【作者】 宋晓国；

【导师】 冯吉才； 曹健；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2012， 博士

【摘要】 Si₃N₄陶瓷以其优异的力学性能及良好的透波性能成为下一代天线罩材料的首选，在天线罩装配和服役过程中，需实现罩体与金属的连接。TiAl合金具有高比强度和比刚度，良好的抗氧化性以及优异的高温力学性能。实现二者的连接制成应用于航空航天、武器装备等领域的构件，不仅可以减重而且能够提高服役温度。本课题旨在实现TiAl合金与Si₃N₄陶瓷的钎焊连接，通过试验确定了合适的钎料体系；基于界面反应产物的表征分析了钎焊过程中钎料与母材之间的反应，深入探讨了钎焊连接机理。根据复合强化理论，设计了纳米Si₃N₄颗粒增强的AgCuTi复合钎料，并采用该钎料实现了TiAl合金和Si₃N₄陶瓷的连接。通过界面结构的分析解明了钎焊过程中纳米颗粒的反应及弥散机理。采用有限元模拟及X射线应力分析的方法，对接头残余应力的分布进行了分析。开发了TiNiNb和TiNiV两种TiNi基高温共晶钎料，并采用这两种钎料围绕TiAl合金和Si₃N₄陶瓷进行了钎焊连接试验，成功实现了TiAl合金的高质量钎焊连接。通过SEM，EDS，TEM以及XRD等分析方法确定了TiAl/TiNiNb界面结构为：TiAl/B2+γ+O-Ti2AlNb/γ+τ3-Al3（Ti,Nb）2Ni+O-Ti2AlNb；TiAl/TiNiV界面结构为：TiAl/B2+γ/B2+τ3-Al3NiTi2+γ/B2+τ3-Al3NiTi2。采用TiNiNb钎料在1200°C和1220°C钎焊时获得的接头分别具有最高的室温及高温抗剪强度为308MPa和172MPa。采用TiNiNb和TiNiV钎料钎焊Si₃N₄陶瓷自身及TiAl合金时发现钎料与两侧母材均能发生冶金反应，并在钎缝中形成了连续的TiN、Ti-Si及Ni2Ti脆性化合物层。然而，在接头残余应力的作用下于脆性化合物中形成了贯穿钎缝的裂纹，即使采用Nb箔作为应力缓解层也无法实现TiAl合金和Si₃N₄陶瓷的可靠连接。采用具有塑性变形能力的AgCu共晶钎料实现了TiAl合金与Si₃N₄陶瓷的可靠连接。接头典型界面结构为：TiAl/B2/AlCuTi/AlCu₂Ti/Ag（s,s）+AlCu₂Ti/TiN+Ti₅Si₃+AlCu₂Ti/Si₃N₄。钎焊过程中，活性元素Ti与陶瓷反应形成了连续的TiN+Ti₅Si₃反应层；一部分Cu与TiAl反应在TiAl侧形成了连续的AlCuTi及AlCu₂Ti反应层。V与Al反应形成了弥散分布的细颗粒Al3V化合物，作为AlCu₂Ti相析出的形核质点，导致了颗粒状AlCu₂Ti相在钎缝中的弥散分布。基于试验结果，获得了TiN反应层成长的动力学方程。另外，在研究钎焊工艺对TiAl/AgCu/Si₃N₄界面结构的影响时发现，当钎焊温度较低或时间较短时，反应层厚度较薄，颗粒状AlCu₂Ti化合物在Ag基体中弥散程度较差；当钎焊温度较高或时间较长时，反应层厚度显著增加，颗粒状AlCu₂Ti化合物聚集长大，钎缝中出现裂纹。当钎焊工艺合适时（860°C/5min），在钎缝中形成了弥散分布的颗粒状AlCu₂Ti化合物增强的Ag基复合材料组织。为了缓解接头残余应力及提高接头高温性能，采用机械球磨的方法制备了纳米Si₃N₄增强的AgCuTi复合钎料（AgCuTic），并采用该复合钎料成功实现了TiAl合金和Si₃N₄陶瓷的连接，接头典型界面结构为：TiAl/AlCu₂Ti/Al₄Cu₉+TiN+Ti₅Si₃+Ag（s,s）/TiN+Ti₅Si₃/Si₃N₄。钎焊过程中，液相钎料中的Ti元素与纳米Si₃N₄反应形成了纳米尺寸的TiN和Ti₅Si₃颗粒，这些颗粒作为微米尺度Al₄Cu₉化合物的形核质点，使得钎缝中形成了微纳米颗粒增强的Ag基复合材料组织。研究发现Ti元素的溶解及扩散是Ag基复合材料组织钎缝形成的关键因素，基于试验结果，获得了880°C条件下TiAl合金的溶解厚度方程。复合钎料中增强相含量、钎焊温度、钎焊时间等工艺参数对接头界面结构和力学性能影响较大，当增强相含量为3wt.%，钎焊温度为880°C，钎焊时间为5min时，接头室温及高温（400°C）抗剪强度最大分别为115MPa和156MPa，比采用AgCuTi钎料获得的接头强度提高一倍。研究了复合钎料使用对接头性能改善的原因，一方面复合钎缝中弥散分布的细颗粒TiN及Ti₅Si₃化合物作为第二相通过剪切滞后、位错强化及Orowan强化等方式强化了Ag基体，提高了钎缝性能；另一方面通过降低钎缝的热膨胀系数在一定程度上缓解了接头残余应力，从而提高了接头室温及高温性能。接头残余应力有限元模拟结果表明：复合材料的使用对接头的应力分布形式影响不大，但减小了残余应力分布区域以及应力峰值。X射线应力分析表明：增强相含量为3wt.%时，Si₃N₄陶瓷表面压应力峰值降低70MPa左右，与模拟结果相吻合。更多还原

