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In this paper, we have demonstrated that very perfect thin multilayers of the Cu/Ni system can be prepared with coherent interfaces if the layer modulation wavelength is in the 10 nm range. At modulation thicknesses above about 60 nm, the interfaces become incoherent. We have injected a crack into a coherent interface in the 10 nm case which has generated dislocations into the interface forming the crack plane, as well as into the layers adjacent to the crack plane. The dislocations injected into the crack plane presumably form misfit dislocations on that interface, and are grouped so close to the crack tip that individual dislocations are not completely imaged. The dislocations injected into the adjacent layers are distributed rather widely. We have analyzed the dislocation emission from a crack in the fee geometry appropriate to the multilayers using a simplified elastic theory developed for cracks in homogeneous materials. The mixed mode loading which the misfit stresses are expected to produce lead one to expect these materials to be ductile and to have high toughness. Very high dislocation densities on the crack plane near the crack, however, may lead to a brittle mode of failure, which is beyond the purview of the elastic theory. The dislocations are observed to have strong interactions with alternating interfaces in the multilayers, and this effect could be due to elastic bunching of the dislocations at alternating interfaces caused by the misfit stress.

I. INTRODUCTION

II. EXPERIMENTAL

The unusual plastic behavior of compositionally modulated multilayered alloys (CMA) has been of interest since 1921 when Blum first observed enhanced tensile properties in electrochemically produced alloys consisting of alternating layers of copper and nickel.1 He ascribed the observed enhancement to grain refinement strengthening. More recently with the work of Hilliard, Tsakalakos, and Baral a number of experiments were conducted on sputtered (111) textured Cu/Ni CMA that suggested an apparent enhancement in the elastic properties as well as in the plastic properties.2 This effect was also observed in other materials and has been discussed by Cammarata.3 A number of theories have been proposed that attempt to explain this apparent elastic effect.4 More recently Davis et al.5 have examined in more detail, elastic properties of Cu/Pd and Cu/Ni CMA, which in past studies exhibited a significant enhancement in elastic properties. They concluded, by using a combination of Brillouin scattering, surface-acoustic wave techniques, and uniaxial tensile testing, that there was no significant change in elastic properties. However, the plastic properties of these materials have been demonstrated to be significantly affected as the modulation wavelength is varied.6 This effect manifests itself in significant increases in the tensile strength, microhardness, and improvements in both lubricated and unlubricated sliding wear.7

Copper nickel superlattices have been electrochemically produced by a number of different investigators.8a This