The theme of the Nanomechanics and Nanometallurgy of Boundaries project at Sandia National Labs is to understand the connection between mechanical deformation and grain boundary instability in nanocrystalline metals. Our prior observations have led to the hypothesis that fatigue-induced abnormal grain growth is a precursor to crack initiation in nanocrystalline metals [Padilla et al, Metall. Mater. Trans. A, 2011]. The development of this new technique is an instrumental step forward in connecting mechanical performance to rare microstructural events. Capturing the abnormal grain growth process associated with nanocrystalline fatigue was particularly problematic because it had only ever been observed by post-mortem fractography where the final fracture was needed to locate a ‘needle in the haystack’. While we have been developing a parallel effort to observe grain boundary mechanisms during high-cycle fatigue in the TEM, that effort will always be impeded somewhat by the limited interrogation volume of the TEM and hence will require a pre-existing starter notch/crack to localize the failure process.
While the experiments described in this paper were performed as a user at SSRL with a beamline limited to a ~100 μm monochromatic beam, we are currently exploring the extension of this technique to white beam experiments at the ALS or CHESS with either a microbeam or a broad ~1 mm scale beam, respectively. Both new sources also offer much faster area detectors that could greatly improve the techniques temporal ability to capture fatigue-induced (or otherwise thermomechancially induced) grain boundary instability. The technique could also be extended to cryogenic experiments where the kinetics for conventional boundary motion mechanisms are glacial. Finally, it is worth noting that this technique could apply to other materials science studies where there is a strongly bimodal grain size distribution, such as electrical steels (so-called Goss texture) or heterogeneous recrystallization phenomena.
In parallel with these in-situ characterization techniques, we are also exploring various metallurgical schemes to control grain boundary instability. While the current study is focused on a nanocrystalline Ni-based alloy, this technique could also be applied to emerging ‘thermodynamically stabilized’ alloys where grain boundary segregation is thought to eliminate the driving force for boundary motion [Abdeljawad and Foiles, Acta Materialia, 2015]. Such a stabilization scheme may one day offer nanocrystalline metals that are impervious or highly resistant to fatigue crack initation.