【作者】 宋广平；

【导师】 赫晓东；

【作者基本信息】 哈尔滨工业大学 ， 材料学， 2009， 博士

【摘要】 金属热防护系统是可重复使用飞行器的关键技术之一。作为未来新一代高速飞行器的主要防热部件,与陶瓷瓦相比,金属热防护系统具有高的强度、韧性、高可靠性、良好的抗热震性能及耐潮湿,且同时具备良好的工艺性能,便于制备较大尺寸的结构件等优势而成为本领域的研究热点。本文中采用电子束物理气相沉积技术成功制备出大尺寸、高强度、高韧性和具有良好焊接性能的镍基高温合金薄板。采用X射线衍射（XRD）、扫描电镜（SEM）、原子力显微镜（AFM）和透射电子显微镜（TEM）以及万能拉伸实验机和高温蠕变实验机等实验设备和测试手段深入地研究了制备态和热处理态镍基合金薄板的组织结构和力学性能;研究了基板温度、转速对沉积材料组织性能和致密性的影响以及锭料蒸发速率比和靶基距对沉积薄板厚度分布和质量蒸发效率的影响,并建立了厚度分布数学模型;制备态和热处理态镍基合金薄板的高温氧化行为。研究发现基板温度700℃时（Ts/Tm=0.6）,沉积薄板组织为细小的等轴晶组织,具有很好韧性;基板温度450℃时（Ts/Tm=0.45）,沉积薄板组织为柱状晶组织,材料表现为明显的脆性。在基板温度700℃时,基板转速选择为5rpm、12rpm和30rpm,实验结果表明随基板转速增加,沉积材料的致密性有所的降低,但下降程度很小。薄板厚度分布的均匀性是决定沉积合金薄板性能的一个重要指标,均匀性的好坏将直接影响到薄板使用性能。根据真空蒸镀中小平面蒸发源理论和EB-PVD工艺特点,建立了在基板旋转时靶基距和锭料蒸发速率比与厚度分布关系的数学预测模型,厚度分布符合cosnθ规律,其中n=5.3,模型与实际厚度分布符合很好。预测模型显示,随靶基距的降低,薄板的厚度分布不均匀性增大;通过调整两个蒸发源的蒸发速率比,可以改变薄板沿半径方向厚度分布的均匀性,根据实际需要从而获得最佳的厚度分布。当靶基距一定时,随着4号坩埚锭料蒸发量占两个坩埚总蒸发量百分比的增加,质量蒸发效率线性增加;当两个坩埚蒸发量占总蒸发量的比值一定时,随着靶基距的降低,质量蒸发效率单调增加。优化沉积工艺参数后获得的制备态合金薄板为等轴晶组织,晶粒尺寸从几百纳米到1²微米不等,晶粒生长没有明显的择优取向,XRD衍射分析表明合金为单一的镍基固溶体。为了获得好的高温力学性能,研究了合金薄板的后续热处理工艺,确定处理工艺为1020℃固溶0.5h,水冷;760℃时效48h,空冷。时效热处理后,合金薄板晶粒发生长大,晶粒尺寸在4⁵μm左右,同时在晶界处有尺寸为纳米级（20⁵0nm）细小的碳化物析出,其成分为（Cr,Fe）23C6,晶体结构为面心立方（FCC）。时效热处理后合金薄板试样的高温拉伸力学性能有了显著的提高,制备态合金薄板800℃时的抗拉强度为64MPa,而时效热处理合金薄板试样800℃时的抗拉强度达到了275MPa。制备态镍基合金薄板800℃恒温氧化结果表明,在氧化初期合金薄板表面首先生成Cr2O3氧化物,随着氧化时间的增加,内氧化物Al2O3逐渐在基体和Cr2O3界面处生成,并在横向方向上不断长大而形成准连续的内氧化层。900℃恒温氧化制备态合金薄板表面首先生成的为Cr2O3氧化物层,氧化96h后,基体和Cr2O3层内侧有内氧化现象发生,内氧化物颗粒成分为Al2O3,但在界面处没有形成连续的内氧化层。1000℃恒温氧化初期制备态合金薄板表面首先生成的氧化物为Cr2O3和TiO₂颗粒。随着氧化时间的增长,处于外表面层的TiO₂颗粒快速长大,氧化100h后,外层TiO₂颗粒逐渐相连,近似形成连续的TiO₂外氧化层。内层为Cr2O3氧化物层,同时在Cr2O3层内侧基体内,有内氧化现象发生,内氧化物为Al2O3。制备态镍基合金薄板800℃恒温氧化动力学曲线遵循立方规律。由于在氧化过程中其氧化物晶粒尺寸长大缓慢,晶界体积分数很高,因此氧化膜生长的主要方式是金属离子通过晶界扩散（短路扩散）,理论推导出其三次方规律,理论与实验结果相符合。制备态镍基合金薄板900℃和1000℃恒温氧化动力学曲线遵循抛物线规律。时效热处理试样800℃恒温氧化初期首先生成Cr2O3氧化物,随着氧化时间的增长,在氧化层内部与基体之间,发生内氧化现象,在Cr2O3氧化层与基体界面处有Al2O3氧化物生成,但界面处Al2O3氧化物没有形成连续的内氧化物层。基体内部也有少量内氧化现象发生。其恒温氧化动力学遵循抛物线规律。更多还原

