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INTRODUCTION

EFFECTS of specimen size on the mechanical behavior of amorphous metals have recently attracted attention following the extensive work done on crystalline metals during the last decade. Plastic deformation in crystalline metals occurs by the nucleation, motion, and multiplication of dislocations. Therefore, size effects tend to become manifest when some characteristic length scale associated with dislocations (e.g., their mean free path) becomes comparable to a characteristic dimension of the specimens. Size effects have been observed in crystalline metals both in the presence[1–4] H. BEI, Research Staff Member, is with the Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN, 37831-6115. Contact e-mail: [email protected] Z.P. LU, Professor, is with the State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing, 100083 P.R. China. S. SHIM, Senior Researcher, is with the Steel Structure Research Division, Research Institute of Industrial Science and Technology, Gyunggi 445-813, Korea. G. CHEN is Visiting Scientist, Materials Science and Technology Division, Oak Ridge National Laboratory, and Professor, Engineering Research Center of Materials Behavior and Design, Ministry of Education, Nanjing University of Science and Technology, Nanjing 210094, P.R. China. E.P. GEORGE is Distinguished Research Staff Member, Materials Science and Technology Division, Oak Ridge National Laboratory, and Professor, Department of Materials Science and Engineering, The University of Tennessee, Knoxville, TN 37996-2200. This article is based on a presentation given in the symposium ‘‘Bulk Metallic Glasses VI,’’ which occurred during the TMS Annual Meeting, February 15–19, 2009, in San Francisco, CA, under the auspices of TMS, the TMS Structural Materials Division, TMS/ASM: Mechanical Behavior of Materials Committee. Article published online September 23, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A

and absence[5–13] of plastic strain gradients. In bulk metallic glasses (BMGs), there is no dislocation-related internal length scale that can give rise to a size effect. Therefore, the underlying mechanisms of size effects in BMGs would be quite different from those in crystalline materials. There are two classes of BMG specimens that have been investigated by uniaxial compression to study size effects. One class consists of micropillars with diameters in the micrometer range.[14–20] For example, using the focused ion beam (FIB) milling technique, Lee et al.[15] carried out room-temperature compression tests on 1- to 4-lm micropillars of Mg65Cu25Gd10 and observed a 60 to 100 pct increase in strength compared to bulk specimens. In contrast, Schuster et al.[16] conducted a similar test on ~2-lm Pd40Ni40P20 micropillars prepared using the FIB technique and found only a modest (9 pct) increase in the 0.2 pct offset yield strength compared to bulk specimens. They attributed this increase to defects rather than any intrinsic size dependence. Another class con