@article {1346,
	title = {Fatigue deformation in a polycrystalline nickel base superalloy at intermediate and high temperature: Competing failure modes},
	journal = {Acta Materialia},
	volume = {152},
	year = {2018},
	pages = {16{\textendash}33},
	abstract = {<p>
	The microstructural configurations that favor early strain localization and fatigue crack initiation at intermediate and high temperature (400 \&deg;C\&ndash;650 \&deg;C) have been investigated using novel experimental techniques, including high resolution digital image correlation and transmission scanning electron microscopy. Cyclic fatigue experiments in the high and low cycle fatigue regimes have been performed on a Ren\&eacute; 88DT polycrystalline nickel-base superalloy at temperatures up to 650 \&deg;C and compared to previous fatigue results obtained from tests in the very high cycle fatigue regime. Competing failure modes are observed along with an inversion in the temperature fatigue life dependence of fatigue strength from the low to high cycle fatigue regime. Oxidation-assisted processes are dominant at high applied stresses while cyclic plastic localization and accumulation govern fracture at low applied stresses. In addition, a second competing mode exists in the high and very high cycle fatigue regime from non-metallic inclusions as compared to internal intrinsic initiation sites. The grain-scale features that exhibit strain localization and crack initiation were investigated in detail. Transmission electron microscopy (TEM), transmission scanning electron microscopy (TSEM) and electron channeling contrast imaging have been conducted on samples removed from targeted regions with microstructural configurations that favor crack initiation to characterize the associated dislocation sub-structure and its evolution with temperature. Plasticity is observed to be less localized during cyclic loading at high temperature compared to room temperature. The microstructural features that drive initiation across the temperature range investigated are: twin-parent grains pairs that are at the upper end of the size distribution, are oriented for near maximum elastic modulus mismatch, and have high stresses along planes parallel to the twin boundaries.
</p>
},
	keywords = {Coherent twin boundary, Elastic anisotropy, Fatigue crack initiation at intermediate and high temperature, High resolution digital image correlation, Microstructural effect, Polycrystalline microstructure, Ren{\'e} 88DT polycrystalline superalloy, Strain localization},
	issn = {13596454},
	doi = {10.1016/j.actamat.2018.03.035},
	url = {https://doi.org/10.1016/j.actamat.2018.03.035},
	author = {Stinville, Jean Charles and Martin, Etienne and Karadge, Mallikarjun and Ismonov, Shak and Soare, Monica and Hanlon, Tim and Sundaram, Sairam and Echlin, Mclean P. and Callahan, Patrick G. and Lenthe, William C. and Miller, V. M. and Miao, Jiashi and Wessman, Andrew E. and Finlay, Rebecca and Loghin, Adrian and Marte, Judson and Pollock, Tresa M.}
}
@article {1486,
	title = {Femtosecond laser rejuvenation of nanocrystalline metals},
	journal = {Acta Materialia},
	volume = {156},
	year = {2018},
	pages = {183{\textendash}195},
	abstract = {<p>
	Nanocrystalline metals are distinct from traditional engineering materials due to their high concentration of grain boundaries and corresponding structural disorder at grain boundaries. The effect of local disorder in nanocrystalline materials manifests in ways reminiscent of fully amorphous materials, such as mesoscale shear localization and pressure-dependent yielding, owing to the high concentration of grain boundaries and their predominance in governing plasticity. Relaxation processes in nanocrystalline materials that facilitate reconfigurations of grain boundaries and lower their energy, such as low temperature annealing, have been shown to enhance mechanical strength. However, processes that raise the energy of a nanocrystalline metal have not been observed, limiting the tunability of properties and the prospect for suppressing shear localization. Here, we use femtosecond laser processing as a unique non-equilibrium process that can generate complex stress states due to ultrafast electronic excitation and subsequent relaxation events. Experiments on nanocrystalline Al-O and Cu-Zr alloys indicate that sub-ablation femtosecond laser pulses cause up to an 87\% reduction in hardness with no change in grain size, which can be ascribed to grain boundary-mediated processes. Parallels between our results and rejuvenation processes in glassy systems will be discussed in the context of controlling metastable structural configurations through novel processing routes.
</p>
},
	keywords = {Femtosecond lasers, Grain boundaries, Nanocrystalline, Rejuvenation},
	issn = {13596454},
	doi = {10.1016/j.actamat.2018.06.027},
	url = {https://doi.org/10.1016/j.actamat.2018.06.027},
	author = {Balbus, Glenn H. and Echlin, Mclean P. and Grigorian, Charlette M. and Rupert, Timothy J. and Pollock, Tresa M. and Gianola, Daniel S.}
}
