Importance of dislocation pile-ups on the mechanical properties and the Bauschinger effect in microcantilevers
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C. Kirchlechnera) Max-Planck-Institut für Eisenforschung, Düsseldorf 40237, Germany; and Montanuniversität Leoben, Department Materialphysik, Leoben 8700, Austria
R. Pippan Erich Schmid Institute of Material Science, Austrian Academy of Sciences, Leoben 8700, Austria
G. Dehm Max-Planck-Institut für Eisenforschung, Düsseldorf 40237, Germany; Montanuniversität Leoben, Department Materialphysik, Leoben 8700, Austria; and Erich Schmid Institute of Material Science, Austrian Academy of Sciences, Leoben 8700, Austria (Received 21 October 2014; accepted 28 January 2015)
Copper microcantilevers were produced by focused ion beam milling and tested in situ using a scanning electron microscope. To provide different interfaces for piling up dislocations, cantilevers were fabricated to be single crystalline, bicrystalline, or single crystalline with a slit in the region of the neutral axis. The aim of the experiment was to study the influence of dislocation pile-ups on (i) strength and (ii) Bauschinger effects in micrometer-sized, focused ion beam milled bending cantilevers. The samples were loaded monotonically for several times under displacement control. Even though the cantilevers exhibited the same nominal strain gradient the strength varied by 34% within the three cantilever geometries. The Bauschinger effect can be promoted and prohibited by the insertion of different interfaces.
I. INTRODUCTION
Mechanical properties of micrometer-sized, single crystalline copper bending beams are strongly influenced by the arrangement of geometrically necessary dislocations (GNDs) leading to two main phenomena inherent to the bending geometry: (i) size effects of the flow stress1,2 and (ii) a pronounced Bauschinger effect or nonlinear elastic unloading behavior.3–5 Size effects and the importance of internal material length scales are well known and have been widely discussed6,7 and investigated over several decades. Studies have also been performed for geometries with inherent strain gradients.8 For instance, Stölken and Evans9 performed the first foil bending of 12.5–50 lm thick Ni foils to study the influence of length scales on the mechanical behavior, which had been found to be in the range of 3–5 lm in pure Ni foils. The flow stress increase with reduced foil thickness was well explained by strain gradient plasticity.8 Recently, the possibility to produce micrometer- and submicrometer-sized samples Contributing Editor: Yang-T. Cheng a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2015.49 J. Mater. Res., Vol. 30, No. 6, Mar 28, 2015
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by focused ion beam (FIB) milling and to test them either ex situ or in situ allows for sample size effects in a broad context down to the submicrometer range to be studied. These types of experiments have been commonly performed over the last decade in micro torsion,8,10 micro compression,11,12 micro tensile,13,14 and micro bending1,2 geometries. Motz et al.1 were the first who applied micro bending onto s
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