strength, low thermal conductivity, good compatibility to medium and good resistant to hydrogen embrittlement, which makes a titanium alloy suitable for the use in liquid fuel tank and tube at cryogenic temperature. The two near a titanium alloys CT20 andCT20A, developed by NIN, are for application of structural parts at cryogenic temperature. It is meaningful toe arry o ut s tudy one ffect o f microstructure and interstitial c ontent on cryogenic properties of these two near a titanium alloys and will promote the application of newly developed alloys in cryogenic engineering.In order to accomplish the main targets, the following jobs were done: the annealing of CT20 alloy tube at 6 different temperatures was carried out and the specimens with different microstructure were tested at room temperature (RT) and 20K. The tensile fractures were analyzed by means of SEM and TEM. The C T20A alloy specimens with 3 kind of Oxygen Equivalent (Oeq) were prepared, and the tensile properties at RT and 20K, the impact toughness at RT and 77K were tested. SEM and TEM observation were carried out on CT20A alloy too. At last, the deformation mechanism at cryogenic temperature was probed on the basis of the work done.The studying results indicated that:Three typical microstructures were abtained when CT20 alloy were annealed under different temperature, that is, equiaxed microstructure (annealed at T1, T2 and T3), bimodal microstructure (annealed at T4 and T5) and lamellar microstructure (annealed at T6). The effect of microstructure on RT tensile properties was weak, while at 20K was notable. At 20K, the strength of CT20 alloy with various microstructuresincreased while elongation obviously dropped down, and the lamellar microstructure had the best elongation. The fracture of CT20 alloy was of plastic dimples character at RT, while some holes and cleavage appeared at 20K, showing some brittle feature. When CT20 alloy was tensioned at 20K, the main deformation mechanism in equiaxed microstructure was the slip of dislocations, and in bimodal microstructure the slip of dislocations and a small amount of twinning and in lamellar microstructure a large amount of twinning. Corresponding to the amount rising of twins, the ductility at cryogenic temperature of CT20 alloy increased.The changing of interstitial content (Oeq varies from 0.15% to 0.23%) had no obvious influence on the microstructure of CT20A alloy, while the properties at 20K were strongly influenced. With interstitial content increasing, the strength of CT20A alloy increased while the elongation dropped down. The alloy with low interstitial content had better ductility and toughness. The fracture of CT20A alloy, whose Oeq is 0.15%, was occupied by dimples, showing good ductility; while the distribution and size of dimples in the fracture of alloy with Oeq 0.23% were not uniform and localized quasi-cleavage feature was found, indicting bad ductility and toughness. With Oeq increasing, the impact toughness of CT20A alloy decreased rapidly, especially at 77K. Quite notable {11 22 } twins with the appearance of strip piercing through lamellar colony were found in the tensile microstructure of CT20A alloy by TEM analyzing, while no deformation twinning was found in the microstructure of alloy with high Oeq.The deformation of near a titanium alloy at RT is controlled by dislocation slip while at 20K is under the mutual control of dislocation slip and twinning. In this investigation, three types of twins were observed in the deformation microstructures of CT20 alloy at cryogenic temperature. That is, twins of {11 2~l }, {10 T2 } and {li 22 } type. While only {11 2~2 } type twin was found in CT20A alloy. The role that twinning plays in the deformation of near a titanium alloy at cryogenic temperature can be divided into two parts, that is, direct part and indirect part. The direct part is, twinning itself can bring some deformation, which is beneficial to the improvement of plasticity. While this con更多还原