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In this communication we have reported experimental evidence of a change of behavior in plastic deformation with the structure dimension. It appears that dislocation activity is less localized as material confinement increases. An analytical model providing deformation mechanism maps is proposed, which is further investigated by using numerical simulation of dislocation dynamics. With decreasing layer thickness, both the effect of more and more discrete pile ups of dislocations and a decrease in the interface strength can explain the deviation at small scale from the Hall±Petch law. The softening observed in ultrafine Cu/Ni (h < 50 ) is explained by interdiffusion effects. ExperimentalIn-situ reflective high-energy electron diffraction was used in an original way to access both in-plane and out-of-plane lattice parameters during growth [15], and therefore to give information about the stress relaxation state of the layer. These measurements were in good agreement with the in-situ laser curvature method for residual stress measurements [7]. Local probe microscopy such as scanning tunneling microscopy (STM) and atomic force microscopy (AFM) was used to characterize the roughness and the grain size of the deposited films. Auger spectroscopy on wedge deposited films was used to measure the composition profile across the interfaces. This was coupled with X-ray diffraction techniques using the Bragg±Brentano setup. The periodicity of the layer structure give rises to a satellite peak and the fit to experimental data of the recorded spectrum gives access to layer roughness and composition profiles. We used high-resolution TEM to study the interface structure (misfit dislocations) and the layer structure (Cu is BCC in a Cu/Nb multilayer for h £ 2 nm) [16]. TEM in-situ deformation observations were also carried out on these small-scale structures to study deformation mechanisms [17].The mechanical properties were measured by nanoindentation techniques using an MTS Nanoindenter in continuous stiffness mode. For testing films, stiff and hard substrates are required: hence the use of Si in our experiments, so that there should be no interference from the substrate. The Berkovich indenters (triangular prism) were calibrated on Al and fused silica using the Oliver±Pharr procedure [18]. AFM was used to image the remaining indent and characterize the area of contact. Nanoindentation was used in different ways: to measure elastic and plastic properties (Young's modulus and hardness), and to determine the strain-rate sensitivity at room temperature. Moreover, we designed specific experiments to observe the slip patterns of deep indents in the films. Tensile tests on free standing films were also performed [19] and dedicated experiments on dislocation injections from single crystal substrates were done [6].Applications of reinforced composites and heterogeneous solids are widespread, spanning technological areas of various aerospace and mechanical industries. A real challenge concerning these materials is their life time predictio...