【Abstract】 Metallic thermal protection system （MTPS） is one of the key techniques for developing reusable launch vehicle （RLV）. As a new generation key thermal protection component, MTPS has high strength and toughness, high reliability, excellent thermal shock resistance and moisture resistance compared with ceramic tile. At the same time it also has advantages of good processing propoties and producing large size component easily. Therefore, MPTS has become a research focus in this area. In this paper, large-scale Ni-based alloy sheets are fabircatied by electron team physical vapor deposition （EB-PVD）, with high strength and toughness, good jointing propoties. Their microstructure and mechanical properties are systematically investigated by means of X-ray diffactometry （XRD）, sanning electron microscopy （SEM）, atom force microscopy （AFM） and transmission electron microscopy （TEM） as well as mechanical test method. Effection on temperature and rotational speed of the substrate on microstructure and density is studed. Effection of ingots evaporation speed ratio and source-to-substrate distance on sheet thickness distribution and mass evaporation efficiency is studied and the mathematic model of thickness distribution is presented. In addition, high temperature oxidation behaviors of as-deposited and annealed Ni-based alloys are studied.In this paper, the Ni-based alloys were deposited at the substrate temperature 700℃and 450℃. The results showed that the as-deposited sheet with good toughness consisted of equiaxed grains at 700℃and with brittleness cnsisted of columnar microstucture at 450℃. The sheets were fabricated at 700℃with substrate rotational speed 5rpm, 12rpm and 30rpm. The results showed that density of the sheets decredsed with rotational speed increase, but a small decline in the extent.The thickness distribution uniformity is an important target which determines the service performance of Ni-based alloy sheet. According to EB-PVD process feature, a mathematical model of thickness distribution of the sheet deposited on rotary substrate surface is presented by taking the effection of ingots evaporation speed ratio and source-to-substrate distance on sheet thickness distribution uniformity into consideration. The theory of small plate evaporation source under the condition of vacuum evaporation is used in the predicted mathematical model. The predicted model follows cosnθlaw and agrees well with the measured data when n = 5.3. The model shows that the thickness distribution uniformity is worse with decreasing source-to-substrate distance. Thickness distribution can be adjusted through changing ingots evaporation speed ratio according to factual requirement. When source-to-substrate distance is not changed, mass evaporation efficiency increases linearly with increasing evaporaiton speed ratio of the ingot in No.4 crucible. When ingots evaporaiton speed ration is invariable, however, mass evaporation efficiency decreases linearly with decreasing source-to-substrate distance.The study results indicated that as-deposited Ni-based alloys consisted of equiaxis grains. The grain size varies from several nanometers to 1²μm. As-deposited Ni-based alloys sheet consisted of single Ni-based solution and exhibited no texture. The heat treatment experiments were planned to confirm standard heat treatment regime of solution heat treatment at 1020℃for 0.5h, water quenching and aging treatment at 760℃for 48h, air-colled. After standard heat treatment, the grains of alloy sheet grew up to 4⁵μm and fine carbide particles with 20⁵0nm in size separated out on grain boundary. The carbide composition was （Cr,Fe）23C6, whose crystal structure was face-centered cubic （FCC）. High temperature tensile strength of the heat treatment samples was improved obviously, such as from 64MPa for as-deposited samples to 275MPa for heat treatment samples at 800℃.The oxidation results show that oxide Cr2O3 is formed on the surface of as-deposited Ni-base alloy sheet firstly at 800℃. Secondly, inside oxide Al2O3 paticals are formed on the interface between matrix and Cr2O3 sacle. Lastly, oxide Al2O3 paticals grow up in landscape orientation and form Quasi continuous inside oxide Al2O3 scale. The oxide Cr2O3 scale is formed when as-deposited Ni-base alloy sheet is oxidezed at 900℃for 96h. There are inside oxide Al2O3 paticals formed on the interface and in matrix, but no Quasi continuous inside oxide Al2O3 scale is formed. TiO₂ and Cr2O3 are formed on the surface of the alloy sheet at 1000℃firstly. The oxide TiO₂ particles grow up shaply with oxidation time increasing. After 100h, the outer oxide TiO₂ forms Quasi continuous oxide scale. The oxide below TiO₂ is Cr2O3 scale. There are inside oxide formed in matrix, whose composition is Al2O3.The oxidation kinetics of as-deposited Ni-based alloy sheet at 800℃follows cubic law. The oxide grains grow up slowly at 800℃, so volume fraction of grain boundary is high. Therefore, the diffusion of elements through grain boundaries plays an important role in the oxidation scale growth on the Ni-based alloy sheet surface. A cubic law is derived and agrees well with the experiment data. The oxidation kinetics of as-deposited Ni-based alloy sheet at 900℃and 1000℃follows a parabolic power law.The oxide Cr2O3 is formed on the surface of aging heat treatment sample at the beginning of oxidation for 800℃. There are inside oxide formed on the interface between Cr2O3 scale and matrix and in matrix. After 100h, Quasi continuous inside oxide Al2O3 scale is not formed. The oxidation kinetics of heat treatment samples at 800℃follows a parabolic power law.更多还原