Interfacial effect on strengthening nanoscale metallic multilayers - a combined Hall-Petch relation and atomistic simulation study

Besides the modulus difference, coherent stress and structural barrier mechanisms, the critical shear stress (CSS), which represents the resistance to the transmission of dislocation cross the interface, is another key factor influencing the hardness of nanoscale metallic multilayers (NMMs). However, it is very difficult to measure the CSS experimentally. In this study, we estimate the CSS based on the Hall-Petch relation via the available experimental hardness versus layer-thickness data. The obtained values are then verified by molecular dynamics simulations. With the proposed approach, 20 different NMMs are investigated systematically. The relative hardening contribution from the layers of the two constituent elements are quantified and these 20 NMMS grouped into two types except for one NMM. One group includes 12 NMMs whose interface hardening is contributed from both constituent element layers with different percentage. The other group includes 7 NMMs whose hardening contribution is mainly from the soft layer. We find that the peak hardness is not very sensitive to the CSS between the two constituent elements, rather is mainly determined by the average hardness of constituent layers. Moreover, it is more beneficial if the hard layer is about twice as hard as the soft layer for achieving high peak hardness.