Structure and Elastic Properties of Ni/Cu AND Ni/Au Multilayers
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STRUCTURE AND ELASTIC PROPERTIES OF Ni/Cu AND Ni/Au MULTILAYERS
ADEMOLA TAIWO"+, HONG YAN, AND GRETCHEN KALONJI" *Dept. of Materials Science & Engineering, University of Washington, Seattle, WA 98195 +Department of Nuclear Engineering, MIT, Cambridge, MA 02139 ABSTRACT
The structure and elastic properties of Ni/Cu and Ni/Au multilayer systems are investigated as a function of the number of Ni monolayers built into the systems. We employed lattice statics simulations with the interatomic potentials described by the embedded-atom method. For the Ni/Cu systems, coherent interfaces and FCC structure are maintained, and no elastic anomaly is found. For the Ni/Au systems, when the Ni layers are thick enough, they undergo a strain-induced phase transformation from FCC to HCP structure. An enhancement of Young's modulus of these systems is found to be associated with this structural change. INTRODUCTION
The observation of elastic anomalies in composition-modulated metallic superlattices, the socalled "supermodulus effect" [ 1,2], has inspired a lot of interest and enthusiasm over the past two decades. These anomalies include both an enhancement of certain moduli, such as Young's modulus and biaxial modulus, and a weakening of others, such as C 44. While this phenomenon has pointed to possibilities of engineering desirable mechanical properties by materials design, the understanding of the underlying mechanism is at best tentative and far from complete. A number of proposals has been put forward to explain the phenomenon. Earlier theories suggest that a modified electronic structure due to the additional periodicity in the layering would modify the mechanical properties [3]. "Coherency" strain caused by the in-plane mismatch at the interface has also been proposed as a possible source of elastic anomalies [4]. In addition, electron transfer between adjacent metallic layers [5] and surface tension at each interface [6] are all thought to be important. A consensus, however, seems to be that interfacial effects should contribute predominantly to the mechanism. Exploring the relationship between interfacial structure and resulting elastic properties, Wolf et al. have carried out a series of computer simulations of grain-boundary superlattices [7], and correlated the elastic anomalies with structural disorder around the interface. Later, they studied the effect of coherency on the elastic moduli by using the Lennard-Jones potential to model strained-layer systems with both coherent and incoherent interfaces [8], and reinforced their earlier conclusions on the role of structural disorder. In these simulations, the number of layers of the different materials are kept equal, and the bulk structures remain intact. In this paper, we report some preliminary results from our simulations of Ni/Au and Ni/Cu multilayer systems, in which the number of layers of different materials is varied systematically. The embedded-atom method (EAM) for interatomic potentials [9, 10] is used in our simulations. Similar simulations of Ni/Cu systems w
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