The effect of microstructure heterogeneity on the microscale deformation of ultrafine-grained aluminum
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Jittraporn Wongsa-Ngam Department of Mechanical Engineering, Faculty of Engineering, King Mongkut’s Institute of Technology Ladkrabang, Bangkok 10520, Thailand
Terence G. Langdon Departments of Aerospace & Mechanical Engineering and Materials Science, University of Southern California, Los Angeles, California 90089-1453, USA; and Materials Research Group, Faculty of Engineering and the Environment, University of Southampton, Southampton SO17 1BJ, United Kingdom
Samantha Dalya) Department of Materials Science & Engineering, The University of Michigan, Ann Arbor, Michigan 48109, USA; and Department of Mechanical Engineering, The University of Michigan, Ann Arbor, Michigan 48109, USA (Received 22 April 2014; accepted 21 July 2014)
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. I. INTRODUCTION
Ultrafine-grained (UFG) metals have an average equiaxed grain size of between 100 nm and ;1 lm,1–3 and are of interest due to their ability to achieve increased strength over their coarse-grained counterparts while maintaining significant ductility.3–6 These unique properties make UFG materials interesting from a fundamental perspective and attractive for numerous commercial applications.7,8 One of the most common procedures for producing UFG metals is equal-channel angular pressing (ECAP).3,9–13 ECAP processing leads to a strong torsion texture in the resulting UFG material14–16 with the final microstructure, which may be inhomogeneous,15–18 strongly influenced by the initial coarse-grained microstructure.19,20 The microstructure after four passes can consist of ultrafine grains separated by primarily high angle grain boundaries a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2014.207 1664
J. Mater. Res., Vol. 29, No. 15, Aug 14, 2014
http://journals.cambridge.org
Downloaded: 28 Mar 2015
(HAGBs),21 separated by primarily low angle grain boundaries (LAGBs), or arranged in bands that have been designated in prior literature as “deformat
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