The microstructure length scale of strain rate sensitivity in 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, UK
Samantha Dalya) Department of Mechanical Engineering, The University of Michigan, Ann Arbor, Michigan 48109, USA; and Department of Materials Science & Engineering, The University of Michigan, Ann Arbor, Michigan 48109, USA (Received 2 September 2014; accepted 24 February 2015)
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 2.5 105 s1 and 3.0 103 s1. 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.
I. INTRODUCTION
Ultrafine-grained (UFG) metals possess an exceptional combination of mechanical properties including considerable ductility and higher strength as compared to their coarse grained counterparts.1–4 While the strength increase is a result of the high fraction of grain boundaries impeding dislocation motion as described by the Hall– Petch relationship, the high ductility is generally associated with an enhanced strain rate sensitivity.5–10 This research was motivated by the need to provide a better understanding of the relationship between this enhanced strain rate sensitivity and the material microstructure. UFG metals are commonly produced by the severe plastic deformation technique of equal-channel angular pressing (ECAP).1,11–15 ECAP-processed Al has a shear/ torsion texture16–18 and consists primarily of ultrafine
Contributing Editor: Jürgen Eckert a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2015.58 J. Mater. Res., Vol. 30, No. 7, Apr 14, 2015
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grains separated by high-angle grain boundaries (HAGBs).19 But it may also contain “supergrains” (defined by Davidson20 as groups of grains in which the slip planes in adjacent
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