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Author: search.filters.author.Langdon, Terence G.Subject: search.filters.subject.nanostructure

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    The microstructure length scale of strain rate sensitivity in ultrafine-grained aluminum
    (2015-04-14)
    Kammers, Adam D.
    ;
    Wongsa-Ngam, Jittraporn
    ;
    Langdon, Terence G.
    ;
    Daly, Samantha
    The mechanical properties of ultrafine-grained aluminum produced by equal-channel angular pressing (ECAP) are strongly influenced by strain rate. In this work, an experimental investigation of local strain rate sensitivity as it relates to microstructure was performed using a combination of scanning electron microscopy and digital image correlation. Uniaxial tension tests were carried out at 200 °C and strain rates alternating between 2.5 × 10<sup>-5</sup> s<sup>-1</sup> and 3.0 × 10<sup>-3</sup> s<sup>-1</sup>. The results demonstrate that the heterogeneous microstructure generated by ECAP has a strong effect on the microstructure scale strain rate sensitivity. Deformation centered at grain boundaries separating regions of banded microstructure exhibits the greatest strain rate sensitivity. Strain rate sensitivity is limited in deformation occurring in regions of microstructure composed of ultrafine grains separated by low-angle grain boundaries. The tensile specimens all failed by shear bands at 200 °C and at room temperature they failed by necking with little plastic deformation apparent outside of the neck.
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    The effect of microstructure heterogeneity on the microscale deformation of ultrafine-grained aluminum
    (2014-01-01)
    Kammers, Adam D.
    ;
    Wongsa-Ngam, Jittraporn
    ;
    Langdon, Terence G.
    ;
    Daly, Samantha
    A combined approach of scanning electron microscopy and digital image correlation was used to examine microstructure-scale strain localization and active deformation mechanisms in ultrafine-grained (UFG) high purity (99.99%) aluminum processed by equal-channel angular pressing (ECAP). The results from tensile tests demonstrate a strong relationship between the heterogeneous microstructure and strain localization. The localized deformation was investigated in areas that contain significantly different microstructural features typical of ECAP processed aluminum. It was found that areas of the UFG microstructure containing primarily low angle grain boundaries deformed by dislocation slip and behaved similarly to a coarse-grained material. The greatest strain localization occurred at high angle grain boundaries (HAGBs) separating distinct microstructure regions and with median surface trace angles of approximately 26.6°. In areas of banded microstructure, shear strain localization as high as 30% and shear displacements of up to 500 nm occurred at the HAGBs separating bands, suggesting grain boundary sliding. Copyright © 2014 Materials Research Society.