Strengthening mechanisms of the nanolayered polycrystalline metallic multilayers assisted by twins

• Introducing nano-twinned lamellae into NPMMs can drastically enhance their strengths.
• Decreasing twin thickness leads deformation mechanism to transit from hairpin-like dislocation gliding to necklace-like one.
• Two deformation mechanisms coexist in NPMMs with non-uniform distribution.
• The rule of mixture can predict well the strengths of NPMMs with non-uniform twin lamella thickness.

To study the assistance of growth twin in nano-layered polycrystalline metallic multilayers to their strength, a series of uniaxial tensile modeling of nano-twinned Cu//Ag and Cu//Ni multilayers is performed via molecular dynamics method, with especial attentions to the influence of the twin lamella thickness and its spatial distribution on the strengthening mechanism. The results indicate that the strengths of nano-twinned multilayers can be enhanced significantly by introducing nano-twin lamellae into them and show strong twin thickness effect. By checking the atomic details, it is found there exists a critical twin lamella thickness above which the deformation mechanism of metallic multilayers is the hairpin-like partial dislocation gliding dominated and below which it becomes the necklace-like multiple jogged dislocation gliding dominated, however. The formation and transition of different deformation mechanisms are discussed in detail and analyzed theoretically, and the effect of the distinct deformation mechanisms on the strength of multilayers is also depicted quantitatively. In addition, the strengths and deformation mechanisms of two type nano-twinned Cu//Ag multilayers with non-uniform twin lamellae distribution are also discussed. The results show that the necklace-like dislocation mechanism and the hairpin-like dislocation mechanism can coexist in those non-uniform twinned multilayers and the necklace-like multi-jogged extended dislocations are unexpectedly observed in thicker twin lamellae. Although there are synergetic interactions between two mechanisms, the strengths of multilayers with non-uniform nano-twin lamella thickness can be well predicted by the rule of mixture.