损伤容限型钛合金微细α+β片层结构形成及其对强韧性的影响

Forming a fine α+β layer structure of lamellar structure is one of the most effective way to achieve high strength toughness and high damage tolerance for titanium alloy, which can not only possess high tolerance, but also lead to the fine grain strengthening and micro lamellar toughening. In the present project, the combination technology of multi-directional forging (MDF) and β heat treatment for typical titanium alloy will be investigated carefully，in order to fabricate a fine α+β lamellar structure by using of both the constraint effect of β fine grain formed by MDF on the lamellar structure and the regulation effect of the β heat treatment on the lamellar structure. The formation mechanism and forming conditions of β fine grain during MDF process will be studied. Then, the phase transformation from β→α, the α laths growth behavior and the formation of ultra-fine lamellar microstructure by the coordination technology of MDF and heat treatment will be investigated. Based on using room temperature tension, fracture toughness test and fatigue crack growth resistance test, the dependence of room temperature tensile properties and damage tolerance on lamella microstructure characteristics will be researched. After that, the process control technology of ultra-fine lamella microstructure with high strength-toughness and better damage tolerance will be investigated. This aims to explore a preparation of alpha + beta fine lamellar structure with high strength and toughness and damage tolerance technology of titanium alloy, and is helpful to provide theoretical and technical support for the aircraft adapted to modern damage tolerance design standards.

具有微细α+β片层结构不仅可使片层组织保留高损伤容限性，还可实现微细晶强化和微细片层韧化，是获得高强高韧损伤容限型钛合金的有效途径。本项目以TC21-DT钛合金为研究对象，采用多向锻造（MDF）+β热处理工艺方法，利用MDF形成的β微细晶（约束片层形成和发展）与β热处理（调控片层组成和结构）的协同作用，研究β微细晶与β热处理联合机制下的β→α相变行为及α片层形核/生长机制，揭示α+β微细片层结构的形成机制及其演变规律；结合室温拉伸实验、断裂韧性实验和抗疲劳裂纹扩展实验，研究α+β微细片层结构特征与强韧性能和损伤容限性能的关联及其微观机理，探索高强高韧损伤容限型钛合金α+β微细片层结构形成机制及其调控方法。研究结果可为发展适应现代损伤容限设计准则要求的航空飞行器提供理论和技术支撑。

本项目采用多向锻造（MDF）+热处理工艺方法，利用MDF形成的β微细晶与热处理的协同作用，探索高强高韧损伤容限型钛合金α+β微细片层结构形成机制及其调控方法。主要研究结果如下：①模拟优化了TC21钛合金多向锻造过程。提高锻造温度、降低锻造速率，合金等效应力降低、提高变形均匀性。提高变形量和锻造道次并适当降低锻造速率有利于提高合金的等效应变值和促进动态再结晶。②研究了锻造工艺参数对合金微观组织的影响规律。随多向锻造温度升高，合金片层组织球化体积分数和球化晶粒尺寸增加。锻造温度高于940℃时球化组织会粗化，锻造温度低于850℃时片层组织不易球化，且变形组织不均匀。增加锻造周期和单道次锻造变形量会促进合金片层组织球化，球化体积分数增加，球化晶粒尺寸减小。合金在锻造温度910℃、单道次锻造变形量50%、锻造3周期时可获得晶粒尺寸为2μm的球化组织。③研究了进一步后续热处理对锻造组织晶粒细化的影响规律。随加热温度升高，球化体积分数增加，但加热温度高于940℃时，球化组织会粗化，低于860℃时，静态球化效果不佳。随保温时间延长，合金静态球化越明显，但在较高加热温度（>940℃）时过长保温时间（>2h）会导致静态球化晶粒快速长大，不能获得均匀细小组织；880℃~910℃下三周期多向锻造，热处理加热温度在900℃左右，保温时间在1~2h，在该条件下可获得微米级均匀球化组织。④揭示了合金多向锻造晶粒细化机理。多向锻造时片层组织动态球化是由α片内动态再结晶与β相向α/α边界楔入共同作用的结果。⑤研究了β→α相变行为及α片层形核/生长机制。α片只在原始β晶粒范围内生长，不能穿过原始β晶界，且α片呈集束式生长。较小原始β晶粒也有助于获得细小(α+β)片层组织。过小和过大冷却速度均不利于(α+β)片层组织性能。在低冷速时，片层组织的特征参数均较大，对合金强塑性不利。在最佳锻造参数和热处理后合金抗拉强度可达1100MPa、延伸率可达17.8%。