Size Effects Associated with Microcompression Experiments on Single-Crystal Magnesium Cynthia M. Byer*, K.T. Ramesh* *Johns Hopkins University, 3400 N. Charles Street, Baltimore, MD 21218 ABSTRACT Microcompression is becoming an increasingly popular technique to investigate the orientation dependence and size effects associated with single crystals under uniaxial compression. Literature shows that for certain materials, as the diameter of these micro-scale pillars decreases, yield stresses and strain hardening rates may increase; however, this phenomenon has not yet fully been investigated for hexagonal close packed (hcp) materials. In this study, microcompression experiments are conducted on micropillars that are fabricated using focused ion beam (FIB) milling. These single crystal magnesium specimens are loaded in compression along the [0001] c-axis, and the stress-strain curves reveal that there are no significant size effects. INTRODUCTION Hexagonal close-packed (hcp) Mg and its alloys are very attractive for automotive and aerospace applications because of their low density (1740 kg m3 for pure Mg). However, due to the low symmetry, the deformation behavior of these materials is much more complicated than that of high-symmetry facecentered cubic (fcc) metals, such as aluminum and copper. Due to this complexity, prior work on the fundamental deformation mechanisms has often been controversial or inconclusive. A related issue is that of size effects in hcp metals, and in Mg in particular. Many microcompression studies on fcc metals have shown that as pillar diameters decrease, the flow stress increases. Decreasing diameters have sometimes been shown also to increase the amount of strain hardening micropillars exhibit. Similar studies have been conducted on bcc metals, as well as on other materials such as alloys, intermetallics, nanocrystalline metals, nanoporous foams, and metallic glasses. The explanation for these phenomena is still under debate. We have performed microcompression experiments on Mg to investigate the possibility of a size effect in hcp metals, given that their deformation processes are very different from those of fcc crystals. EXPERIMENTAL TECHNIQUE Micropillars, ranging in size from 0.6 to 10 micrometers, are fabricated on a 99.999% pure single crystal of magnesium (1 cm in diameter and 2 mm thick), oriented along the [0001] c-axis, using a dual-beam FEI Nova 600 focused ion beam (FIB). Rough pillars are milled using a series of concentric circular cuts with decreasing diameters and currents. However, this fabrication process alone yields Figure 1: (a) Pillar with no taper, made by lathe-type milling. (b) Pillar made by top-down annular milling, resulting in taper. Note the magnifications are not the same for the two images. (a) (b) Proceedings of the SEM Annual Conference June 7-10, 2010 Indianapolis, Indiana USA ©2010 Society for Experimental Mechanics Inc. 41 T. Proulx (ed.), MEMS and Nanotechnology, Volume 2, Conference Proceedings of the Society for Experimental Mechanics Series 2, DOI 10.1007/978-1-4419-8825-6_6, © The Society for Experimental Mechanics, Inc. 2011
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