【Abstract】 Due to the excellent mechanical properties and good wave penetratingperformance, Si₃N₄ceramics are preferred as the next generation materials forradomes, which need be connected with metallic materials in the assembly andservice process. TiAl alloys possess high specific strength, high specific stiffness,good oxidation resistance, as well as excellent high-temperature mechanicalproperties. The components made by joining the two materials could be applied tothe fields of aerospace and weaponry, which can not only reduce weight but alsoincrease service temperature. The objective of this study is to achieve the joiningof TiAl alloys and Si₃N₄ceramics. In the present study, appropriate brazing alloysystems were determined by experiments. The interactions between molten brazingalloys and parent materials as well as the brazing mechanism was investigatedbased on the characterization of interfacial microstructure. In addition, accordingto the composite reinforcement theory, AgCuTi+nano-Si₃N₄composite brazingalloys were developed and used to achieve the brazing of the two materials. Thedispersion mechanism of fine compounds in brazing seams was discussed and thedistribution of residual stress in joints was analyzed by finite element methodsimulation and X-ray analysis.TiNiNb and TiNiV eutectic brazing alloys were developed and reliablejoining of TiAl alloys was realized with the two brazing alloys. The typcialinterfacial microstructure of TiAl joints brazed using TiNiNb and TiNiV brazingalloys were TiAl/B2+γ+O-Ti2AlNb/γ+τ3-Al3（Ti,Nb）2Ni+O-Ti2AlNb and TiAl/B2+γ/τ3-Al3NiTi2+B2+γ/B2+τ3-Al3NiTi2resepectively, which were determined bySEM, EDS, TEM and XRD. The highest shear strength at room temperature andhigh temperature （600°C） reached308MPa and172MPa when joints were brazedusing TiNiNb brazing alloy at1200°C and1220°C resepectively. Metallurgicalreactions occurred when the two eutectic brazing alloys were used to braze Si₃N₄ceramics to themselves and to TiAl alloys, which resulted in a continuous TiNlayer was formed adjacent to Si₃N₄side and a brittle Ni2Ti layer in the brazingseam. However, cracks always formed in these brittle reaction layers because oflarge residual stress in joints, which resulted in the failure of brazed joints eventhough Nb foil was introduced as a stress-release interlayer.Brazing of TiAl alloys and Si₃N₄ceramics was achieved using AgCu eutucticbrazing alloy. The interfacial microstructure was TiAl/B2/AlCuTi/AlCu₂Ti/Ag（s,s）+AlCu₂Ti/TiN+Ti₅Si₃+AlCu₂Ti/Si₃N₄. During brazing, active element Ti reactedwith Si₃N₄to form a continuous TiN+Ti₅Si₃layer and a part of Cu reacted withTiAl to form AlCuTi and AlCu₂Ti reaction layers. V reacted with Al to form fine dispersed Al3V particles, as the nucleations of AlCu₂Ti phase, resulting in thedispersion of granular AlCu₂Ti compounds in brazing seam. In addition, thekinetic equation of TiN layer was obtained based on the experimental datas. As theincrease of brazing temperature or brazing time, the thickness of reaction layersincreased and granular AlCu₂Ti compounds aggregated, even some microcrackswere formed in brazing seams. The optimal process parameter was860°C/5min.AgCuTi+nano-Si₃N₄composite brazing alloys （AgCuTic） were developed andused to braze TiAl alloys and Si₃N₄ceramics in order to reduce residual stress injoints and improve joining properties. The typical interfacial microstructure ofTiAl/AgCuTic/Si₃N₄joint was TiAl/AlCu₂Ti/Al₄Cu₉+TiN+Ti₅Si₃+Ag（s,s）/TiN+Ti₅Si₃/Si₃N₄. During brazing, nano-TiN and nano-Ti₅Si₃compounds were formedby the reaction of Ti and nano-Si₃N₄, which acted as nucleations of Al₄Cu₉compounds resulting in the formation of Ag based composite structure in brazingseams. The investigation shows that the dissolution and diffusion of element Tiwere the key foctors in the formation of composite brazing seams and thedissolution equation of TiAl was obtained. In addition, reinforcement content,brazing temperature, as well as brazing time had a significant impact on theinterfacial microstructure and joining properties. The highest shear strength atroom temperature and high temperature （400°C） were115MPa and156MParespectively when brazed at880°C for5min using AgCuTic with3wt.%nano-Si₃N₄addition, which was2times as high as that of joints brazed usingAgCuTi brazing alloy.The effects of composite brazing alloy on improving joining properties wereinvestigated and the results show that brazing seams were strengthened by loadbearing effect, dislocation strengthening and Orowan strengthening. Furthermore,the residual stress in joint was reduced due to the reduction of the CTE coefficientof brazing seams. FEM simulation results show that residual stress distributionform unchanged but both the stress area and stress peaks were reduced whencomposite brazing alloys were utilized. X-ray stress analysis show thatcompressive stress peak on the surface of Si₃N₄ceramics was reduced by70MPawhen AgCuTic with3wt.%nano-Si₃N₄addition was used in this research, whichwas coincide with the simulation results.更多还原