Evolution of residual stress and interface coherency and their impact on deformation mechanisms in Al/Ti multilayers

Hardness of nanostructured metallic multilayers (NMMs) are often understood from their dependence on individual layer thickness (h), yet little is known about the impacts exerted by other microstructural factors. In this work, the effects of residual stress and interface coherency on the deformation mechanisms of Al/Ti NMMs with h = 2.5–52 nm were studied via experiments and molecular dynamics simulations. The residual stress was found to be tensile in Ti layers and compressive in Al layers in general, both of which tended to increase with decreasing h. Tensile stress of more than 2 GPa were measured in the Ti layers at h < 10 nm. Such high stress was related to the more coherent interfaces at small h, as confirmed by transmission electron microscopy analysis. Finally, molecular dynamics simulations showed that at h = 2.5 nm, the coherent interfaces were more effective barriers to dislocation initiation and transfer than the incoherent ones.