Tailored elastic behavior of multilayers through controlled interface structure

Atomistic simulations are reviewed that elucidate the causes of the anomalous elastic behavior of thin films and composition-modulated superlattice materials. The investigation of free-standing thin films and of superlattices, composed of grain boundaries, shows that the elastic anomalies are not an electronic but a structural interface effect that is intricately connected with the local atomic disorder at the interfaces. The consequent predictions that (i)coherent strained-layer superlattices should show the smallest elastic anomalies and (ii) making the interfaces incoherent should enhance all anomalies, are validated by simulations of dissimilar-material superlattices. Such simulations can be an effective aid in tailoring the elastic behavior of composite materials because, in contrast with experiments, they allow one to systematically investigate simple, but well-characterized model systems with increasing complexity. This unique capability of simulations has enabled us to elucidate the underlying driving forces and, in particular, (i) to deconvolute the distinct effects due to the inhomogeneous atomic disorder, localized at the interfaces from the consequent interface-stress-induced anisotropic lattice-parameter changes and (ii) to separate the homogeneous effects of thermal disordering from the inhomogeneous effects due to the interfaces