Correlation between dislocation density and pop-in phenomena in aluminum studied by nanoindentation and electron channeling contrast imaging Afrooz Barnoush, a, * Markus T. Welsch b and Horst Vehoff a a Saarland University, Department of Materials Science, Bldg. D22, PO Box 151150, D-66041 Saarbruecken, Germany b Brueck GmbH, Brueckstraße 16, D-66131 Saarbruecken, Germany Received 24 March 2010; revised 21 April 2010; accepted 29 April 2010 Available online 5 May 2010 A combination of electron channeling contrast imaging and nanoindentation tests was conducted to reveal the intercon- nected nature of the pop-in (yield point phenomena) with the underlying dislocation substructure in aluminum. Using digital image processing, it was possible to reveal the dislocation substructure in a bulk sample. It is shown that a very low dislocation density is necessary to observe the pop-in, and at high dislocation density the dislocation sources become activated instead of dislocation nucleation. Ó 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Dislocation structure; Homogeneous nucleation; Yield phenomena; Aluminum Recently, nanoindentation has been recognized as the most appropriate method of material testing to quantify a characteristic length of the scale dependency [1]. Nanoindentation experiments have become a stan- dard method to measure the mechanical properties of small volumes of materials, particularly properties such as hardness and elastic modulus. An indenting tip made of a hard material and having a defined geometrical shape (usually spherical) is placed in contact with the surface of the material being studied. In some metals, the initial indentation behavior is completely elastic, with fully reversible loading [2]. At some point, as the load increases, the material undergoes irreversible plas- tic deformation that, in load-controlled-instrumented indentation, manifests as a ‘‘pop-in,’’ or excursion in depth. The distinctive finding of pop-in observed in the load displacement (L–D) curves commonly thought to be linked to surface oxide breakdown [3–6] or dislo- cation emission phenomena, particularly homogenous dislocation nucleation [7–12], dislocation source activa- tion [13–15], and point defect source (i.e., a vacancy) activation [16,17]. Plastic deformation of metal results in an increase in dislocation density, which should have an effect on pop-in behavior if pop-in is controlled only by dislocation nucleation or dislocation source activa- tion. Unfortunately, available methods for evaluation of dislocation density in bulk metals are destructive and not applicable for all metals. One exceptional non- destructive method for qualitative evaluation of disloca- tion density near surface regions of metal is the electron channeling contrast imaging (ECCI) technique using a scanning electron microscope (SEM). The electron channeling effect denotes that the intensity of the back- scattered signal depends on the angle between the inci- dent electron beam and the crystal lattice. Under ECCI conditions (low working distance, high magnifica- tion, and 25 kV acceleration voltage) the tilting of the beam during scanning is negligible, and therefore con- trast arises from the tilting in the crystal lattice. This tilt- ing can be caused by distortions of the lattice, e.g. orientation differences in single grains or local lattice tilting by means of accumulations of lattice defects, dis- location bundles, or grain boundaries. Thus, the ECCI measurements in SEM are directly comparable with transmission electron microscopy (TEM) images, but al- low investigations on bulk samples instead of thinned samples. In order to determine the source of pop-in during nanoindentation, a set of experiments was performed on high purity aluminum in annealed and cyclic de- formed conditions. The dislocation sub-microstructure 1359-6462/$ - see front matter Ó 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2010.04.048 * Corresponding author; E-mail: a.barnoush@matsci.uni-sb.de Available online at www.sciencedirect.com Scripta Materialia 63 (2010) 465–468 www.elsevier.com/locate/scriptamat