γ-TiAl合金钎焊接头中原位TiB晶须生长机制及韧化机理研究

This project will explore a novel brazing technology for γ-TiAl alloy joints (consisting of complete intermetallics) strengthened and toughened by the in situ synthesized TiB whiskers. Formation mechanism and controlling method of the in situ synthesized TiB whiskers, solidification process of the filler metal in the presence of the TiB whiskers, mechanical property and toughening mechanism of the brazed joints, are chosen as core research contents. To determine the evolution mechanism and controlling method of the interfacial microstructure of the brazed joint and reveal toughening mechanisms of the composite brazing seam, and fundamentally control the quality of γ-TiAl brazed joints, this project will study dynamic behavior of the in situ synthesized TiB whiskers in the brazing seam and illuminate the nucleation and growth mechanism of intermetallics in the presence of TiB whiskers, clarity the effect of TiB whiskers on the solidification process of liquid filler and master the existential state of TiB whiskers in the brazed joint as well as interfacial bonding mechanisms between the TiB whiskers and the matrix phase, establish the interrelationship of the brazing parameters, the morphology, content and distribution of the TiB whiskers, microstructure of the brazed joint as well as its bonding strength and toughness. It is expected that this study will largely solve the main problem to improve the performance of the γ-TiAl brazed joint and develop new ideas for materials connection, which also can supply basic supporting theory and key reserved technology for the connect manufacturing of new high temperature critical structural components and the development of new weapon equipment.

本项目以γ-TiAl合金钎焊连接为研究对象，以探索通过原位自生TiB晶须强韧化全金属间化合物γ-TiAl合金接头的钎焊技术为研究主线，以研究原位反应生成TiB晶须的形成机制及控制方法、TiB晶须存在条件下钎料合金凝固过程、钎焊接头力学性能及韧化机制等为核心内容。拟研究钎缝中原位形成TiB晶须的动力学行为，阐明金属间化合物在TiB晶须存在下的形核、生长机制，明晰TiB晶须对液态钎料体系凝固过程的影响，掌握TiB晶须在钎焊接头中的存在状态及其与基体相之间的界面结合机理，建立钎焊工艺参数、TiB晶须形态、含量、分布、接头微观组织以及钎焊接头强度、韧性等力学性能间的内在联系，确定钎焊接头界面组织的演化机理及控制方法，揭示钎焊接头中复合钎缝的韧化机制，从根本上控制γ-TiAl合金钎焊接头的连接质量。该研究将为新型耐高温关键结构部件的连接制造及新型武器装备的研制提供基础理论支撑和关键技术储备。

该项目面向航空航天和汽车工业领域发动机轻量化制造国家重大需求，针对增压涡轮中采用TiAl基合金取代Ni基合金所涉及到的γ-TiAl高温合金的钎焊展开研究，致力于解决γ-TiAl钎焊接头组织与性能调控技术瓶颈背后的组织形成机理及演变、连接界面冶金行为、组织和性能调控机制和服役性能评价等基础科学问题。针对发动机涡轮上Ti-47Al-2Nb-2Cr-0.15B (at.%)合金的高温服役需求，通过向共晶成分Ti-61Ni (at.%)和Ti-24Ni(at.%)中添加TiB2或B设计出理论上可在钎缝中原位自生0～40 vol.%的TiB晶须的新型钛基粉末和铸态钎料，并分析研究钎料的显微组织和形成机理、熔化特性及钎焊性。系统研究钎焊温度、钎焊时间、原位自生TiB体积分数对γ-TiAl钎焊接头界面组织、接头性能和断裂行为的影响规律的基础上，从热力学及动力学、自由能及扩散理论等角度深入研究接头界面组织（反应物类型、形态及分布）及形成机理，并在详尽分析接头的界面组织与力学性能关联性基础上揭示TiB晶须在接头中的强化机制。结果表明，TiAl钎焊接头均由三个界面反应层组成且主要物相为Al3NiTi2、Ti3Al和TiB。过低的钎焊温度与保温时间都会造成母材与钎料的冶金结合不充分，过高的钎焊温度或过长的保温时间会造成钎料的流失或钎料对母材的溶蚀。TiB晶须细化组织、强韧化作用随其体积分数（0～20 vol.%）的增加而增强，有利于接头抗剪强度的提高，而生成30 vol.%和40 vol.%TiB晶须时易发生团聚而降低其细化晶粒的作用和增大液态钎料的粘度，导致TiAl钎焊接头脆化和产生孔洞。因此，随着钎焊温度、钎焊时间和原位自生TiB晶须体积分数的增加，接头抗剪强度先增加后减小，Ti-61Ni (at.%) + TiB2铸态钎料1200℃/5 min/20 vol.%TiB时抗剪强度达374 MPa，高于同成分粉末钎料10%。TiH2-24Ni (at.%) + B粉末钎料1080℃/10 min/10 vol.%TiB时接头抗剪强度达390.73 MPa。另外，高弹性模量的TiB晶须主要是通过非均质形核细化晶粒、裂纹偏转和晶须拔出增加断裂能、骨架桥接等方式来强韧化钎焊接头的。该项目在发动机涡轮结构材料轻量化和先进连接工艺推广应用方面具有科学意义与工程应用价值。