Abstract A novel in situ scanning electron microscope (SEM), high frequency fatigue testing methodology is developed using a combination of laser milling, focused ion beam fabrication and nanoindentation. This methodology is used to investigate crack initiation, propagation, fracture, fatigue life, and the mechanical response of microcantilever samples of a Ni-based superalloy (Rene-N5) under different cyclic strain amplitudes. The crack initiation and propagation in the microcantilever is monitored by observing changes in the beam's dynamic stiffness and continuous SEM imaging. The dynamic stiffness response of the micro-beams exhibits a transition from softening to hardening at a critical strain amplitude of 7 × 10 − 3 . Theoretical analysis indicates that this transition corresponds to the stress required to shear γ ′ precipitates. SEM imaging reveals the evolution of significant extrusions, intrusions, and slip traces during cyclic loading above this critical strain amplitude. Below this strain amplitude, very little surface roughening is observed. In addition, the measured dynamic stiffness is observed to exhibit two regimes of decrease after crack initiation. These two regimes correspond to short and large crack propagation. Finally, an overall increase in fatigue life is observed when comparing to bulk scale experiments on nickel-base superalloys. It is proposed that this is an inherent size effect in the small-volume, single crystal specimens tested.

中文翻译：

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镍基高温合金 René-N5 微晶的高频原位疲劳响应

摘要 结合激光铣削、聚焦离子束制造和纳米压痕，开发了一种新型原位扫描电子显微镜 (SEM) 高频疲劳测试方法。该方法用于研究 Ni 基高温合金 (Rene-N5) 微悬臂梁样品在不同循环应变幅度下的裂纹萌生、扩展、断裂、疲劳寿命和机械响应。通过观察梁的动态刚度和连续 SEM 成像的变化来监测微悬臂梁中的裂纹萌生和扩展。微梁的动态刚度响应在临界应变幅度为 7 × 10 - 3 时表现出从软化到硬化的转变。理论分析表明，这种转变对应于剪切 γ ' 析出物所需的应力。SEM 成像揭示了在高于此临界应变幅度的循环加载期间显着挤压、侵入和滑动痕迹的演变。在该应变幅度以下，观察到很少的表面粗糙。此外，观察到测得的动态刚度在裂纹萌生后表现出两种降低状态。这两种状态对应于短裂纹和大裂纹扩展。最后，与镍基高温合金的大体积实验相比，观察到疲劳寿命总体增加。建议这是测试的小体积单晶样品中固有的尺寸效应。此外，观察到测得的动态刚度在裂纹萌生后表现出两种降低状态。这两种状态对应于短裂纹和大裂纹扩展。最后，与镍基高温合金的大体积实验相比，观察到疲劳寿命总体增加。建议这是测试的小体积单晶样品中固有的尺寸效应。此外，观察到测得的动态刚度在裂纹萌生后表现出两种降低状态。这两种状态对应于短裂纹和大裂纹扩展。最后，与镍基高温合金的大体积实验相比，观察到疲劳寿命总体增加。建议这是测试的小体积单晶样品中固有的尺寸效应。