【作者】 马李；

【导师】 赫晓东；

【作者基本信息】 哈尔滨工业大学 ， 材料学， 2008， 博士

【摘要】 金属热防护系统(TPS)是可重复使用运载器(RLV)的关键技术之一;由于直接与外部的环境接触,金属TPS面板材料的研究成为重中之重。按照能够替代Ni基高温合金薄板的轻量化选材要求,本文采用了电子束物理气相沉积(EB-PVD)技术成功地制备了0.1~0.3mm厚、尺寸为直径为1000mm的圆形Ti/Ti-Al微叠层薄板。文中不仅对EB-PVD的工艺特点进行了考察,同时还借助一些现代分析测试手段研究了微叠层材料的组织结构和宏观力学性能,为材料安全、可靠地使用提供了理论依据。研究内容主要包括:Ti/Ti-Al微叠层薄板材料结构参数的优化设计、微叠层薄板的残余应力分析、工艺参数对材料蒸发和沉积过程的影响、制备工艺方法、微叠层材料组织结构特点及物相组成、微观组织在高温下的演变及层状结构的退化、宏观力学性能及其变形特征、应力对材料高温蠕变性能的影响。利用三点弯曲模拟试验考察了微观结构参数变化对材料断裂性能的影响。结果表明,随着材料层数或层厚比的增加,材料的断裂功随之增加,但增加的趋势逐渐减小,当二者超过一定数值时,断裂功不再发生明显变化。利用材料力学相关理论和有限元方法考察了材料在成型过程中形成的残余应力。结果表明,微层板的残余应力是位置的函数,在叠层板中心处的轴向残余应力值最大,在边缘处的值最小;Ti层所受的都是残余压应力,TiAl层受到的是残余拉应力;随着层厚比或层数的增加,Ti所受的残余压应力逐渐增大,TiAl层所受的残余拉应力逐渐减小,且变化趋势越来越小。利用X射线衍射法测量了微层板最外两层的残余应力随距离基板中心的变化,结果表明其变化趋势与理论预测结果相一致。讨论了工艺参数对制备Ti/Ti-Al微叠层材料工艺过程的影响及可行性。根据靶基距与碰撞几率的函数关系,并综合考虑工艺要求与靶材利用率,将最终靶基距值为310mm;推导了沉积材料中Ti和Al元素的含量的计算公式;试验表明在熔池中加入铌片后,材料蒸发达到稳态的时间明显缩短,达到稳态后的成分与靶材成分更为接近;对沉积过程中Al的再蒸发量和沉积量的比值进行了计算,结果表明可以忽略Al的二次蒸发对材料成分变化的影响。Ti/Ti-Al微层板表面质量良好,不同尺度下的AFM形貌表明样品表面层在一定尺度范围内具有较明显的生长动力学过程导致的分形特征。截面的调制结构十分清晰,具有完整的层间镶嵌式界面;沉积Ti、Al元素的T/Tm(Tm为沉积材料的熔点)不同导致了组元层有明显的结构差别,其中Ti-Al层为以等轴晶为主、柱状晶与等轴晶混合型结构为特征;而Ti层主要为柱状晶构成;材料经热压致密化处理后,孔隙明显减少,致密度由94.91%提高到了98.07%。Ti、Al元素的饱和蒸气压的差异导致了Ti-Al镀层成分波动,进而造成了Ti-Al镀层成分的周期变化,并形成多个亚层。构成Ti/Ti-Al微层板的Ti层、Ti-Al层以及界面区分别由α相,γ+α2相、α2相构成。高温退火试验表明,Al在高温下向Ti中的扩散,导致了有序相含量的降低;根据Fick第二定律推导了Ti层中Al元素的浓度分布与扩散温度、时间以及Ti层厚度之间的关系方程,可求得不同温度退火不同时间后Al在Ti层内的浓度梯度分布。微观结构演化表明层状结构的退化受到孔洞形成及长大、晶粒的长大以及Ti层被Ti-Al层夹断过程的影响。纳米压痕试验结果表明界面区的硬度最高,Ti-Al层其次,Ti层最低,且硬度随着距界面区的距离改变呈现梯度变化,分析认为pile-up效应和界面处的应力集中是造成上述现象的原因。添加金属韧化层对TiAl基合金的常高温力学性能均有改善,且热压致密化处理后的材料在性能上有了很大的提高。层间距和层厚比的改变分别通过沉积工艺特点和复合材料的混合律对材料的力学性能有所影响。微层板中的裂纹多次沿着层间界面或层中拐折,表现出良好的断裂延迟特性,其增韧机制则为韧化层的存在导致裂纹发生偏转、微桥接等使裂纹扩展阻力增加。考察了650℃时不同应力水平下对Ti/Ti-Al微层板蠕变性能的影响,给出了能够描述减速蠕变阶段和稳态蠕变阶段的本构模型。稳态蠕变阶段的蠕变应力指数n从应力为60～70MPa之间的1.53变到了应力为70～100MPa之间的7.66,表明随着应力的增加,蠕变机制由高密度界面滑移到位错攀移控制的回复蠕变机制的改变。更多还原

