Size effects in strengthening of NiCo multilayers with modulated microstructures

M Daly and S Haldar and VK Rajendran and J McCrea and GD Hibbard and CV Singh, MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING, 771, 138581 (2020).

DOI: 10.1016/j.msea.2019.138581

The influence of size effects on the mechanical properties of a new class of multilayer architectures are reported. In contrast to traditional multilayers, which are constructed from alternating layers of differing metallic species, the multilayer architectures examined in this study are comprised of a periodic layering of coarse-grained and nanocrystalline microstructural features of the same composition. Results of mechanical testing indicate that multilayer architectures exhibit a rule of mixtures response when the thicknesses of coarse- grained layers are large. However, an anomalous strengthening effect is observed in architectures where coarse-grained layer thicknesses approach their crystal size. This strengthening phenomenon is found to be directly correlated to the increased interfacial density between coarse-grained and nanocrystalline layers. Using a combination of mechanical testing and molecular dynamics simulations, an interface- mediated deformation twinning mechanism is uncovered, which is determined to underpin strengthening effects. Consequently, the relative influence of this deformation mechanism on flow behavior becomes increasingly significant in multilayer architectures with higher interfacial densities. For this reason, the associated strengthening effects are not significant in multilayers with thick coarse-grained layers but become apparent in architectures with the finest coarse- grained layers. Overall, this study explores deformation behavior in a new design-space within the multilayer architecture and provides insight into the interplay between size effects (i.e, crystal size and layer thickness), deformation mechanisms, and resultant mechanical performance.

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