Deformation Behaviour of NiCo Multilayers with Modulated Microstructures

Metallic multilayers, which possess a material architecture of modulated coarse-grained and nanocrystalline features stacked in a laminate configuration, represent an ideal structure to study the influence of abrupt heterogeneities on deformation behaviour. While the deformation mechanisms of homogeneous coarse-grained and nanocrystalline materials are well documented, it is not clear from the current literature how these mechanisms may interact in a heterogeneous structure, and how these interactions manifest in the measured mechanical properties. The objective of this thesis is to characterize the dominant deformation mechanisms in NiCo multilayers with modulated microstructures. More specifically, a targeted study is performed to understand the influence of interfacial plastic interactions between multilayer features on the overall deformation behaviour. For this purpose, a wide range of multilayer architectures, which possess varying fractions of coarse-grained-nanocrystalline interfaces, are manufactured. The mechanical properties of these multilayers are measured using uniaxial tensile testing and nanoindentation, and the as-deposited structures are characterized through a combination of electron microscopy and crystallographic analysis. Interestingly, the flow stress of the multilayers is observed to significantly exceed rule of mixtures expectations as the fraction of structural interfaces is increased. These results indicate a strong contribution of interfacial effects to the overall deformation behaviour. Inspired by these findings, targeted molecular dynamics simulations are performed on representative architectures to investigate interfacial deformation mechanisms. Based on molecular dynamics simulations, a novel deformation mechanism, whereby strain relaxation is achieved in nanocrystalline features through emission of deformation twins into the coarse-grained microstructure, is elucidated. Consequently, the coarse-grained microstructure undergoes dynamic grain size refinement, leading to an increase in the intragranular boundary density. This deformation behaviour effectively reduces the dislocation mean free path of coarse-grained features, subsequently enabling additional strengthening. Based on the observed deformation mechanisms, a generalized phenomenological model is developed to predict multilayer work hardening, delivering significant improvements to rule of mixtures estimates. The emergence of deformation twinning in coarse-grained features is a unique consequence of microstructure modulation. This result offers a pathway to engineer deformation mechanisms and tailor mechanical performance through heterogeneous architectural design.Ph.D