Influence of bulk pre-straining on the size effect in nickel compression pillars A.S. Schneider a,n , D. Kiener b , C.M. Yakacki c , H.J. Maier d , P.A. Gruber e , N. Tamura f , M. Kunz f , A.M. Minor g , C.P. Frick h a INM—Leibniz Institute for New Materials, Campus D2 2, 66123 Saarbr¨ ucken, Germany b University of Leoben, Department of Materials Physics, Jahnstr. 12, 8700 Leoben, Austria c Department of Mechanical Engineering, University of Colorado Denver, Denver 80217, USA d University of Paderborn, Lehrstuhl f¨ ur Werkstoffkunde (Materials Science), 33098 Paderborn, Germany e Karlsruhe Institute of Technology, Institute for Applied Materials, Kaiserstr. 12, 76131 Karlsruhe, Germany f Advanced Light Source (ALS), Lawrence Berkeley National Laboratory (LBNL), 1 Cyclotron Road, Berkeley, CA 94720, USA g Department of Materials Science and Engineering, University of California, Berkeley, and National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA h University of Wyoming, Mechanical Engineering Department, 1000 East University Avenue, Laramie, WY 82071, USA article info Article history: Received 17 May 2012 Received in revised form 9 August 2012 Accepted 13 August 2012 Available online 19 August 2012 Keywords: Nickel Plasticity Dislocations Size effect Micropillar compression Electron microscopy abstract Micro-compression tests were performed on pre-strained nickel (Ni) single crystals in order to investigate the influence of the initial dislocation arrangement on the size dependence of small-scale metal structures. A bulk Ni sample was grown using the Czochralski method and sectioned into four compression samples, which were then pre-strained to nominal strains of 5, 10, 15 and 20%. Bulk samples were then characterized using transmission electron microscopy (TEM), micro-Laue diffrac- tion, and electron backscatter diffraction. TEM results show that a dislocation cell structure was present for all deformed samples, and Laue diffraction demonstrated that the internal strain increased with increased amount of pre-straining. Small-scale pillars with diameters from 200 nm to 5 mm were focused ion beam (FIB) machined from each of the four deformed bulk samples and further compressed via a nanoindenter equipped with a flat diamond punch. Results demonstrate that bulk pre-straining inhibits the sample size effect. For heavily pre-strained bulk samples, the deformation history does not affect the stress–strain behavior, as the pillars demonstrated elevated strength and rather low strain hardening over the whole investigated size range. In situ TEM and micro-Laue diffraction measure- ments of pillars confirmed little change in dislocation density during pillar compression. Thus, the dislocation cell walls created by heavy bulk pre-straining become the relevant internal material structure controlling the mechanical properties, dominating the sample size effect observed in the low dislocation density regime. & 2012 Elsevier B.V. All rights reserved. 1. Introduction It is well known that microstructural features which influence dislocation motion dictate mechanical strength in metals [1]. However, as specimen dimensions approach the micron and nanometer regime, metals that are manufactured such that they have a pristine microstructure demonstrate yield strength values near the theoretical strength [2–6]. In such cases, it is believed that these ultra-high stress values are associated with dislocation nucleation in an otherwise defect-free crystal [3,5–8]. Often it is observed that once this strength has been reached, the stress required to continue plastic deformation decreases drastically [2]. However, fabrication processes that produce relatively defect-free metals represent only a fraction of the possible microstructures over this size scale. A wide range of small-scale manufacturing techniques continue to emerge, which inherently produce micro- structural features likely to dominate dislocation nucleation and motion. A powerful technique used for small-scale deformation studies is the compression of focused ion beam (FIB) manufactured micro/ nano-pillars (e.g. see recent reviews [9–11]). To date, this techni- que has primarily been used to test single crystal samples cut from bulk face centered cubic (FCC) metals, ranging in diameter from tens of microns down to hundreds of nanometers [12–17]. Experimental results have clearly shown mechanical strength values well below the theoretical strength, although a strong correlation to the pillar diameter has been observed. As diameter decreases, the relationship that best fits the collective yield strength data is approximately s y  d 0.6 [9]. However, deforma- tion characteristics inherent to FIB manufactured compression Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/msea Materials Science & Engineering A 0921-5093/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2012.08.055 n Corresponding author. Tel.: þ49 681 9300 312; fax: þ49 681 9300 279. E-mail address: Andreas.schneider@inm-gmbh.de (A.S. Schneider). Materials Science & Engineering A 559 (2013) 147–158