由于陆地资源的紧缺，人类攫取深海资源的需求日益迫切。深海载人潜水器是达成深海任务的重要工具，Ti-6Al-4V ELI合金因具备优异的力学性能而成为潜水器耐压舱壳体的首选材料。由于作业环境的特殊性，深潜器会经历多次下潜-定深度巡航-上浮的过程，在定深度巡航阶段，深潜器会承受一段时间的载荷保持作用（后文称为“保载”），因此该过程可视为承受保载疲劳载荷过程。因制备工艺或海水腐蚀等因素，深潜器耐压舱局部区域可能会存在应力集中或缺陷，然而目前还未见针对缺口和缺陷对钛合金保载疲劳行为影响的系统研究。此外，尽管钛合金的保载疲劳研究已有大量的报导，钛合金的保载疲劳机理仍未被完全揭示。鉴于此，本论文研究了缺口和缺陷对深海载人潜水器耐压舱用Ti-6Al-4V ELI合金保载疲劳行为的影响，并借助扫描电子显微镜(Scanning Electron Microscope，SEM)、电子背散射衍射(Electron Backscatter Diffraction，EBSD)和聚焦离子束(Focused Ion Beam，FIB)技术等对Ti-6Al-4V ELI合金的保载疲劳机理进行了分析，主要研究结果如下：

（1）缺口对Ti-6Al-4V ELI合金的保载疲劳行为有显著影响，相同局部最大应力下，缺口试样的保载疲劳寿命高于光滑试样，并且缺口试样保载疲劳呈现疲劳破坏模式，最大应力的保载时间对缺口试样的疲劳寿命、失效机制和累积应变没有影响。Ti-6Al-4V ELI合金的保载疲劳行为对缺陷尺寸不敏感，缺陷试样保载疲劳断口呈现混合失效模式，最大应力保载降低缺陷试样的疲劳寿命。

（2）当疲劳裂纹起源于引伸计测量段内时，引伸计的施加位置会显著影响（保载）疲劳过程中应变值的测量结果。当引伸计的位置与裂纹萌生位置在同一侧时，应变测量结果偏大；反之，应变测量结果偏小。这种影响随着裂纹的扩展而增大，并且在缺口试样中较光滑试样更为明显。研究结果对于（保载）疲劳过程中应变的测量和分析具有重要意义。

（3）准原位EBSD与SEM观测表明，在疲劳和保载疲劳中，基面滑移或柱面滑移起主导作用，滑移系倾向于在施密特因子较大的α晶粒中开动。一定程度的最大应力下保载有利于滑移系统的激活，但并不是变形越大越有利于滑移。孪晶发生在柱面施密特因子不大于0.2且c轴与轴向应力的夹角小于46 °的α晶粒中，与最大应力是否保载无关。疲劳载荷作用下，微裂纹倾向于从柱面施密特因子较大的α晶粒中萌生，而在保载载荷作用下，微裂纹倾向于从基面施密特因子较大的α晶粒或c轴与轴向应力夹角较小的α晶粒晶界处萌生。保载产生的塑性变形可以抑制滑移或解理引起的微裂纹生长，并导致延性失效模式。

Due to the scarcity of land resources, the demand for human exploitation of deep-sea resources is increasingly urgent. Manned submersibles are an important tool for achieving deep-sea missions, and Ti-6Al-4V ELI alloy is a preferred material for pressure hulls of submersibles due to its excellent mechanical properties. Due to the particularity of the operating environment, the submersible will undergo the process of multiple diving-steady depth cruising-surfacing. During the steady depth cruising stage, the submersible will bear a load holding action for a period of time (hereinafter referred to as “dwell”), so this process can be regarded as a dwell fatigue load process. Due to factors such as preparation process or seawater corrosion, there may be stress concentration or defects in the local area of the pressure hull of submersibles, but there is currently no systematic research on the influence of notches and defects on the dwell fatigue behavior of titanium alloys. In addition, although there are many reports on the dwell fatigue research of titanium alloys, the dwell fatigue mechanism of titanium alloys has not been fully revealed. In view of this, this thesis studies the influence of notches and defects on the dwell fatigue behavior of Ti-6Al-4V ELI alloy for pressure hulls of manned submersibles, and analyzes the dwell fatigue mechanism of Ti-6Al-4V ELI alloy by SEM (Scanning Electron Microscope), EBSD (Electron Backscatter Diffraction) and FIB (Focused Ion Beam) technology. The main results are as follows:

(1) Notches have a significant effect on the dwell fatigue behavior of Ti-6Al-4V ELI alloy. Under the same local maximum stress, the dwell fatigue life of notched specimens is higher than that of smooth specimens, and the notched specimens show fatigue failure mode. The dwell time of maximum stress has no influence on fatigue life, failure mechanism and cumulative strain of notched specimens. The dwell fatigue behavior of Ti-6Al-4V ELI alloy is insensitive to defect size. The fracture surface of specimens with defects shows mixed failure mode. The maximum stress dwell reduces the fatigue life of specimens with defect.

(2) When fatigue cracks initiate within the extensometer measurement segment, the position of the extensometer will significantly affect the measured result of the strain during (dwell) fatigue process. When the position of the extensometer and the crack initiation site are of the same side, the strain measurement is higher; otherwise, the strain measurement is lower. This effect increases with the crack growth and it is more pronounced in notched specimens than in smooth specimens. This result is of great significance for the strain measurement and analysis during (dwell) fatigue process.

(3) Quasi in-situ EBSD and SEM observations show that the basal slip or prismatic slip dominates for both fatigue and dwell fatigue of Ti-6Al-4V ELI alloy, and the slip system tends to activate in α grains with bigger SFs for basal slip or prismatic slip. A certain degree of maximum stress dwell favors the activation of slip system, but it is not the greater the deformation the more favorable the activation of slip system. Twinning occurs in α grains with Schmid factors not bigger than 0.2 for prismatic slip and their c-axis to the applied axial stress smaller than 46 °, which is irrespective of the maximum stress dwell. Microcracks tend to initiate from prismatic plane of α grains with bigger SFs for prismatic slip under fatigue loadings, while microcracks tend to initiate from basal plane of α grains with bigger Schmid factors for basal slip or the boundaries of α grains with small angle of the c-axis to the applied axial stress under dwell fatigue loadings. The plastic strain by the dwell stress could restrain the growth of the microcrack induced by sliding or cleavage and result in the ductile failure mode.