Nanoindentation investigation of the mechanical behaviors of nanoscale Ag/Cu multilayers
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The microstructure, hardness, elastic modulus, and indentation creep of Ag/Cu multilayers prepared by magnetron sputtering were investigated by x-ray diffraction, transmission electron microscopy, and nanoindentation. The hardness values obey the Hall–Petch relationship as the periodicity decreases to 20 nm. For multilayers with periodicity smaller than 20 nm, the Hall–Petch relationship breaks down and the hardness values saturate at about 4.6 GPa; moreover, there are shear bands formed around their indents and strain bursts occurring during the load-holding process of indentation creep. These results imply that there is a transition of the deformation mechanism in the region where the periodicity is equal to 20 nm. This transition of the deformation mechanism can be ascribed to grain-size-dependent competition between the dislocations-mediated plasticity and grain-boundary sliding-mediated plasticity.
I. INTRODUCTION
Nanocrystalline materials exhibiting outstanding physical and mechanical properties have been studied intensively over the past couple of decades. Significant advancements have been made in the understanding of their fundamental properties, especially in the last few years. 1 In particular, the mechanical properties of nanocrystalline materials have received considerable attention in recent years.2,3 For materials with such smallfeature grain size ( 20 nm, there are no SBs formed around the indents. This reveals a tendency that SBs are more easily formed in multilayer with smaller periodicity. Moreover, SBs only occur in
multilayers with ⌳ 艋 20 nm, and it may be correlated to the breakdown of the Hall–Petch relationship. Indentation creep was also carried out to investigate the deformation mechanism. Although the use of indentation creep to quantify creep behavior is a bit risky, considering the complex and transient state of stress under the indentation,22 it may give some qualitative information on deformation mechanism. All of the samples were loaded with a constant strain rate (0.05 s−1) to a predetermined depth of 200 nm, and then the load was held constant for 10 min to record the creep deformation. Two kinds of typical indentation creep curve are shown in Figs. 8(a) and 8(b) for multilayers with ⌳ ⳱ 10 and 50 nm, respectively. As a whole, there is an initial sharp rise in creep depth at the early part of the creep curves followed by a region showing a smaller and steady rate of increase in creep depth. The general profile of these curves is similar to the strain-versus-time plot obtained from uniaxial tensile creep testing of bulk materials subjected to power law creep. However, there is a strain burst in the creep curve for a multilayer with ⌳ ⳱ 10 nm. The creep curves of multilayers with ⌳ ⳱ 4 and 20 nm also have this kind of phenomenon, while creep curves of multilayers with ⌳ > 20 nm are continuous. Moreover, for multilayers with ⌳ ⳱ 4 and 10 nm, strain bursts occurred in all 10 creep curves; for a multilayer with ⌳ ⳱ 20 nm, strain burst only occurred in 6 of 10 creep curves. This implies
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