Strain Rate Dependent Behavior of Pure Aluminum and Copper Micro-Wires
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Strain Rate Dependent Behavior Of Pure Aluminum And Copper Micro-Wires P. A. El-Deiry and R. P. Vinci Department of Materials Science and Engineering, Lehigh University Bethlehem, PA 18015, U.S.A.
ABSTRACT In order to shed light on the role that grain boundaries and dislocations play in anelastic relaxation of thin films and small-scale structures, we measured the effective elastic moduli of 99.99% pure Al and Cu 10 µm diameter micro-wires in the as-received (drawn and slight tempered) and annealed states. Moduli were determined using microtensile tests at various strain rates (6.7x10-6 s-1, 1.3x10-5 s-1, 2.6x10-5 s-1, 4.5x10-5 s-1, 2.5x10-4 s-1, 4.5x10-4 s-1). Focused-ion beam scanning electron microscopy was used for imaging grain sizes. Results from the asreceived wires are compared with the annealed wires to illustrate the effects of grain size and dislocation density on effective moduli, which closely relates to grain boundary sliding and dislocation motion, respectively. We conclude that microstructure is more significant than scale in inducing anelasticity in small-scale wires and, by extension, thin films.
INTRODUCTION It is well known that as material dimensions become smaller, physical mechanisms for deformation and failure are sometimes unlike those of bulk materials. One prominent difference between the mechanical behavior of thin films and bulk materials is the low elastic modulus measured under microtensile testing conditions. Many variables such as texture, voids, dislocation microplasticity, reversible microplasticity, highly compliant grain boundaries, grain boundary voiding, and microcracking can cause a reduction of Young’s modulus [1, 2]. Recently, it has been shown that modulus values determined by microtensile testing can depend strongly on strain rate [1, 2, 3]. These results suggest anelastic mechanisms as another source of low moduli, possibly related to grain boundary sliding or dislocation motion. Anelastic deformation is any portion of the total deformation of a body that occurs as a function of time when load is applied and which disappears completely after a period of time when the load is removed [4]. Our experiments were devised to look at the effect of grain structure, dislocation density and geometry on anelastic behavior. To this end, 10 µm diameter 99.99% pure Al and Cu microwires were heat treated in vacuum and tested under uniaxial tension using a microtensile system. To study the effect of geometry, 10 µm diameter 99.99% pure Al micro-wires were annealed at high temperature (350°C) for comparison to previous results obtained from 99.99% pure Al thin films with a similar grain size [2, 3]. The wires measured in this study and the thin films measured in previous studies have similar cross-sectional areas and purities. Nonetheless, there are also several differences: initial dislocation density, texture (preferred orientation along the loading axis vs. perpendicular to it), grain shape, absolute grain size, and the ratio of grain size to film/wire thickness. As-received wires ha
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