带有气膜孔的镍基单晶合金热梯度机械疲劳损伤机理及寿命评估

For aero-engines, internal cooling is important to reduce the components’ temperature. In the course of service, the turbine components are not only subjected to alternating mechanical loads, but also to the impact of high temperature and the effect of cooling air on the surface and inside of the components, respectively. So the turbine parts suffer from thermomechanical fatigue damage caused by the varying loads and temperatures. Moreover, the temperature gradient brings the multiaxial stress state into the mechanical parts and makes the fatigue multiaxial. For internally cooled parts, the temperature gradients produce additional stresses, which are expressed as multiaxial compressive loads on the hotter surface, while a multiaxial tensile load is displayed on the colder surface. Few works on the TGMF were published. How to quantify the effects of temperature gradients on the stress distribution and fatigue life of the components is an urgent problem. This project is aimed at the working condition of aero-engine turbine blades and turbine disks. The nickel-base single crystal alloy DD6 is used as the research object. The mechanism of deformation, damage and failure of the nickel-based single crystal alloy is studied theoretically and experimentally under the thermal gradient mechanical fatigue (TGMF) loading. The temperature gradient, thermomechanical fatigue evolution law and their interaction are studied by the TGMF tests. Meanwhile, the fatigue, phase difference and temperature gradient characteristics and mutual coupling relationship are revealed. Finally, the constitutive model and fatigue life prediction model of thermal gradient mechanical fatigue of Ni-based superalloy are proposed. The model can consider the influence of material deformation evolution and temperature gradient under different temperature and mechanical load coupling conditions, and it can describe hysteresis curve cycle characteristics and damage laws, which are used to evaluate the fatigue life of nickel-based superalloy under TGMF loading.

对于航空发动机，内部冷却对于降低温度和确保部件的正常运行非常重要，温度梯度会导致部件承受多轴载荷，如何量化内部冷却引起的温度梯度对零件的应力分布和疲劳寿命的影响是一个亟待解决的问题。本项目针对航空发动机涡轮叶片的工作状态，以第二代镍基单晶合金DD6为研究对象，采用理论分析和试验研究相结合的方法，研究镍基单晶合金在热梯度机械疲劳(TGMF)载荷下的变形、损伤和失效机理。进行不同相位和载荷幅值的热梯度机械疲劳试验，重点研究温度梯度和热机械疲劳演化规律及其相互作用关系，揭示疲劳、相位差和温度梯度特性以及相互耦合关系。最终提出镍基单晶合金的热梯度机械疲劳的本构模型和寿命预测模型，该模型能够考虑材料在不同温度和机械载荷耦合条件下的材料变形演化特点和温度梯度的影响，同时能够描述滞回曲线循环特性和损伤规律，实现镍基单晶合金在TGMF载荷下的疲劳寿命评估。

镍基单晶合金的热梯度机械疲劳（TGMF）是航空发动机涡轮叶片结构完整性的关键问题之一。本项目针对涡轮叶片的服役工况，以二代镍基单晶合金DD6为研究对象，采用试验表征和理论分析相结合的方法，研究镍基单晶合金DD6在TGMF载荷下的微结构演化和疲劳损伤机理。改进了TGMF实验系统，通过镜面辐射传热实验，标定辐射加热装置的功率系数，并通过实验和数值计算相结合的方法设计了试件气冷内表面的间接温度测量方法。开展带有气膜孔的镍基单晶合金热梯度机械疲劳实验，通过长焦镜头蓝光补偿的方法，记录孔边裂纹生长过程。揭示DD6单晶合金在高温时效和热梯度机械疲劳下的筏化机理，建立基于微结构演化的双屈服混合强化晶体塑性本构模型。通过断口分析、裂纹尖端观测和元素含量分析，建立热梯度机械疲劳载荷幅值、热机相位角、裂纹扩展速率和裂纹表面氧化速率之间的关系，量化温度梯度对损伤演化的影响，建立镍基单晶合金的热梯度机械疲劳寿命预测模型。本项目成果将为航空发动机涡轮叶片的结构完整性和可靠性设计提供重要的实验和理论依据。