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High Cycle Fatigue in the Transmission Electron Microscope Daniel C. Bufford, Douglas Stauffer, William M Mook, Syed Asif Syed Amanulla, Brad L. Boyce, and Khalid Hattar Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b01560 • Publication Date (Web): 28 Jun 2016 Downloaded from http://pubs.acs.org on June 30, 2016
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Nano Letters
High Cycle Fatigue in the Transmission Electron Microscope Daniel C. Bufford†, Douglas Stauffer‡, William M. Mook†, S.A. Syed Asif‡, Brad L. Boyce†, and Khalid Hattar*,† †
Sandia National Laboratories, Albuquerque, NM, 87185, USA
‡
Hysitron, Inc., Eden Prairie, MN, 55344, USA
ABSTRACT: One of the most common causes of structural failure in metals is fatigue induced by cyclic loading. Historically, microstructure-level analysis of fatigue cracks has primarily been performed post mortem. However, such investigations do not directly reveal the internal structural processes at work near micro- and nanoscale fatigue cracks, and thus do not provide direct evidence of active microstructural mechanisms. In this study the tension-tension fatigue behavior of nanocrystalline Cu was monitored in real time at the nanoscale by utilizing a new capability for quantitative cyclic mechanical loading performed in situ in a transmission electron microscope (TEM). Controllable loads were applied at frequencies from one to several hundred Hz, enabling accumulations of 106 cycles within one hour. The nanometer-scale spatial resolution of the TEM allows quantitative fatigue crack growth studies at very slow crack growth rates, measured here at ~10-12 m·cycle-1. This represents an incipient threshold regime that is well below the tensile yield stress, and near the minimum conditions for fatigue crack growth. Evidence of localized deformation and grain growth within 150 nm of the crack tip was observed by both standard imaging and precession electron diffraction orientation mapping. These observations begin to reveal, with
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Nano Letters
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unprecedented detail, the local microstructural processes that govern damage accumulation, crack nucleation, and crack propagation during fatigue loading in nanocrystalline Cu.
KEYWORDS: fatigue; TEM; metals; crack propagation
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Nano Letters
The formal study of fatigue began during the late industrial revolution in the 1800s, in the wake of unexpected catastrophic failures of heavy machinery.1 Research continues today to refine predictive models of fatigue in conventional structural materials and to better understand the fatigue behavior of emerging advanced materials.2 The regimes of fatigue behavior include (1) cyclic plasticity and ratcheting during gross yielding and load reversal, (2) low cycle fatigue in the vicinity of the yield stress and fatigue lifetimes of ~10-103 cycles, (3) high cycle fatigue well below the yield stress and fatigue life exceeding 104 cycles, and (4) very high cycle fatigue under conditions when fatigue life may reach >>107 cycles. As the applied stress or strain decreases from (1) to (4), corresponding rates of microstructural change and crack propagation generally fall. The overall fatigue lifetime, or number of cycles to failure, is often of great interest, while more refined approaches examine the lifetime in separate crack initiation and crack propagation stages. Conventional experiments provide invaluable information about overall fatigue lifetimes and microstructural-scale deformation, but open questions remain about the mechanistic processes that govern fatigue crack initiation and incipient propagation at the nanoscale. The effects of these processes may be amplified in nanocrystalline materials with small (
