The Structure and Mechanical Properties of Ru-Cu and Ru-Ti Nanolayer Composites
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THE STRUCTURE AND MECHANICAL PROPERTIES OF Ru-Cu AND Ru-Ti NANOLAYER COMPOSITES H. KUNG, M. NASTASI, T. R. JERVIS, K.M. HUBBARD, R.M. MESSNER*, T.E. MITCHELL, and J.D. EMBURY** Los Alamos National Laboratory, Los Alamos, NM 87545. *3M, Minneapolis, Minnesota **McMaster University, Ontario, Canada. ABSTRACT Multilayers of Ru-Cu and Ru-Ti have been prepared by electron beam evaporation technique. One set of composites has Ru thickness varying from 250 to 2500A alternating with Cu or Ti of 15A. The other set has 250A of Ru and the Cu or Ti layer varies between 15 and 200A. Nanoindentation measurements show that there is no significant change in hardness as either Ru or Cu/Ti thickness varies. However, the Ru-Cu multilayer has twice the hardness of the Ru-Ti system. High resolution transmission electron microscopy discloses that there is an epitaxial orientation relationship between Ru and Ti in Ru-Ti while no such relationship exists in Ru-Cu. The strengthening mechanism proposed by Koehler [1] predicts that Ru-Ti composites should have a higher strength than Ru-Cu due to the larger modulus difference between Ru and Ti. The discrepancy between the prediction and the experimental results suggests that other strengthening mechanism(s) may be operating. We have proposed two models based on a "shear" mechanism to explain the differences observed between these two systems. The effects of these mechanisms in controlling the deformation process in nanolayer composites are discussed. INTRODUCTION Thin film nanolayer composites composed of alternating layers of two metals have been shown to possess high strengths. These strengthening effects were predicted by Koehler [1] and confirmed experimentally for Al-Cu and Al-Ag nanolayers by Lehoczky [2,3]. An example from the work of Lehoczky on Al-Cu [2] of thickness t is given in Fig. 1. These data show that the tensile yield strength increases with 1/t until a maximum value is reached at a critical thickness of 70nm; the tensile fracture strength also peaks at this critical thickness. In fine scale structures, we need to consider both scale and elastic modulus mismatch constituents. According to Koehler's theory, the strengthening phenomena comes from the significant differences in elastic constants between the two components and also the thin layer thickness, which result in the large applied force needed to drive dislocations from the material with a low modulus into the high one. Using Koehler's model, Lehoczky showed that for a dislocation moving on a glide plane in a softer metal, A, into a stiffer metal, B, the minimum stress required for yield is given by: Cy > (VA+VB XA)!81(P'B+'A( ByA 8nc '9114'
where VA and VB are the volume fractions, YA and YB are the Young's moduli, and gt Aand PgBare the shear moduli of the softer and stiffer materials respectively. Taking into account both of the strengthening constraints discussed above, Kelly [4] predicted that Frank-Read sources will be restricted from operating and Koehler strengthening will be observed when the layer thickness
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