高强韧钛合金多层次组织对缺口疲劳损伤行为的影响机理研究

Titanium alloys with high strength & toughness, in recent decades, have been successfully developed as the key structural materials in airplane and spacecraft industries. It has been demonstrated that the fatigue damage behavior is always the critical problem strongly affecting the long life and high reliability of titanium alloys with high strength & toughness during the airplane and spacecraft performing process. Our recent research results display that different level microstructure indicates different deformation behavior, which controls the notch fatigue damage on different notch fatigue stage of high strength & toughness Ti-55531 titanium alloy with multi-level lamellar microstructure. Therefore, the aim of this work is to reveal the co-ordinational influence mechanism of multi-level microstructure and notch factors on fatigue damage behavior of titanium alloys with high strength & toughness. The interrupted fatigue test method, high resolution ultrasonic microscope and in-situ SEM fatigue test system will be used to performed systematical researches on cyclic deformation and notch fatigue crack initiation behavior of Ti-55531 alloys with different multi-level microstructures at plastic zone of notch root during notch fatigue damage process. The influence mechanism of multi-level microstructure on microstructural deformation behavior at plastic zone of notch root and fatigue crack initiation of the alloy will be expounded. Key microstructure factors of controlling the cyclic deformation mechanism and notch fatigue crack initiation of titanium alloys with high strength & toughness will also be revealed. According to above results, the model of coupling relationship between notch fatigue damage behavior and multi-level microstructure of titanium alloys with high strength & toughness will be built, which can provide long life and high reliability in aerospace engineering performance of titanium alloys with high strength & toughness with theoretical basics.

高强韧钛合金是航空航天领域不可或缺的关键结构材料，其疲劳损伤问题一直是制约高强韧钛合金构件服役过程长寿命和高可靠性的核心问题。前期研究发现，多层次片层组织的高强韧Ti-55531合金在缺口疲劳过程，不同的疲劳阶段受到不同层次的组织控制。为此，本项目采用中止疲劳试验法、高分辨超声波显微镜和扫描电镜原位疲劳等测试手段，对多层次片层组织合金的缺口疲劳行为进行系统的研究分析，首先研究缺口根部多层次组织的循环变形行为，阐明多层次组织在缺口疲劳变形过程起控制作用的组织单元及其变形机制；其次研究缺口根部疲劳裂纹萌生与多层次组织的关系，揭示多层次组织对缺口疲劳裂纹萌生的影响机制。通过上述研究，构建多层次组织对缺口疲劳损伤行为影响的耦合模型，为高强韧钛合金结构件的长寿命和高可靠性设计提供借鉴。

高强韧钛合金是航空航天领域不可或缺的关键结构材料，其疲劳损伤问题一直是制约钛合金构件服役过程长寿命和高可靠性的核心问题。前期研究发现，高强韧Ti-55531合金片层组织具有多层次结构，在缺口疲劳过程，不同阶段受不同层次的组织控制。本项目首先研究了热处理对Ti-55531合金多层次片层组织结构参数的影响规律，该类组织与室温拉伸、冲击及断裂韧性之间的关系，揭示了片层组织的强韧化相关机制，阐明了控制强度、塑性及韧性的关键组织参数，获得了制备优异强韧性的钛合金片层组织的优化工艺参数。然后研究了不同缺口根部半径下，在缺口根部循环变形过程起控制作用的组织单元、变形机制和对缺口根部疲劳裂纹萌生全过程的影响规律；相同的缺口根部半径下，多层次组织中不同层次组织的参数对缺口根部塑性应变区内组织的循环变形和疲劳裂纹萌生全过程的影响规律。最后通过原位SEM拉伸和有限元模拟，进一步分析了合金片层组织的疲劳变形及断裂行为。结果显示，固溶温度增加，β晶粒和α丛束直径增加，α片减小；退火温度增加，α丛束直径和α片厚度增加，而α相体积分数减少。控制合金多层次片层组织强度、塑性、韧性的关键组织分别是α集束、β晶粒及α片尺寸。缺口显著降低合金的疲劳极限，随Kt增加(2,3,4)，合金疲劳极限分别下降至220，130和120MPa。疲劳裂纹都萌生于缺口根部α束集内的α片和α/β界面处，且大多数微裂纹沿α/β界面扩展。但随缺口根部半径降低，缺口根部应力集中程度增加，塑性变形更严重，起协调变形作用的组织尺度也减小，α片内更高密度的位错缠结、孪晶密度和更剧烈的破碎断裂，微孔和微裂纹萌生的α束集宽度也逐渐减小。缺口疲劳的有限元模拟分析和原位SEM拉伸分析进一步证实，显微组织和缺口参数对合金的形变、裂纹萌生及扩展有显著影响。上述研究结果可为高强韧钛合金结构件的长寿命和高可靠性设计提供借鉴。