Chapter 12 Watching High-Cycle Fatigue with Automated Scanning Electron Microscope Experiments Nathan M. Heckman, Timothy A. Furnish, Christopher M. Barr, Khalid Hattar, and Brad L. Boyce Abstract Fatigue is a multistep process where cyclic loading causes damage within materials that eventually leads to crack formation and propagation. In nanocrystalline metals, a dominant damage mechanism is the abnormal growth of grains up to 100 times their original size. Previous in situ synchrotron experiments have revealed that this grain growth process precedes crack formation and takes up a majority of the fatigue lifetime. The growth of nanocrystalline grains leads to the formation of protrusions on the surface of a material, which can be resolved in scanning electron microscopy. Based on this concept, an automated in situ scanning electron microscope tension–tension fatigue test method has been developed to observe the evolution of crack formation and propagation in materials. In this study, this method was applied to understand the high-cycle fatigue behavior in nanocrystalline Ni- and Pt-based alloys. Fatigue tests between 105 and 107 cycles were performed, and in combination with postmortem characterization through grain orientation mapping and transmission electron microscopy, we identified differences in resistance to damage and crack propagation in the various alloys, and observed varying damage levels prior to crack formation, strongly dependent on the number of cycles to failure. Keywords Fatigue · In situ · Scanning electron microscopy · Crack initiation · Crack propagation 12.1 Introduction Fatigue is often the cause for mechanical failure of components, where a vast majority of mechanical service failures in metals are due to cyclic loading events [1–4]. While this is the case, most studies investigate uniaxial monotonic properties of materials instead of fatigue properties. Due to this, the monotonic tensile deformation behavior of simple and complex face centered cubic (FCC) metals are well understood across a wide range of microstructures, where dislocation motion tends to dominate when grain sizes are in the coarse-grained regime [5], while at the nanocrystalline scale, grain-boundary-mediated mechanisms tend to dominate the monotonic deformation behavior [6]. Monotonic deformation behavior is typically well understood even in advanced metals across these grain sizes. There is a much lesser understanding of complex mechanisms in fatigue. In this manuscript, we discuss a newly developed method to understand the evolution of fatigue damage in metals utilizing automated in situ fatigue experiments in the scanning electron microscope (SEM). This method is utilized in preliminary studies on nanocrystalline Ni-based alloys to understand the evolution of fatigue damage in these material systems. This serves as a tool to understand both simple and complex mechanisms in the formation and propagation of damage in fatigue. 12.2 Background The two primary stages of fatigue failure are crack formation and crack propagation. Each stage is often mechanistically dissimilar, where different mechanisms may dominate during each. In comparison to coarse-grained metals, nanocrystalline metals tend to spend a much larger portion of their fatigue life in the crack formation stage. In most simple nanocrystalline systems, abnormal grain growth has been observed as the dominant mechanism for crack formation [7]. Note, this mechanism N. M. Heckman ( ) · T. A. Furnish · C. M. Barr · K. Hattar · B. L. Boyce Sandia National Laboratories, Albuquerque, NM, USA e-mail: nheckma@sandia.gov; tafurni@sandia.gov; cbarr@sandia.gov; khattar@sandia.gov; blboyce@sandia.gov © The Society for Experimental Mechanics, Inc. 2021 S. Xia et al. (eds.), Fracture, Fatigue, Failure and Damage Evolution, Volume 3, Conference Proceedings of the Society for Experimental Mechanics Series, https://doi.org/10.1007/978-3-030-60959-7_12 73
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