In this study, a dual-scale numerical procedure is developed to reveal the creep-fatigue damage mechanisms and estimate the crack initiation life for notched structures made of Inconel 718 superalloy at 650 °C. The macro-scale simulation solves the creep-fatigue deformation behavior with viscoplastic constitutive models, and the local deformation histories are supplied to the micro-scale simulation as boundary conditions. In the micro-scale simulation, the local damage evolutions are solved based on crystal plasticity combined with grain boundary cavity model. The creep damage is calculated by a special formulation in the form of cavity nucleation, growth and coalescence. The fatigue damage is represented by accumulated energy dissipation originated from crystal plasticity finite element simulation. Experimentally, the creep-fatigue tests of notched structures are carried out for Inconel 718 superalloy at 650 °C to validate the feasibility and robustness of the proposed numerical procedure. Moreover, the crack initiation behavior, including transgranular cracks under fatigue loading and intergranular cracks under creep-fatigue loading, is explained through different types of microstructure observations together with a dual-scale numerical procedure. In detail, the crack initiation sites transferred from the grain interior at notched surface to the grain boundaries at notched subsurface with an increase in hold times can be well predicted by the proposed numerical procedure. In addition, the simulated life based on the developed life prediction approach agrees well with the experimental data within an error band of ±2. Parametric studies show that the creep damage is more sensitive to grain boundary diffusion than to the external conditions of strain level and hold time.

在这项研究中，开发了一个双尺度数值程序来揭示蠕变疲劳损伤机制并估计由 Inconel 718 高温合金制成的缺口结构在 650 °C 时的裂纹萌生寿命。宏观模拟使用粘塑性本构模型求解蠕变疲劳变形行为，并将局部变形历史作为边界条件提供给微观模拟。在微尺度模拟中，基于晶体塑性结合晶界腔模型求解局部损伤演化。蠕变损伤是通过空腔成核、生长和聚结形式的特殊公式计算的。疲劳损伤由源自晶体塑性有限元模拟的累积能量耗散表示。实验上，在 650 °C 下对 Inconel 718 高温合金进行了缺口结构的蠕变疲劳试验，以验证所提出的数值程序的可行性和稳健性。此外，裂纹萌生行为，包括疲劳载荷下的穿晶裂纹和蠕变疲劳载荷下的晶间裂纹，通过不同类型的微观结构观察和双尺度数值程序来解释。详细地说，随着保持时间的增加，裂纹萌生点从带缺口表面的晶粒内部转移到带缺口次表面的晶界，可以通过所提出的数值程序很好地预测。此外，基于开发的寿命预测方法的模拟寿命与实验数据吻合良好，误差范围为±2。