TiAl变形合金片层界面取向调控机理及其对力学性能的影响研究

Full lamellar microstructures of TiAl alloys with most grains’ lamellar boundaries paralleled to loading direction have the best comprehensive mechanical properties, and their manufacture technologies, such as directional solidification method, are always one of the hottest topics in TiAl alloys research. A novel idea to produce such microstructure is proposed in this proposal, that is, control the texture evolution and grain growth behavior of high temperature phase, α phase, during thermolmechanical processing and subsequent heat treatment. However, the evolution and formation mechanism of α deformation texture and recrystallization texture in TiAl alloys are not clear, and the relationship between α2 texture and mechanical properties of full lamellar microstructure is not studied. The aim of this work is to clarify the α phase texture evolution law and grain growth behavior during deformation and heat treatment of TiAl alloys, establish the scientific mechanism of controlling lamellar boundaries’ orientation, build the quantitative relationship between texture of α2 and mechanical properties of full lamellar microstructure, and determine the texture strengthen mechanism of full lamellar microstructure. The outcome of this work not only promotes the understanding of texture evolution, texture formation mechanism and texture strengthen mechanism of TiAl alloys, but also provides theoretical references on optimizations of processing parameters, microstructure and mechanical properties of TiAl wrought alloys and related wrought components. It has both theoretical and practical significance.

片层界面择优分布的TiAl合金全片层组织具有优异的综合力学性能，因此该类组织的制备，如定向凝固方法，一直是TiAl合金研究的热点之一。本申请书提出制备该类组织的新思路，即通过调控高温α相变形织构及其在后续热处理的再结晶和晶粒长大行为，得到片层界面沿目标方向择优分布的全片层组织。本申请书的主要目标聚焦于TiAl合金中高温α相在变形和后续热处理过程的织构演变规律和晶粒长大行为，以及织构与全片层组织力学性能的定量关系。通过阐明TiAl变形合金片层界面控制的科学原理，实现片层界面沿目标方向择优分布；通过建立织构与全片层组织力学性能定量关系，完善织构强化理论；通过阐明织构组织的晶粒长大动力学行为，减少易萌生裂纹晶粒的体积，提高力学性能。这既加深了对六方α相织构形成机制和演变规律的认识，丰富了织构强化理论，又将指导TiAl变形合金及相关部件的工艺优化、组织调控和性能提高，具有重要的科学意义和应用价值。

TiAl合金的力学性能与片层界面和加载方向间夹角密切相关，控制多晶TiAl合金片层界面沿特定取向择优分布，可有效提高TiAl合金的力学性能。针对这一优势，本研究系统深入研究了TiAl合金在热变形和热处理过程中的α/α2相织构演变规律，分析了α2相织构对TiAl合金全片层组织力学性能的影响。.α2相织构的形成与片层晶粒的流动变形以及破碎球化有关，其中片层晶粒流动变形过程中形成片层界面平行于金属流动方向的显微组织，相应α2相为{0001}织构；随着变形量的增大，α2相片层破碎球化，并随着柱面滑移系的开动而形成{0001}<10-10>织构，局部温度升高发生选择生长再结晶而形成{0001}<11-20>织构。.本研究系统研究了坯料初始组织对TiAl合金变形组织及其α2相织构的影响，确定了全片层组织坯料有利于显微组织与α2相织构调控。对比了锻造和挤压工艺对变形组织中α2相变形织构的影响，发现挤压变形更容易形成织构单一的变形制品。系统研究了挤压温度和挤压比等工艺参数对α2相变形织构的影响，发现温度过高或过低时，变形组织中易形成再结晶片层晶粒或者残余片层晶粒，二者均在α单相固溶热处理中诱发二次再结晶而形成粗大片层晶粒。增加变形量有利于片层晶粒破碎，促使α2相形成单一的{0001}<10-10>变形织构。.TiAl合金变形组织热处理时，变形量不大的残余α2相片层仍保持原有晶体取向，而变形量较大得到的等轴α2相晶粒发生选择生长再结晶。变形组织中α2相小角度晶界不利于晶界迁移而限制了晶粒长大，大角度晶界有利于晶界迁移而导致晶粒迅速长大而发生二次再结晶。固溶热处理温度和热处理时间过高也会导致二次再结晶。.对比研究了α2相织构不同的全片层组织的力学性能，发现室温抗拉强度、室温塑性、断裂韧性、持久寿命均随着α2相基面垂直于加载方向晶粒体积含量的减小而增大。断口形貌观察和XRT分析表明，片层界面垂直于加载方向的晶粒更易萌生裂纹导致力学性能下降，这与该取向下的α2相与γ相临界剪切应力相差较大而导致应力集中诱发裂纹萌生有关。.