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 2500Å alternating with Cu or Ti of 15Å. The other set has 250Å of Ru and the Cu or Ti layer varies between 15 and 200Å. 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  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.