  Molecular dynamics (MD) simulations were employed with empirical potentials
to study the effects of multilayer interfaces and interface spacing in Al-Ti
nanolayers. Several model interfaces derived from stacking of close-packed
layers or face-centered cubic \{100\} layers were investigated. The simulations
reveal significant and important asymmetries in defect production with
$\sim$60% of vacancies created in Al layers compared to Ti layers within the
Al-Ti multilayer system. The asymmetry in the creation of interstitials is even
more pronounced. The asymmetries cause an imbalance in the ratio of vacancies
and interstitials in films of dissimilar materials leading to $>$90% of the
surviving interstitials located in the Al layers. While in the close-packed
nanolayers the interstitials migrate to the atomic layers adjacent to the
interface of the Al layers, in the \{100\} nanolayers the interstitials migrate
to the center of the Al layers and away from the interfaces. The degree of
asymmetry and defect ratio imbalance increases as the layer spacing decreases
in the multilayer films. Underlying physical processes are discussed including
the interfacial strain fields and the individual elemental layer stopping power
in nanolayered systems. In addition, experimental work was performed on
low-dose (10$^{16}$ atoms/cm$^2$) helium (He) irradiation on Al/Ti nanolayers
(5 nm per film), resulting in He bubble formation $\sim$1 nm in diameter in the
Ti film near the interface. The correlation between the preferential flux of
displaced atoms from Ti films to Al films during the defect production that is
revealed in the simulations and the morphology and location of He bubbles from
the experiments is discussed.
