Nanoindentation strain-rate jump tests for determining the local strain-rate sensitivity in nanocrystalline Ni and ultra

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in­rate jump tests for determining the local strain­ rate sensitivity in nanocrystalline Ni and ultrafine­grained Al Verena Maier, Karsten Durst, Johannes Mueller, Björn Backes, Heinz Werner Höppel and Mathias Göken Journal of Materials Research / Volume 26 / Issue 11 / 2011, pp 1421 ­ 1430 DOI: 10.1557/jmr.2011.156

Link to this article: http://journals.cambridge.org/abstract_S0884291411001567 How to cite this article: Verena Maier, Karsten Durst, Johannes Mueller, Björn Backes, Heinz Werner Höppel and Mathias Göken (2011).  Nanoindentation strain­rate jump tests for determining the local strain­rate sensitivity in nanocrystalline Ni and ultrafine­ grained Al. Journal of Materials Research,26, pp 1421­1430 doi:10.1557/jmr.2011.156 Request Permissions : Click here

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Nanoindentation strain-rate jump tests for determining the local strain-rate sensitivity in nanocrystalline Ni and ultrafine-grained Al Verena Maier, Karsten Durst,a) Johannes Mueller, Björn Backes, Heinz Werner Höppel, and Mathias Göken Department of Materials Science and Engineering, Institute 1: General Materials Properties, University Erlangen-Nuremberg, 91058 Erlangen, Germany (Received 10 February 2011; accepted 13 April 2011)

A nanoindentation strain-rate jump technique has been developed for determining the local strainrate sensitivity (SRS) of nanocrystalline and ultrafine-grained (UFG) materials. The results of the new method are compared to conventional constant strain-rate nanoindentation experiments, macroscopic compression tests, and finite element modeling (FEM) simulations. The FEM simulations showed that nanoindentation tests should yield a similar SRS as uniaxial testing and generally a good agreement is found between nanoindentation strain-rate jump experiments and compression tests. However, a higher SRS is found in constant indentation strain-rate tests, which could be caused by the long indentation times required for tests at low indentation strain rates. The nanoindentation strain-rate jump technique thus offers the possibility to use single indentations for determining the SRS at low strain rates with strongly reduced testing times. For UFG-Al, extremely fine-grained regions around a bond layer exhibit a substantial higher SRS than bulk material.

I. INTRODUCTION

Over the last decades, ultrafine-grained (UFG) and nanocrystalline (NC) materials with grain sizes below 1 lm and 100 nm, respectively, have gained significant attention. These materials exhibit superior strength at room temperature and in some cases enhanced ductility for UFG material. This exceptional behavior is strongly related to their enhanced strain-rate sensitivity (SRS), compared to coarse-grained (CG) and single crystalline (SX) materials.1–4 With conventional uniaxial macroscopic testing like compression4–6 and tensile testing,7–9 the deformation resistance is studied on a macroscopic scale, averaging over many microstructural length scales and features. However, nanoindentat