A Review of Fatigue Behavior in Nanocrystalline Metals

Abstract

Nanocrystalline metals have been shown to exhibit unique mechanical behavior, including break-down in Hall-Petch behavior, suppression of dislocation-mediated plasticity, induction of grain boundary sliding, and induction of mechanical grain coarsening. Early research on the fatigue behavior of nanocrystalline metals shows evidence of improved fatigue resistance compared to traditional microcrystalline metals. In this review, experimental and modeling observations are used to evaluate aspects of cyclic plasticity, microstructural stability, crack initiation processes, and crack propagation processes. In cyclic plasticity studies to date, nanocrystalline metals have exhibited strongly rate-dependent cyclic hardening, suggesting the importance of diffusive deformation mechanisms such as grain-boundary sliding. The cyclic deformation processes have also been shown to cause substantial mechanicallyinduced grain coarsening reminiscent of coarsening observed during large-strain monotonic deformation of nanocrystalline metals. The crack-initiation process in nanocrystalline metals has been associated with both subsurface internal defects and surface extrusions, although it is unclear how these extrusions form when the grain size is below the scale necessary for persistent slip band formation. Finally, as expected, nanocrystalline metals have very little resistance to crack propagation due to limited plasticity and the lack of crack path tortuosity among other factors. Nevertheless, like bulk metallic glasses, nanocrystalline metals exhibit both ductile fatigue striations and metal-like Paris-law behavior. The review provides both a comprehensive critical survey of existing literature and a summary of key areas for further investigation.

Anomalous Fatigue Behavior and Fatigue-Induced Grain Growth in Nanocrystalline Nickel Alloys

“…However, a survey of the available literature on the fatigue of NC metals reveals that fatigue failure still occurs for NC metals, often with nothing more than a modest improvement in performance over CG metals. [26,27] Perhaps the explanation for the lack of dramatic improvements in fatigue performance is related to the instability of NC grain structures, which are known to evolve even during storage at room temperature. [28] Previous experimental observations of NC grain growth during plastic deformation, [29,30] indentation experiments [31] and as a result of fatigue loading [14,32] warrant serious consideration.…”

Fatigue-induced grain coarsening in nanocrystalline platinum films

“…In many metallic films (e.g. Ni, Cu and Ag) the grains spontaneously coarsen ("self-annealing"), a phenomenon that can also be driven by stress [1][2][3][4][5][6]. Nanocrystalline Ni, for example, has been shown to display a typical power law exponent (between 2 and 3.5) [2,7] and grain coarsening during cyclic contact [8].…”

“…The earliest experiment to report grain coarsening in response to fatigue is by Witney et al [1], who reported on the microstructural evolution of nanocrystalline copper. Since then, other experimental works involving fatigue on nanocrystalline nickel and nickel-tungsten alloys [2], ultrafine-grained copper [3,4] and nickel-manganese alloys [5] have noted grain coarsening. The coarsening has been cited as the reason for the observed cyclic softening in nanocrystalline materials [3] and for contributing to crack initiation [5], but the mechanisms for the growth are still unclear.…”

Nanotwin-assisted grain growth in nanocrystalline gold films under cyclic loading

“…Furthermore, Rupert et al 4 have experimentally uncovered the shear-stress-driven GB migration behaviour in the aluminium film using the specimens smartly designed with stress and strain concentrators. Until now, the grain growth has been observed under various loading modes including tension 5,6 , compression 7 , nanoindentation 3,8 and cyclic deformation 9,10 .…”