Size-dependent strength in nanolaminate metallic systems
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The effect of layer thickness on the hardness of nanometallic material composites with both coherent and incoherent interfaces was investigated using nanoindentation. Then, atomistic simulations were performed to identify the critical deformation mechanisms and explain the macroscopic behavior of the materials under investigation. Nanocomposites of different individual layer thicknesses, ranging from 1–30 nm, were manufactured and tested in nanoindentation. The findings were compared to the stress–strain curves obtained by atomistic simulations. The results reveal the role of the individual layer thickness as the thicker structures exhibit somehow different behavior than the thinner ones. This difference is attributed to the motion of the dislocations inside the layers. However, in all cases the hybrid structure was the strongest, implying that a particular improvement to the mechanical properties of the coherent nanocomposites can be achieved by adding a body-centered cubic layer on top of a face-centered cubic bilayer.
I. INTRODUCTION
Understanding the mechanical properties of nanoscale composite thin films has become an increasingly important topic. The potential applications of nanoscale thin films cover a broad range of fields, including fuel cells, solar technology, flexible electronics, and even biomechanics. Since all the aforementioned applications require materials with their characteristic length being at the same scale as their microstructure, the mechanical behavior of these new materials will be significantly different than their bulk counterparts. Several studies (both computational and experimental) on copper (Cu)/ niobium (Nb) and Cu/nickel (Ni) multilayers1–7 have shown that, when the individual layer thickness approaches a few nanometers, the films exhibit yield or flow strengths in excess of 2 GPa, significantly higher than that of bulk Cu, Ni, or Nb. These multilayers are “stacked” to form structures with a fraction of millimeter’s total thickness and strengths that approach the theoretical limit. However, despite experimental evidence showing high strengths, the understanding of the underlying mechanisms responsible for this unique behavior is still limited. Better comprehension of the mechanical behavior of this class of materials and its dependence on the layer thickness is crucial in the development of enhanced and more reliable products. Historically, size effects in polycrystalline materials have been modeled using the Hall–Petch relation where the material strength depends on the spacing of barriers to a)
Address all correspondence to this author. e-mail: [email protected] This paper has been selected as an Invited Feature Paper. DOI: 10.1557/jmr.2011.120 J. Mater. Res., Vol. 26, No. 10, May 28, 2011
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dislocation motion described by the grain size diameter or the film thickness and a material specific strengthening constant. This relation is based on the assumption that the strengthening is caused by the pileup of dislocations against the
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