A comparison of microtensile and microcompression methods for studying plastic properties of nanocrystalline electrodepo
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A comparison of microcompression and microtensile methods to study mechanical properties of electrodeposited nanocrystalline (nc) nickel has been performed. Microtensile tests that probe a volume of more than 2 × 106 m3 show reasonable agreement with results from microcompression tests that probe much smaller volumes down to a few m3. Differences between the two uniaxial techniques are discussed in terms of measurements errors, probed volume and surface effects, strain rate, and influence of stress state. Uniaxial solicitation in compression mode revealed several advantages for studying stress–strain properties.
I. INTRODUCTION
In recent years, the LIGA process (combines x-ray lithography with electroplating and plastic molding) for developing structures from nano- to micrometer sizes has become a routine process.1,2 The reasons for the success of the galvanoforming method in the microelectromechanical systems (MEMS) industry are numerous and include the fact that metallic pieces with a wellcontrolled microstructure, high aspect ratio, and excellent spatial resolution can be obtained. This opens new horizons in research and industry, with possibilities for changing mechanical properties by tailoring microstructure and alloying elements. For a successful application of such microscopic devices, a detailed understanding of their mechanical properties (at small length scales) is indispensable.3 Device design requires the knowledge of the stress–strain behavior of a material over a wide range of strains.4 The accurate determination of complete stress–strain curves on the microscopic scale is therefore of vital interest. Mechanical properties determined with standard, established materials testing procedures at the macroscale cannot be easily extrapolated to smaller length scales. In particular, the errors of the measuring technique are difficult to evaluate if there is a convolution with the socalled extrinsic size effect of the samples.5 Along with nanoindentation measurements (which are, however, mainly used for the determination of hardness and Young’s modulus), other techniques to test materials at a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0162 J. Mater. Res., Vol. 23, No. 5, May 2008
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micrometer scales are emerging, such as microtensile6,7 and microcompression tests.8,9 Size effects in monocrystalline materials were evidenced in the past using mainly nanoindentation and microcompression tests.9,10 Although size effects related to internal length scales of materials are widely observed and simulated11,12 and used for industrial applications (i.e., improvement of a metal yield strength by grain size refinement), there remains a fundamental challenge to systematically investigate external length-scale effects related to the probed volume in the submillimeter to nanometer regime. External length-scale effects may be observed at multiple stages over this wide range of sizes, because the mechanisms ass
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