【Abstract】 Metal thermal protection system (TPS) is one of the key techniques of the reusable launch vehicle (RLV). Since contacting with the external environment directly, the research on materials of metal TPS sheet is the most important one. To choose the lightweight materials that can take place of the Ni-based superalloys sheet, Ti/Ti-Al microlaminated round sheets with a diameter of 1000mm as well as thickness of 0.1~0.3mm are fabricated successfully by electron beam-physical vapor deposition (EB-PVD) in this paper. Not only the preparation process and characteristics of EB-PVD are discussed but also the microstructure and properties of the alloys are studied by modern analysis and test methods, which establishes the base for the engineeringl application of Ti/Ti-Al microlaminated sheet. The main contents of the study include: the optimum design of the microstructure parameter for Ti/Ti-Al microlaminated sheet, residual stresss analysis of for Ti/Ti-Al microlaminated, the effect of processing parameters on the processs of evaporation and deposition, preparation method of the Ti/Ti-Al microlaminated sheet, as well as the microstructure and phase composition, the evolution and collapse of microstructure under high temperature, and the impact of stresss on the creep deformation at high temperature for the microlaminate.The effect of microstructure parameters on failure performance of microlaminate was studied by simulation of three-point bending test. The results showed that the fracture work increases gradually with the increasing of layer number or thickness ratio. However, the extent of increasement reduced gradually and when the two parameters exceeded a certain value, the fracture work would not be changed.Correlative theory in materiall mechanics and FE method were used to study the residual stress formed during the preparation processs of microlaminate sheet. It was demonstrated that the residual stresss is a function of the locateon along radial direction of microlaminate sheet. It was pointed out that the value of residual stress reached its maximum at the centure and reached its minimum at the edge of microlaminated sheet. Residual stresss in Ti layers and Ti-Al layers was compressive stress and tension stress respectively. With increasing of layer number or thickness ratio, compressive stress would rise gradually and compressive stress would reduce gradually. X ray diffraction was used to determine residual stress distribution of outmost two layers of deposited microlaminated sheet. Results showed that residual stress decreases gradually along the radius from center to edge of sheet metal. The trend of residual stress of actual measurement is similar to that gained with numerical method.The influence and the feasibility of processing parameters on the preparation technology have been discussed. According to the function between target-substrate distance and the collision probability, combined with the processs requirements and utilization ratio of target materials, the optimal source-substrate distance was fixed as 310mm. The effect of saturation vapor pressure on the evaporation process of Ti-Al alloy source was discussed, and the experimental result showed that the addition of Nb into molten pool make it earlier to reach the steady-state compare to that without addition of Nb, and the composition of deposit with Nb addition at steady-state was much closer to that of source materials. From the calculation result of the ratio of re-evaporating capacity with depositing capacity of Al on the substrate, it can be concluded that the effect on deposit by re-evaporation of Al could be neglected.The surface quantity of Ti/Ti-Al microlaminated sheet is well. AFM patterns of outer layer at different scales indicate a fractal characteristic due to growing kinetics. The modulation architecture and inlaid interlaminar interfaces can be seen from the cross-section of microlaminate. The difference of melting points between Ti and Al led to the distinctive structure: the Ti-Al layers were mainly constituted of equiaxed grains and Ti layers were constituted of column grains. The porosity decreased obviously when the as-deposited materials were densificated by hot-pressed technology and the density has been increased from 94.91% to 98.07%.Because of the deviation of saturated vapor pressure between Ti and Al element, the component showed a gradient change periodically along the normal direction of Ti-Al layers and results in several sub-layers. The Ti layers, Ti-Al layers and interfacial layers were constituted ofαphase,γ+α2 phase andα2 phase respectively. The test results of high temperature annealing showed that the ordered phase reduced due to the diffusion of Al into Ti. Based on the Fick’s second law, the function between the concentrateion of Al in Ti with diffuseion temperature, time and the thickness of Ti is estabilished from which the concentration distribution of Al in Ti according to different temperature and time. Research results showed that the break down of layered architecture was induced by pore formation, grain growth and the grain boundary grooving.It was found that the nanohardness and elastic modulus of Ti layer and Ti-Al layer are of gradient distribution according to the distance to the interface layer which may be results from the pile-up effect and the stress concentrateion at interface.The addition of Ti ductileity layer can improve the mechanical properties of Ti-Al alloy at room and high temperature and hot-pressed densificated technology is necessary to get a finer performance. The change of interlaminar space and the thickness ratio will affect the mechanical properties by means of deposition characteristic and mixed rule of composite respectively. The cracks will stagger along the interlaminar interface or the layer due to which microlaminate expresses a good characteristic of delayed fracture. The toughening mechanisms are that the crack deflection and micro-bridge connection caused by the toughening layers increases crack propagation resistance.Finally, the impact of vared stresses on the creep property for Ti/Ti-Al microlaminate at 650℃was investigated and the constituteive equations which could be used to describe the slowing creep stage and the steady creep stage was obtained. During the steady creep stage, the creep stresss exponent changed from 1.53 under a stresss of 60～70MPa to 7.66 in the stress range of 70～100MPa, it indicated that with the increasement of stress, the creep mechanism will transmited from the interface slide to the recovery creep controlled by dislocateion climbing.更多还原