Heterogeneous dislocation nucleation from surfaces and interfaces as governing plasticity mechanism in nanoscale metals
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We report the results of constant strain rate experiments on electroplated, single crystalline copper pillars with diameters between 75 and 525 nm. At slow strain rates, 103 s1, pillar diameters with 150 nm and above show a size-dependent strength similar to previous reports. Below 150 nm, we find that the size effect vanishes as the strength transitions to a relatively size-independent regime. Strain rate sensitivity and activation volume are determined from uniaxial compression tests at different strain rates and corroborate a deformation mechanism change. These results are discussed in the framework of recent in situ transmission electron microscopy experiments observing two distinct deformation mechanisms in pillars and thin films on flexible substrates: partial dislocation nucleation from stress concentrations in smaller structures and single arm source operation in larger samples. Models attempting to explain these different size-dependent regimes are discussed in relation to these experiments and existing literature revealing further insights into the likely small-scale deformation mechanisms.
I. INTRODUCTION
Investigations into strengthening mechanisms in metals have demonstrated many different pathways in which the same elemental material can increase its strength. In large systems, standard examples include work hardening, whereby the strength increases due to the evolving dislocation density through the Taylor relation pffiffiffi r } lb q,1 and decreasing grain size to ;100 nm, whereby strengthening occurs via dislocation pile-ups against grain boundaries, known as the Hall–Petch mechanism.2,3 In addition, in the last ;5 years it has been demonstrated that strengthening in metals can be achieved by reducing sample dimensions to the micro- and nanoscale.4–7 Specifically, single crystalline metals containing initial dislocations have been shown to attain much greater strengths compared with bulk as a result of one or more of their external dimensions decreasing to the micron and below dimensions.8 At these small length scales, the strength of cylindrical metallic pillars under uniaxial compression and tension scales with their critical dimensions as r } Dn, where r is the pillar flow stress at some characteristic strain, D is the pillar diameter, and n is a value found to lie between 0.5 and 1.0 for facecentered cubic (FCC) metal.4–7,9–12 In addition to these “1-dimensional” samples, single crystalline metallic thin films on flexible substrates exhibit similar size-dependent
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Address all correspondence to this author. e-mail: [email protected] This paper has been selected as an Invited Feature Paper. DOI: 10.1557/jmr.2011.338 J. Mater. Res., Vol. 26, No. 22, Nov 28, 2011
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mechanical properties, whereby their flow strength scales with the film thickness in a power-law fashion, with the exponent of approximately 0.5.13,14 At both the sub-micron and micron scales, several groups have proposed that collective dislocation behavior is responsible for the size
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