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Nanostructured Cu/304 stainless steel (SS) multilayers were prepared by magnetron sputtering. 304SS has a face-centered-cubic (fcc) structure in bulk. However, in the Cu/304SS multilayers, the 304SS layers exhibit the fcc structure for layer thickness of 艋5 nm in epitaxy with the neighboring fcc Cu. For 304SS layer thickness larger than 5 nm, body-centered-cubic (bcc) 304SS grains grow on top of the initial 5 nm fcc SS with the Kurdjumov–Sachs orientation relationship between bcc and fcc SS grains. The maximum hardness of Cu/304SS multilayers is about 5.5 GPa (factor of two enhancement compared to rule-of-mixtures hardness) at a layer thickness of 5 nm. Below 5 nm, hardness decreases with decreasing layer thickness. The peak hardness of fcc/fcc Cu/304SS multilayer is greater than that of Cu/Ni, even though the lattice-parameter mismatch between Cu and Ni is five times greater than that between Cu and 304SS. This result may primarily be attributed to the higher interface barrier stress for single-dislocation transmission across the {111} twinned interfaces in Cu/304SS as compared to the {100} interfaces in Cu/Ni.

I. INTRODUCTION

Nanostructured multilayers are made up of alternating nanometer-scale layers of two different materials. The layer thickness can be well controlled in the scale of 1 nm or less by physical vapor deposition. These nanostructured multilayers have novel mechanical, electrical, magnetic, and optical properties.1–3 The mechanical properties of these multilayered composites are of particular interest because the strength of these multilayer composites can significantly be increased to about onehalf to one-third of a lower-bound estimate of the theoretical strength limit of approximately ␮/30, where ␮ is the shear modulus.4 In the micrometer to the submicrometer length scale regime, the strengthening in these multilayers can be explained by the Hall–Petch model of dislocation pile-ups at interfaces or grain boundaries. The yield strength, ␴y, is proportional to h−1/2, where h is the layer thickness.5,6 Hall–Petch slope is a measure of the strength of interface barrier for slip transmission and determines the rate of strength increase with decreasing h. However, in the tens of nanometers regime, the Hall–Petch model breaks down. 1 The deformation mechanism may involve glide of single dislocations, in a)

Present address: United States Department of Energy, Germantown, Maryland.

the form of Orowan loops, leading to a ␴y⬀ ln(h)/h relation.4,7 In the limit of a few nanometers, the strength of the multilayer may be determined by the stress to transmit a single dislocation across the interfaces. Factors such as shear-modulus mismatch and lattice-parameter mismatch may determine the transmission stress for single dislocations. For multilayers with a large shear-modulus mismatch, dislocations in the low-shearmodulus phase need to overcome a high repulsive image force (Koehler stress) to enter the high-modulus phase.8 For multilayers with a small lattice mismatch, coherency can exist until a certain laye