An improved long-term nanoindentation creep testing approach for studying the local deformation processes in nanocrystal
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The strain-rate sensitivity of ultrafine-grained aluminum (Al) and nanocrystalline nickel (Ni) is studied with an improved nanoindentation creep method. Using the dynamic contact stiffness thermal drift influences can be minimized and reliable creep data can be obtained from nanoindentation creep experiments even at enhanced temperatures and up to 10 h. For face-centered cubic (fcc) metals it was found that the creep behavior is strongly influenced by the microstructure, as nanocrystalline (nc) as well as ultrafine-grained (ufg) samples show lower stress exponents when compared with their coarse-grained (cg) counterparts. The indentation creep behavior resembles a power-law behavior with stress exponents n being ; 20 at room temperature. For higher temperatures the stress exponents of ufg-Al and nc-Ni decrease down to n ; 5. These locally determined stress exponents are similar to the macroscopic exponents, indicating that similar deformation mechanisms are acting during indentation and macroscopic deformation. Grain boundary sliding found around the residual indentations is related to the motion of unconstrained surface grains.
I. INTRODUCTION
Nanocrystalline (nc)- and ultrafine-grained (ufg)-metals show exceptional mechanical behavior especially regarding their enhanced strength paired with sufficient ductility.1,2 The increased strength is caused by the small grain size and thus the Hall-Petch strengthening, whereas the sufficient level of ductility is explained by the enhanced strainrate sensitivity of ufg face-centered cubic (fcc) metals like aluminum (Al) and nickel (Ni). The enhanced number of grain boundaries directly influences the time-dependent material behavior by either grain boundary sliding and grain boundary migration processes,3 or the evolution of a dislocation structure and thermally activated processes.2,4 However, the specific deformation process that dominates at different deformation conditions is not fully understood. With conventional uniaxial macroscopic testing, these processes can only be studied on a macroscopic scale,2,5,6 averaging over many grains. However, nanoindentation testing is a useful tool for studying these effects on a local scale, allowing a detailed localized analysis of the deformation processes.7 Besides focusing on the local determination of the strain-rate sensitivity,8–11 the development of reliable nanoindentation creep experiments has gained much attention over the last decades. In literature, references can be found relating to studies of different indentation a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2013.39 J. Mater. Res., Vol. 28, No. 9, May 14, 2013
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procedures concerning the relationship between indentation strain-rate and hardness.7,12 The most common experiments are constant-load indentation creep tests at room temperature (RT), where the maximum load is held constant for a preset time between 1 min and 1 h, while the indentation depth is monitored
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