Enhanced tensile ductility in an electrodeposited nanocrystalline copper
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A fully dense nanocrystalline (nc) Cu with mean grain size of 72 nm and a broad grain size distribution was synthesized by electrodeposition. Uniaxial tensile tests were done at different strain rates and room temperature. A very high strength of 1.04 G was obtained at strain rate of 0.1 s−1. The nearly perfect plasticity with a large strain of close to 20% was displayed at specific low strain rates of 4 × 10−5 to 10−4 s−1. With increasing strain rate, the nearly perfect plasticity disappeared. Strain rate sensitivity and activation volume of the nc Cu were estimated from the flow stress at a fixed strain of 1% and a strain rate change (jump) test. It was deduced from the high strain rate sensitivity exponent of 0.08 and small activation volume of 12b3 that both dislocation and grain boundary activities would take place in this nc Cu, which explained the nearly perfect plasticity observed in the tensile test.
I. INTRODUCTION
It is well known that in polycrystalline metals, an increase in hardness and strength can be obtained by reducing the grain size to the nanometer scale.1 In recent years, ultrahigh strength elemental metals and alloys were gained by creating nc grains.2,3 Their ductility at room temperature, on the other hand, was often disappointingly low compared with their coarse-grained (cg) counterparts.4 The strength and ductility would be optimized by several means.4,5 First, artifact-free material made by one-step methods is a prerequisite for the study of inherent mechanical behavior. Second, a thick and large sample would be necessary to evaluate the tensile properties. For thin samples, a minor surface flaw or even roughness becomes a threat for the initiation of sufficiently large cracks to induce catastrophic failure.6 Third, a grain size distribution with some fraction of grains large enough to sustain dislocation activity has been shown to be effective in providing the needed strain hardening and therefore increased ductility.7 Fourth, significantly elevated strain rate sensitivity is effective for sustaining more plastic strain without severe necking in absence of strain hardening. High strain rate sensitivity of nc metal was frequently reported.8–12 But the proof that high strain rate sensitivity of nc metal contributed to ductility was rarely given. Electrodeposition is well known as a technologically a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0280 2238
http://journals.cambridge.org
J. Mater. Res., Vol. 23, No. 8, Aug 2008 Downloaded: 28 Mar 2015
and economically viable route to produce large scale and high purity nanostructured metals with high m and a broad grain distribution in one step.8,9,13 In the present work, a nc Cu with grain size of 72 nm was synthesized by a direct current electrodeposition technique. Large tensile samples were made to evaluate the tensile properties. A pronounced homogeneous deformation was observed in the tensile deformation, which would be related to the broad grain distribution and high strain rate sensitiv
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